The Application of Silica-Based Aerogel Board on the Fire Resistance and Thermal Insulation Performance Enhancement of Existing External Wall System Retroﬁt

energies

Article The Application of Silica-Based Aerogel Board on the Fire Resistance and Thermal Insulation Performance Enhancement of Existing External Wall System Retroﬁt

Kuang-Sheng Liu 1, Xiao-Feng Zheng 2 , Chia-Hsing Hsieh 3 and Shin-Ku Lee 4,*

1 Department of Interior Design, TungFang Design University, Kaohsiung City 829003, Taiwan; [email protected] 2 School of Built Environment, Engineering and Computing, Leeds Beckett University, Leeds LS1 3HE, UK; [email protected] 3 Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 310401, Taiwan; [email protected] 4 Research Center for Energy Technology and Strategy, National Cheng-Kung University, Tainan 701401, Taiwan * Correspondence: [email protected]

Abstract: Due to the need of good thermal performance, external wall insulation (EWI) is usually made of materials that are not fire resistant and sometimes flammable. That restricts its application to a particular circumstance such as limited building height. Hence, a material with good thermal insulation and fire resistance performance would allow EWI to be more widely applied. This paper introduces a novel material: a silica-based aerogel porous board, which differs itself from mainstream products available in the market because of its outstanding properties, such as low density, high Citation: Liu, K.-S.; Zheng, X.-F.; surface area, low thermal conductivity and superhydrophobicity. Herein, its thermal insulation and Hsieh, C.-H.; Lee, S.-K. The Application of Silica-Based Aerogel fire-resistant performance were tested and compared with commercial products. The cone calorimeter Board on the Fire Resistance and analysis results indicated that the aerogel porous board could improve the fire resistance performance. Thermal Insulation Performance Moreover, the evaluation of thermal insulation performance suggested that the application of an Enhancement of Existing External aerogel porous board on the external stone wall of existing buildings can decrease the U-value by Wall System Retrofit. Energies 2021, 60%. Through the detailed insight into the case-study, it is quite clear that the carbon impact of 14, 4518. https://doi.org/10.3390/ building stock could be greatly reduced by means of a coherent set of building envelope retrofitting en14154518 actions based on this innovative heat insulation material, without compromising the fire safety.

Academic Editor: Elena Cerro-Prada Keywords: existing building; external wall insulation; ﬁre resistance; energy efﬁciency; aerogel

Received: 8 June 2021 Accepted: 19 July 2021 Published: 26 July 2021 1. Introduction

Publisher’s Note: MDPI stays neutral Building energy retrofitting has become increasingly important as the energy con- with regard to jurisdictional claims in sumption in the building sector represents a significant proportion in the overall energy published maps and institutional affil- consumption [1–3]. Hence, reducing the energy consumption in the building sector is iations. highly necessary to decarbonize the sector. Delivering new low energy buildings and improving the energy efficiency of existing building stock are equally important to achieving that goal. External wall insulation (EWI) plays a very important role in building the energy- retrofitting process because it is able to reduce the building energy demand, improve the indoor acoustic environment and extend the life span of the buildings by cutting down the Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. heat loss through building fabric, filtering incoming external noises and preventing the This article is an open access article formation of condensation in the wall [4–6]. Hence, as a low-hanging fruit, the measures distributed under the terms and of installing EWI has always been taken in building industries to cut down the carbon conditions of the Creative Commons emission and in some occasion to improve the aesthetic of the buildings. Attribution (CC BY) license (https:// Currently, applying the thermal barrier coating on the outer surface of the wall or creativecommons.org/licenses/by/ roof is a common solution to enhance the heat insulation performance of a building in 4.0/). the regions with a subtropical or tropical climate [5,7,8]. Alternatively, thermal insulation

Energies 2021, 14, 4518. https://doi.org/10.3390/en14154518 https://www.mdpi.com/journal/energies Energies 2021, 14, 4518 2 of 19

materials can be installed in roofs or exterior walls [9]. The EWI with good thermal performance, when installed correctly, can significantly reduce the energy demand of the building. However, the EWI is commonly made of materials that are flammable and therefore poses a fire risk to the building. When a building with conventional EWI is on fire, there may be thus significant risks to the lives of occupants and property safety. Several fire accidents have occurred in buildings around the world [10–12]. A recent and well-known one is the fire that occurred in Grenfell tower in London, United Kingdom in 2017, which was caused by the overlooked fire resistance of the installed EWI. Thus, the building fire safety codes in many countries [13–15] request such materials used in the buildings must meet certain fire resistance standards by testing. Common thermal insulation materials include the polyurethane (PU) foam board, polystyrene (PS) board and mineral wool board, which however come with certain drawbacks in one way or another and therefore have limited applications [9,16]. For instance, mineral wool is a flame-retarded material, but the thermal conductivity of the mineral wool board is much higher than that of the PU or PS foam board. On the other hand, the PU or PS foam board is an excellent material for thermal insulation, it is highly combustible and therefore its application comes with a fire risk. One key property of the EWI material that fundamentally determines its fire risk is the flammability. The flammability of PU materials presents a threat to both the fire integrity of such an EWI product and to human health and safety. Therefore, searching for an appropriate fire retardant or synthesizing an inherently non-flammable PU is a significant scientific challenge [17]. Over the past decades, many research works [18–20] have been done to develop diols or polyols containing phosphorus or halogen, as a means for introducing flame retardancy into the PU structure by chemical bonding. In addition, methods to improve flame retardancy of PUs by using additive type flame retardants, such as expandable graphite and melamine, or reactive type flame retardants, such as organophosphorus compounds, cyclotriphosphazenes, aziridinyl curing agents in aqueous polyurethane dispersions (PUDs), organoboron compounds and organosilicon compounds have also been developed. Incorporation of carbon nanotubes (CNTs) into PUs and the use of functionalized fullerenes in PUs are the emerging methods to obtain good thermal stability and flame retardancy. However, flame retardant PUs are combustible materials that prevent or delay flashover from the surface of combustibles; a flame retardant is not designed to prevent the material from ignition but to keep the flame spread rate to a minimum and prevent sustained burning [20]. Recently, many research results of aerogel application in building materials have been published [21,22]. Aerogel found by Kistler [23] is solid with a porous microstructure. The traditional aerogel’s three-dimensional network structure is mainly composed of silicon dioxide, which accounts for more than 90% of the nanometer-sized holes in the total volume. Aerogels have a low thermal conductivity, high porosity, low density, high surface area, low refractive index and low dielectric constant. These features make aerogel more competitive in different applications such as heat insulation, sound insulation and fire resistance. Thus, some aerogel products, such as coatings [24], blankets [25], glasses [26] and cementitious composites [27,28], have been developed due to the outstanding properties. The weather in Taiwan features hot, wet summers and warm, dry winters due to Taiwan’s subtropical climate and Pacific Ocean location. The maximum daily temperature difference is within 20 ◦C[29]. Therefore, Taiwan’s building regulations regarding energy-saving codes for the building envelope are less stringent than those in cold zone countries. For example, the U-value criteria for the exterior wall and roof are 3.5 W/m2K and 0.8 W/m2K, respectively. It implies that as the Taiwanese government needs to meet the global 2050 carbon-neutrality target, future building energy efficiency regulations must enhance the thermal insulation performance of the building envelope. In addition, due to its unique geographical environment, Taiwan is often hit by natural hazards such as typhoons, torrential rains and earthquakes, so most of the building structures are RC structures with external tiles. However, with the increase in the lifespan of buildings and Energies 2021, 14, 4518 3 of 19

the interaction of external factors such as typhoons, earthquakes and dramatic temperature changes, problems such as falling and defects of tiles will affect the buildings’ appearance and even endanger the safety of the people. Cities in Taiwan are densely populated with 80 percent of the total population living in urban areas. Yet some 34,000 of the 86,000 buildings over 30 years old do not meet the latest building code and seismic resistance standards. In 2020, the Taiwan government launched a new statute for expediting the reconstruction of urban unsafe and old buildings to improve citizens’ living environment and building safety in response to the potential climate change and disaster risk. It is a quite important issue to propose a building envelope retrofitting scheme for such existing buildings to meet the criteria of the latest building codes [30]. Extensive research and industrial practices related to building energy retrofitting have been carried out in countries with a large percentage of old housing stocks [31–36]. For instance, the large uptake of building energy technologies and policy have enabled many member states in Europe to carry out state-wide energy retrofitting to improve the overall building energy efficiency in the building sector with many lessons learnt and lots of relevant knowledge accumulated. However, buildings in Taiwan cannot follow the same paths identified in countries with cold or moderate climates to improve the energy efficiency due to its unique climate and building characteristics. Based on the problems as mentioned above, developing an advanced dry-type EWI solution with good fire safety and thermal insulation performance is imminent. In the study, we prepared the MTMS-based silica aerogel porous board via the emulsion method combined with ambient pressure drying (APD). In addition, a series of experiments have been conducted to examine the basic properties of the aerogel porous board and then compared the thermal and fire-resistant performance between the aerogel porous board and three kinds of typical insulation materials, including the rigid PU foam board, fire-retardant rigid PU foam board [37] and rigid PS foam board [38]. The final objective was to evaluate the energy efficiency of the aerogel porous board applied in the EWI system of a case study in Taiwan. By achieving these objectives, it was expected to reduce the operational energy consumption of either new or existing buildings without compromising the fire safety quality. Due to the integrated properties shown in this study, the as-formed aerogel porous board have demonstrated promising applications in delivering improved energy efficiency and fire safety in existing buildings.

2. Materials and Methods 2.1. Preparation of the Aerogel Porous Board In this study, silica aerogel microspheres are prepared at two-step sol–gel synthesis, which is base-catalyzed condensation of methyltrimethoxysilane (MTMS) as the silica precursor [39]. The method for preparing the aerogel porous board includes the steps of: (a) adding a 50–70 vol.% MTMS containing methyl groups and a 0.1–0.3 vol.% surfactant into water, mixing evenly and carrying out hydrolysis to get a mixed aqueous solution; (b) mixing the 12–16 vol.% of mixed aqueous solution with 5–6 vol.% of 0.1–0.2 M ammonium hydroxide (NH4OH) and a remaining percentage of an organic solvent according to the percentage by volume (vol.%) and mixing by the magnetic stirrer until the solution obtained clear color in the hydrolysis reaction for emulsion polymerization to get a water-in-oil (w/o) lotion emulsion; (c) subsequently, the wet gels were shaped in a 10 cm × 10 cm × 1.5 cm steel mold at 150 ◦C in an oven for 24 h. The aerogel porous board was obtained by the ambient pressure drying (APD) method. Figure1 shows the ﬂow chart of the aerogel porous board manufacture via the sol–gel process. Energies 2021, 14, 4518 4 of 19 Energies 2021, 14, 4518 4 of 19

Figure 1. Schematic description of the formation of the aerogel porous board via the emulsion method combined with APD. Figure 1. Schematic description of the formation of the aerogel porous board via the emulsion method combined with APD. 2.2. Methods of Characterization The morphology of the aerogel was observed by scanning electron microscopy (SEM). 2.2.The Methods SEM images of Characterization were recorded on a FE-SEM microscope JEOL JSM-7001 operated at an acceleratingThe morphology voltage of ofthe 15 aerogel keV. The was specimen observed was by scanning mounted electron on a metal microscopy stub using (SEM). a Theconductive SEM images carbon were adhesive recorded disc on witha FE-SEM a 25 mm microscope diameter. JEOL The JSM-7001 samples’ operated surfaces at were an ac- celeratingsputter-coated voltage with of 15 Pt keV. prior The to analysis. specimen The wa surfaces mounted area on (the a metal measurement stub using range a conductive of the 2 carbonminimum: adhesive 0.01 mdisc/g) with of samples a 25 mm was diameter. determined The samples’ by a BET analysissurfaces fromwere thesputter-coated amount of N with2 Ptgas prior adsorbed to analysis. at various The surface partial pressuresarea (the measurement (0.01 < P/P0 < range 1) (NOVA of the 1000e, minimum: Quantachrome 0.01 m2/g) of Instruments, Boynton Beach, FL, USA). Preheating of samples was performed in a nitrogen samples was determined by a BET analysis from the amount of N2 gas adsorbed at various flow for 3 h at 200 ◦C to remove all the volatile materials. Additionally, the specific partial pressures (0.01 < P/P0 < 1) (NOVA 1000e, Quantachrome Instruments, Boynton Beach, surface areas were determined by using the BET equation with an accuracy of ±10 m2/g. FL, USA). Preheating of samples was performed in a nitrogen flow for 3 h at 200 °C to remove The pore volume (at STP the measurement range of minimum: 0.0001 cc/g) and pore all the volatile materials. Additionally, the specific surface areas were determined by using the size distributions (the measurement of range: 3.5–4000 Å) were measured using the BJH 2 BETcumulative equation pore with volume an accuracy method. of ±10 Fourier m /g. transform The pore infraredvolume spectroscopy(at STP the measurement (FTIR, Thermo range ofiS50, minimum: Madison, 0.0001 WI, USA)cc/g) withand pore 4 cm −size1 resolution distributions and 40(the scans measurement for each curve of range: was employed 3.5–4000 Å) wereto investigate measured theusing molecular the BJH structure cumulative of thepore aerogel volume powder. method. The Fourie aerogelr transform powder infrared was spectroscopymixed in the (FTIR, potassium Thermo bromide iS50, (KBr)Madison, powder WI, matrixUSA) with and then4 cm− placed1 resolution this in and the 40 sample scans for eachcup. curve To determine was employed the porosity, to investigate bulk density the molecular and contact structure angle, theof the mercury aerogel porosimeter powder. The aerogel(AutoPore powder IV 9520, was mixed Micromeritics) in the potassium and contact bromide angle (KBr) meter powd (CAM-121,er matrix Creating and then Nano) placed thiswere in used.the sample cup. To determine the porosity, bulk density and contact angle, the mercury porosimeterThe thermal (AutoPore conductivity IV 9520, ofMicromeritics) the aerogel particle and contact and aerogel angle meter porous (CAM-121, board at 25Creating◦C Nano)ambient were temperature used. were studied using a thermal conductivity analyzer (TCI-3-A, TCi) in accordanceThe thermal with conductivity ASTM D7984 of [40 th].e Thisaerogel device particle employs and theaeroge patentedl porous modified board at transient 25 °C am- bientplane temperature source (MTPS) were technique, studied whichusing isa thermal the only conductivity instrument engineered analyzer (TCI-3-A, for evaluating TCi) the in ac- cordancethermal conductivitywith ASTM D7984 of powder [40]. withoutThis device sample employs preparation. the patented Finally, modified the fire resistancetransient plane of the commercial external wall insulation materials and aerogel porous board were analyzed source (MTPS) technique, which is the only instrument engineered for evaluating the thermal by the cone calorimeter method in accordance with CNS 14705-1-2013 [41], respectively. conductivity of powder without sample preparation. Finally, the fire resistance of the com- The cone calorimeter method is commonly used to examine the reactions of materials in a mercial external wall insulation materials and aerogel porous board were analyzed by the cone calorimeter method in accordance with CNS 14705-1-2013 [41], respectively. The cone calorimeter method is commonly used to examine the reactions of materials in a fire by simulating different fire scenarios (ignition period radiant heat: 15 kW/m2; growth period radiant

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fire by simulating different fire scenarios (ignition period radiant heat: 15 kW/m2; growth period radiant heat: 30 kW/m2; growth period radiant heat: 50 kW/m2). The specimens with a surface area of 88.4 cm2 and 1.5 cm thick were heated by an incident radiant heat flux of 50.0 kW/m2 (about 737 ◦C) in a horizontal orientation. The test duration was 20 min. The size of all specimens for thermal conductivity and fire resistance testing were 10 cm in length, 10 cm in width and 1.5 cm in height. In order to evaluate the applicability of aerogel porous board under high humidity conditions. The water absorption of aerogel porous board was measured in accordance with ASTM D570-98 [42]. For the water absorption test, the aerogel porous board was dried in a chamber for a specified time and at temperature and then the aerogel porous board was weighed. The aerogel porous board was then submerged in water at 23 ◦C for 24 h. The specimen was removed, patted dry with a lint free cloth and weighed. Water absorption of the aerogel porous board can be calculated by (m1 − m0)/A × 100%. All these tests allowed evaluating the microstructural and physical performance, with the sample/test pairing summarized in Table1.

Table 1. Tests conducted for each sample.

Microstructural Physical Material SEM BET BD CA FTIR λ FR WA Aerogel porous X X X X X X X X board P1 P2 P1 R1 P2 R1 R1 R1 X X X X X Aerogel particle - -- P1 P2 P1 P1 R1 X X PU ------R1 R1 X X PS ------R1 R1 Flame-retardant X X ------PU R1 R1

Caption: SEM—scanning electron microscope; BET—N2 adsorption–desorption isotherms; BD—dry bulk density; CA—contact angle; FTIR—Fourier-transform infrared spectroscopy; λ—thermal conductivity; FR—cone calorimeter; WA—water absorption; Specimen size: P1—10 mg powder; P2—0.1 g powder; P3—5 g powder; R1—10 cm × 10 cm × 1.5 cm.

3. Results and Discussion 3.1. Characterization of the Aerogel Porous Board Figure2 shows the images of the aerogel porous board at various magniﬁcations. As shown in Figure2a, the aerogel porous board is white and opaque. The bulk density of the aerogel porous board is 0.0301 g/cm3. A SEM image shown in Figure2b clearly illustrates the presence of the porous network structure with pores of different sizes. The FTIR characterization results of the aerogel porous board is shown in Figure3 . The peaks centered at 3445, 1632 and 952 cm−1 are attributed to the O-H stretching band of hydrogen-bonded water, respectively. The peak at 1050 and 758 cm−1 for Si-O-Si stretching vibration corresponds to the structure of the silica network. The peaks at 2963 cm−1, 1380 cm−1 and 848 cm−1 can be attributed to the absorption of C–H. The presence of −1 Si-CH3 functionalities is conﬁrmed by the Si-C mode at 1260 cm . Furthermore, the aerogel porous board synthesized from MTMS is inherently hydrophobic in this study. Figure4 indicated that the aerogel porous board is hydrophobic with a contact angle as high as 145◦, thus, water can form droplets on the surface of the aerogel porous board. In addition, the water absorption of aerogel porous board was 0.186 g/100 cm2. Energies 2021, 14, 4518 5 of 19

heat: 30 kW/m2; growth period radiant heat: 50 kW/m2). The specimens with a surface area of 88.4 cm2 and 1.5 cm thick were heated by an incident radiant heat flux of 50.0 kW/m2 (about 737 °C) in a horizontal orientation. The test duration was 20 min. The size of all specimens for thermal conductivity and fire resistance testing were 10 cm in length, 10 cm in width and 1.5 cm in height. In order to evaluate the applicability of aerogel porous board under high humidity conditions. The water absorption of aerogel porous board was measured in accordance with ASTM D570-98 [42]. For the water absorption test, the aerogel porous board was dried in a chamber for a specified time and at temperature and then the aerogel porous board was weighed. The aerogel porous board was then submerged in water at 23 °C for 24 h. The specimen was removed, patted dry with a lint free cloth and weighed. Water absorption of the aerogel porous board can be calculated by (m1 − m0)/A × 100%. All these tests allowed evaluating the microstructural and physical performance, with the sample/test pairing summarized in Table 1.

Table 1. Tests conducted for each sample.

Microstructural Physical Material SEM BET BD CA FTIR FR WA X X X X X X X X Aerogel porous board P1 P2 P1 R1 P2 R1 R1 R1 X X X X X Aerogel particle - - - P1 P2 P1 P1 R1 X X PU ------R1 R1 X X PS ------R1 R1 X X Flame-retardant PU ------R1 R1 Caption: SEM—scanning electron microscope; BET—N2 adsorption–desorption isotherms; BD— dry bulk density; CA—contact angle; FTIR—Fourier-transform infrared spectroscopy; λ—thermal conductivity; FR—cone calorimeter; WA—water absorption; Specimen size: P1—10 mg powder; P2—0.1 g powder; P3—5 g powder; R1—10 cm × 10 cm × 1.5 cm.

3. Results and Discussion 3.1. Characterization of the Aerogel Porous Board Figure 2 shows the images of the aerogel porous board at various magnifications. As Energies 2021, 14, 4518 shown in Figure 2a, the aerogel porous board is white and opaque. The bulk density6 of of 19 the aerogel porous board is 0.0301 g/cm3. A SEM image shown in Figure 2b clearly illustrates the presence of the porous network structure with pores of different sizes.

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Energies 2021, 14, 4518 6 of 19 The FTIR characterization results of the aerogel porous board is shown in Figure 3. The peaks centered at 3445, 1632 and 952 cm−1 are attributed to the O-H stretching band of hydrogen-bonded water, respectively. The peak at 1050 and 758 cm−1 for Si-O-Si stretch- The FTIR characterization results of the aerogel porous board is shown in Figure 3. ing vibration corresponds to the structure of the silica network. The peaks at 2963 cm−1, The peaks centered at 3445, 1632 and 952 cm−1 are attributed to the O-H stretching band 1380 cm−1 and 848 cm−1 can be attributed to the absorption of C–H. The presence of Si-CH3 of hydrogen-bonded water, respectively. The peak at 1050 and 758 cm−1 for Si-O-Si stretch- functionalities is confirmed by the Si-C mode at 1260 cm−1. Furthermore, the aerogel po- ing vibration corresponds to the structure of the silica network. The peaks at 2963 cm−1, rous board synthesized from MTMS is inherently hydrophobic in this study. Figure 4 in- 1380 cm−1 and 848 cm−1 can be attributed to the absorption of C–H. The presence of Si-CH3 dicatedfunctionalities that the is aerogel confirmed porous by the board Si-C ismode hydrophobic at 1260 cm with−1. Furthermore, a contact angle the aerogel as high po- as 145°, thus,rous boardwater synthesizedcan form droplets from MTMS on the is surfaceinherent ofly thehydrophobic aerogel porous in this board.study. FigureIn addition, 4 in- the waterdicated absorption that the aerogel of(a aerogel) porous porous board boardis hydrophobic was 0.186 with g/100 a contact cm2. ( bangle) as high as 145°, FigureFigurethus, 2. 2.water TheThe appearance appearancecan form droplets ( (aa)) and and SEMon SEM the image image surface ( (bb)) of of the the aerogelaerogel aerogel porous porousporous board. board.board. In addition, the water absorption of aerogel porous board was 0.186 g/100 cm2.

Figure 3. FTIR spectrum of the aerogel porous board. FigureFigure 3. 3.FTIR FTIR spectrumspectrum of the aerogel aerogel porous porous board. board.

Figure 4. The water contact angle on the surface of aerogel porous board. FigureFigure 4. 4.The The water water contact contact angle angle on on the the surface surface of of aerogel aerogel porous porous board. board. Figure 5 shows the nitrogen adsorption–desorption isotherm and pore size distribu- Figure 5 shows the nitrogen adsorption–desorption isotherm and pore size distribution (PSD) of the aerogel porous board, which are obtained by using the standard BET tion (PSD) of the aerogel porous board, which are obtained by using the standard BET and BJH cumulative pore volume method, respectively. The aerogel porous board exhib- and BJH cumulative pore volume method, respectively. The aerogel porous board exhibited the type-IV adsorption isotherm according to the IUPAC classification [43], which is iteda characteristic the type-IV feature adsorption of the isotherm mesoporous according material. to The the averageIUPAC classificationpore size was [43],found which to is a characteristic feature of the mesoporous material. The average pore size was found to

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Figure5 shows the nitrogen adsorption–desorption isotherm and pore size distribution Energies 2021, 14, 4518 (PSD) of the aerogel porous board, which are obtained by using the standard BET and 7 of 19 BJH cumulative pore volume method, respectively. The aerogel porous board exhibited the type-IV adsorption isotherm according to the IUPAC classiﬁcation [43], which is a characteristic feature of the mesoporous material. The average pore size was found to be be approximatelyapproximately 2.47 2.47 nm nm in in Figure Figure2 b.2b. The The aerogel aerogel porous porous board preparedboard prepared in this work in had this a work 2 had a specificspeciﬁc surface surface area area asas high high as as 547 547 m /g. m2/g.

(a)

(b)

Figure 5. (aFigure) Nitrogen 5. (a) Nitrogen adsorption–desorption adsorption–desorption isotherms isotherms and and ((bb)) pore pore size size distribution distribution curve curve of the of the aerogel porousaerogel board. porous board.

The characteristics of the aerogel porous board compared with the Cabot P300 aerogel The characteristicsparticle are summarized of the inaerogel Table2 .porous Considering board that compared the required with aerogel the porous Cabot board P300 aerogel particlemust are have summarized low density and in highTable porosity, 2. Considering it can be concluded that thatthe therequired performances aerogel of the porous board mustaerogel have porous low density board prepared and high via theporosity emulsion, it methodcan be combinedconcluded with that APD the are performances similar of the aerogelto that porous of the aerogel board particle. prepared via the emulsion method combined with APD are similar to that of the aerogel particle.

Table 2. Comparison of the characteristics between the aerogel porous board and aerogel particle.

Aerogel Particle Property Aerogel Porous Board (Cabot P300) Specific surface area (m²/g) 547 600–800 Porosity 92% 90% Bulk density (g/cm³) 0.035 0.06 Pore diameter (nm) 2.47 20 Thermal conductivity (W/mK) 0.033 0.026 Contact angel 145° 151°

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Table 2. Comparison of the characteristics between the aerogel porous board and aerogel particle.

Aerogel Porous Aerogel Particle Property Board (Cabot P300) Speciﬁc surface area (m2/g) 547 600–800 Porosity 92% 90% Bulk density (g/cm3) 0.035 0.06 Pore diameter (nm) 2.47 20 Thermal conductivity (W/mK) 0.033 0.026 Energies 2021, 14, 4518 Contact angel 145◦ 151◦ 8 of 19

3.2. Thermal Conductivity 3.2. Thermal Conductivity Thermal conductivity is the important indicator for evaluating the thermal perfor- manceThermal of a material conductivity under is constantthe important conditions. indicator The for comparison evaluating the of thermalthermal conductivityperfor- mancebetween of a the material aerogel under porous constant board, conditio silica aerogelns. The particle comparison (Cabot, of thermal P300) and conductivity several thermal betweeninsulation the materials aerogel porous used commercially board, silica aerogel for building particle insulation, (Cabot, P300) such asand the several rigid PUther- and PS malfoam insulation board, are materials made in used Figure commercially6. The thermal for building conductivity insulation, of each such material as the wasrigid measured PU andwith PS three foam samples. board, are As made shown in Figure in Figure 6. The6, thethermal mean conductivity thermal conductivity of each material of the was aerogel measured with three samples. As shown in Figure 6, the mean thermal conductivity of the porous board at 25 ◦C ambient temperature was 0.033 W/mK with a standard deviation of aerogel porous board at 25 °C ambient temperature was 0.033 W/mK with a standard de- 0.00085 W/mK. At ambient temperature and pressure conditions, the trapped air within viation of 0.00085 W/mK. At ambient temperature and pressure conditions, the trapped the mesoporous aerogel is a major contributor to its low thermal conductivity. The thermal air within the mesoporous aerogel is a major contributor to its low thermal conductivity. conductivity (0.033 W/mK) of the proposed aerogel porous board was slightly lower than The thermal conductivity (0.033 W/mK) of the proposed aerogel porous board was slightlythose of lower monolithic than those silica of aerogels monolithic prepared silica withaerogels different prepared surface with modiﬁcation different surface procedures modification(0.036–0.0417 procedures W/mK) [(0.036–0.041744]. However, W/mK) 1 wt [44]. % of However, glass ﬁber 1 wt added % of glass in the fiber aerogel added porous inboard the aerogel for increasing porous board its mechanical for increasing strength its mechanical led to the strength thermal led conductivity to the thermal of the con- aerogel ductivityporous board of the higheraerogel than porous that board of the higher commercially than that available of the commercially aerogel particle available (0.026 aer- W/mK ogelwith particle a standard (0.026 deviation W/mK with of 0.002a standard W/mK). deviation of 0.002 W/mK).

FigureFigure 6.6.Comparison Comparison of the the thermal thermal insulation insulation property property between between thethe aerogel aerogel porous porous board board and andcommercially commercially available available materials. materials. The rigid PU foam board had the lowest thermal conductivity (0.0297 W/mK with a The rigid PU foam board had the lowest thermal conductivity (0.0297 W/mK with a standard deviation of 0.0014 W/mK) among the three types of foam board. The thermal standard deviation of 0.0014 W/mK) among the three types of foam board. The thermal conductivity of the foam board was mainly density-dependent; the lower density foam conductivity of the foam board was mainly density-dependent; the lower density foam board shows comparatively lower thermal conductivity than the higher density ones. The board shows comparatively lower thermal conductivity than the higher density ones. The expandable graphite was added as an intumescent flame retardant into PUs, the density expandable graphite was added as an intumescent flame retardant3 into PUs, the density of the flame-retardant rigid PU foam board increased from 70 to 80 kg/m . Thus, the3 ther- malof theconductivity flame-retardant of the flame-retardant rigid PU foam rigid board PU increasedfoam board from (0.040 70 W/mK to 80 kg/mwith a stand-. Thus, the ardthermal deviation conductivity of 0.0024 W/mK) of the was flame-retardant 35% higher than rigid that PU of foamthe rigid board PU foam (0.040 board. W/mK Pol- with a ystyrenestandard foams deviation (PSs) ofare 0.0024 also now W/mK) widely was used 35% as higherthermal than insulators that of in the building rigid PU and foam con- board. struction.Polystyrene The foamsthermal (PSs) conductivity are also of now the widelyPS board used mainly as thermal depends insulators on the blowing in building agent and andconstruction. the concentration The thermal of IR-attenuators. conductivity The of measured the PS board thermal mainly conductivity depends of on the the com- blowing mercial PS board was in the range of 0.0295–0.035 W/mK. Compared with the other typical foam boards, the thermal conductivity of the aerogel porous board was slightly higher than that of the other ones, but the U values of the EWI system with anyone should be similar to each other. In addition, the density of the aerogel porous board was the lightest of the three. These results imply that the aerogel porous board prepared via the emulsion method combined with APD has potential application prospects in building thermal insulation.

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agent and the concentration of IR-attenuators. The measured thermal conductivity of the Energies 2021, 14, 4518 9 of 19 commercial PS board was in the range of 0.0295–0.035 W/mK. Compared with the other typical foam boards, the thermal conductivity of the aerogel porous board was slightly higher than that of the other ones, but the U values of the EWI system with anyone should be similar to each other. In addition, the density of the aerogel 3.3. Fireporous Resistance board was the lightest of the three. These results imply that the aerogel porous boardIt is observed prepared viafrom the emulsionFigure 7 methodand Table combined 3 that with the APD rigid has PU potential foam board application burned very fast afterprospects ignition, in building the pk-HRR thermal insulation.value of the rigid PU foam board was 388.3 kW/m2 and the whole3.3. combustion Fire Resistance time of the rigid PU foam board was 182 s. The flame-retardant rigid 2 PU foamIt board is observed due fromto the Figure addition7 and Table of EG3 that could the rigid depress PU foam the board HRR burned value very to 83.25 fast kW/m , whichafter was ignition, 78.6% thelower pk-HRR than value that of thethe rigid rigid PU PU foam foam board board. was The 388.3 HRR kW/m curve2 and of the the flame- retardantwhole rigid combustion PU foam time board of the rigidis typical PU foam of boardan intumescent was 182 s. The system, flame-retardant exhibiting rigid two peaks. 2 The firstPU foam peak board is related due to to the the addition fast formation of EG could of depress an expanded the HRR valueprotective to 83.25 layer kW/m structure, on the surfacewhich was of 78.6%PU that lower prevents than that heat of the penetration, rigid PU foam while board. the The second HRR curve peak of the is flame-due to the de- retardant rigid PU foam board is typical of an intumescent system, exhibiting two peaks. structionThe first of the peak intumescent is related to the structure fast formation and th ofe an formation expanded of protective a carbonaceous layer structure residue [45]. The effectiveon the surface heat of of PU combustion that prevents (EHC) heat penetration, also revealed while the the burning second peak degree is due of to volatile the gases in thedestruction gas phase of theduring intumescent combustion. structure The and theav-E formationHC value of aof carbonaceous the flame-retardant residue [45]. rigid PU foamThe board effective was heat 16.12 of combustionMJ/kg, which (EHC) decreased also revealed by the25.1% burning compared degree of to volatile that of gases the rigid PU foamin board. the gas The phase HRR during curve combustion. of the Theaerogel av-EHC porous value board of the flame-retardantin Figure 8 indicates rigid PU that the foam board was 16.12 MJ/kg, which decreased by 25.1% compared to that of the rigid PU HRRfoam values board. increased The HRR rapidly curve of theto the aerogel peak porous and boardthen indeclined Figure8 indicatesslowly to that near the HRR0. It is generally believedvalues increased that heat rapidly generated to the peak when and the then organic declined materials, slowly to near such 0. as It isn-hexane, generally trapped in aerogelbelieved were that evaporated. heat generated The when HRR, the pk-HRR organic materials, and THR such values as n-hexane, show a trapped significant in reduction aerogelcompared were with evaporated. the rest The of HRR, building pk-HRR insulation and THR values materials. show a It significant is worth reduction noting that the compared with the rest of building insulation materials. It is worth noting that the THR THR value of the aerogel porous board was 3.0 MJ/m2, a drop of 77.4% compared with the value of the aerogel porous board was 3.0 MJ/m2, a drop of 77.4% compared with the 2 flameflame retardant retardant rigid rigid PU PU foam foam boardboard (13.3 (13.3 MJ/m MJ/m2). These). These results results indicated indicated that the that aerogel the aerogel porousporous board board can can dramatically dramatically inhibit inhibit the the combustion combustion intensity intensity and restrict and the restrict heat release the heat re- leaseduring during combustion. combustion. It could It could reach thereach non-flammable the non-flammable class 1 in accordanceclass 1 in with accordance CNS with CNS14705-1-2013. 14705-1-2013.

2 FigureFigure 7. 7.HRR HRR curves curves of different of different EWIs underEWIs the under 50 kW/m the 502 radiant kW/m heat radiant ﬂux. heat flux.

Energies 2021, 14, 4518 10 of 19

Table 3. Combustion parameters obtained from the cone calorimeter.

Time to Flameout pk-HRR THR av-EHC Samples Ignition (s) (s) (kW/m2) (MJ/m2) (MJ/kg) Rigid PU foam board 7 182 388.30 44.3 21.52 Flame-retardant rigid 2 61 83.25 13.3 16.12 PU foam board Rigid PS foam board 2 279 361.09 56.8 35.74 Energies 2021, 14, 4518 10 of 19 Aerogel porous board 0 0 22.11 3.0 16.59

(a) (b)

FigureFigure 8. 8. StreetStreet view view of of the the case case study study building building before before (a (a) )and and after after (b (b)) retrofit. retroﬁt.

Table3.4. 3. Thermal Combustion Transmittance parameters obtained from the cone calorimeter.

InTime order to toIgnition evaluate theFlameout impact of thepk-HRR application of theTHR aerogel porousav-EHC board on the Samples thermal insulation(s) performance(s) of the external(kW/m wall,2) the thermal(MJ/m transmittance,2) (MJ/kg) also known Rigid PU foam board as the U-value, 7 was used in this 182 study. The thermal 388.30 transmittance 44.3 calculated by 21.52the formula Flame-retardant rigid PU foam boardin accordance 2 with ISO 10292 61 [46 ] is shown 83.25 in Equation (1): 13.3 16.12 Rigid PS foam board 2 279 361.09 56.8 35.74 1 Aerogel porous board 0 0 U = 22.11 3.0 16.59 (1) 1 + ∑ di + 1 he λi hi 3.4. Thermal Transmittance where h (W/m2K) and h (W/m2K) are the external and internal heat transfer coefficients In ordere to evaluate thei impact of the application of the aerogel porous board on the respectively. λ (W/mK) and d (m) are the thermal conductivity and thickness of each thermal insulationi performancei of the external wall, the thermal transmittance, also material. known as the U-value, was used in this study. The thermal transmittance calculated by The external heat transfer coefficient is a function of the wind speed, v, near the outdoor the formula in accordance with ISO 10292 [46] is shown in Equation (1): surface of the wall system given by the following approximate formula: he = 10.0 + 4.1v. The internal heat transfer coefficient was given1 by h = 3.9 + 6.3ε. The value h and h Ｕ i e i equal to 23 and 9 W/m2K were used in this1 study.d 1 (1) ∑ Table4 lists the thermal transmittanceh of dryλ stoneh cladding with the aerogel porous board alongside two typical external wall types including 15 cm of RC external wall and dry where he (W/m2K) and hi (W/m2K) are the external and internal heat transfer coefficients stone cladding on the external wall. U-value rates in dry construction technologies were respectively. λ (W/mK) and di (m) are the thermal conductivity and thickness of each material.comparatively low when compared to traditional wall systems, such as reinforced concrete 2 structureThe external (15 cm heat thickness, transfer 3.495 coefficient W/m K).is a Thefunction U value of the of wind the dry speed, construction v, near the system out- for 2 cm stone cladding on the 15 cm RC wall contained in a gap of 8 cm could decrease door surface of the wall system given by the following approximate formula: h 10.0 to 2.413 W/m2K. The utilization of dry construction with stone cladding on the external 4.1. The internal heat transfer coefficient was given by h 3.9 6.3ε. The value he and wall with an additional aerogel porous board unit demonstrated its capacity to reduce hi equal to 23 and 9 W/m2K were used in this study. the U-value to as low as 1.394 W/m2K, representing a promising assembly for insulating Table 4 lists the thermal transmittance of dry stone cladding with the aerogel porous purposes when its overall thickness is not a restriction for building implementation. The board alongside two typical external wall types including 15 cm of RC external wall and thermal transmittance of this proposed dry construction wall system was already lower dry stone cladding on the external wall. U-value rates in dry construction technologies were comparatively low when compared to traditional wall systems, such as reinforced concrete structure (15 cm thickness, 3.495 W/m2K). The U value of the dry construction system for 2 cm stone cladding on the 15 cm RC wall contained in a gap of 8 cm could decrease to 2.413 W/m2K. The utilization of dry construction with stone cladding on the external wall with an additional aerogel porous board unit demonstrated its capacity to reduce the U-value to as low as 1.394 W/m2K, representing a promising assembly for insulating purposes when its overall thickness is not a restriction for building implementation. The thermal transmittance of this proposed dry construction wall system was already lower than the evaluation criterial (1.8 W/m2K) of the green building materials rating system in Taiwan.

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than the evaluation criterial (1.8 W/m2K) of the green building materials rating system Energies 2021, 14, 4518 Table 4. The U value of the building wall system. 11 of 19 in Taiwan. Energies 2021, 14, 4518 Thermal Thermal11 of 19 Wall Table 4. The U value of the buildingThickness wall system. U Value Architectural Details Conductivity Resistance Assembly Table 4. The U value of the building wall system.dx (m) (W/m2K) Thermal(W/mK) Thermal(m2K/W) Wall ThicknessThermal Thermal U Value Wall Table 4. TheExternal U value Heat of the TransferbuThicknessilding wall system. U Value ArchitecturalArchitectural Details Details ConductivityConductivity Resistance Resistance 2 AssemblyAssembly dx (m) dx (m)- - (W/m2 0.04352K) (W/m K) Coefficient Thermal (W/mK)Thermal2 (m K/W) Wall Thickness (W/mK) (m K/W) U Value Architectural Details Conductivity Resistance Assembly ExternalExternal Heat TransferTile Heat dx (m) 0.010 1.3 (W/m0.00772K) - -(W/mK)- -(m0.04352K/W) 0.0435 TransferCoefficientCement Coefficient Mortar 0.015 1.5 0.0100 External HeatTile Transfer 0.010 1.3 0.0077 Reinforced Concrete- 0.150 - 1.40.0435 0.1071 CementCoefficient MortarTile 0.015 0.010 1.5 1.3 0.0100 0.0077 15 cm ReinforcedCementTile Concrete Mortar 0.1500.010 0.01 1.41.3 1.5 0.10710.0077 0.0067 CementCement Mortar Mortar 0.015 0.015 1.5 1.5 0.0100 0.0100 3.495 RC WALL15 cm Cement Mortar 0.01 1.5 0.0067 15 cm ReinforcedReinforced Concrete 0.150 1.4 0.1071 3.495 RC WALL 0.150 1.4 0.1071 3.495 RC WALL15 cm CementConcrete Mortar 0.01 1.5 0.0067 Internal Heat Transfer 3.495 RC WALL Internal Heat Transfer Cement Mortar- 0.01- - 1.50.1111- 0.00670.1111 CoefficientCoefficient Internal Heat Transfer Internal Heat - --- 0.1111 Coefficient 0.1111 Transfer Coefficient External Heat Transfer External Heat Transfer- - 0.0435 CoefficientExternal Heat - - 0.0435 External HeatCoefficient Transfer - - 0.0435 TransferCladding Coefficient 0.020- 0.4- 2.50000.0435 CoefficientAir LayerCladding - 0.020 - 0.4 0.0500 2.5000 Cladding 0.020 0.4 2.5000 CementCladding MortarAir Layer 0.015 0.020 - 1.5 0.4 0.0100- 2.5000 0.0500 dry stone clad- ReinforcedAir Layer Concrete 0.150 - 1.4 - 0.10710.0500 CementAir Layer Mortar -0.015 - 1.5 0.0500 0.0100 ding on the ex- Cement Mortar 0.015 0.01 1.5 0.00670.0100 2.413 drydry stone stonedry stone clad- clad- ternal wall ReinforcedReinforcedCement Concrete Mortar Concrete 0.150 0.015 0.150 1.4 1.5 1.4 0.1071 0.0100 0.1071 claddingding on on the the ex- 2.413 2.413 ding on the ex- CementCement Mortar Mortar 0.01 0.01 1.5 1.5 0.0067 0.0067 2.413 externalternal wall wall ternal wall Internal HeatReinforced Transfer - 0.150- 1.40.1111 0.1071 CoefficientConcrete Internal Heat Transfer - - 0.1111 InternalCoefficientCement Heat Mortar Transfer 0.01 1.5 0.0067 - - 0.1111 ExternalInternal HeatCoefficient Transfer Heat - -- -0.0435 0.1111 TransferCoefficient Coefficient ExternalCladding Heat Transfer 0.020 0.4 0.0500 - - 0.0435 AerogelCoefficientExternal Porous Board Heat 0.01 0.033 0.3030 External Heat Transfer - - 0.0435 dry stone clad- TransferAirCladding Layer Coefficient 0.020 - - 0.4 - 0.0860-0.0500 0.0435 ding on the ex- AerogelCement Porous MortarCoefficient Board 0.015 0.01 0.033 1.5 0.01000.3030 dry stone clad- Cladding 0.020 0.4 0.0500 ternal wall ReinforcedAir Layer ConcreteCladding 0.150 - 0.020 1.4 - 0.4 0.10710.0860 1.394 0.0500 ding on the ex- with aerogel CementAerogelAerogel Mortar Porous Porous Board 0.015 0.01 0.01 1.5 0.033 0.00670.0100 0.3030 porousternal boardwall Reinforced Concrete 0.150 0.01 1.4 0.033 0.1071 0.30301.394 drydry stone stone clad- Board with aerogel Cement MortarAir Layer 0.01 - 1.5 -0.0067 0.0860 claddingporous on the board Internal Heat Transfer ding on the ex- CementAir Layer Mortar - -0.015 - - 1.50.1111 0.0860 0.0100 external wall Coefficient 1.394 ternal wall InternalReinforced Heat Transfer Concrete 0.150 1.4 0.1071 1.394 with aerogel Cement Mortar- 0.015- 1.50.1111 0.0100 with aerogel CoefficientCement Mortar 0.01 1.5 0.0067 porous board Reinforced porous board 0.150 1.4 0.1071 4. Case Study: The RetrofittingConcrete of a Typical Multistory Building Internal Heat Transfer 4. CaseIn thisStudy: case The study,Cement Retrofitting the selected Mortar of residentiala Typical 0.01Multistory building- for Buildin retrofitting 1.5g - is located 0.0067in down-0.1111 town Kaohsiung, an areaCoefficient with extremely high real estate values. Therefore, the retrofit In this case study,Internal the selected Heat residential building for retrofitting is located in down- interventions have to include only the compact- and easy/fast-to-install - building 0.1111envelope system.town Kaohsiung, This building Transfer an areais representative Coefficientwith extremely of ty highpical real residential estate values. buildings Therefore, in Taiwan, the thereby retrofit presentinginterventions an haveexemplary to include retrofitting only the case compact study andfor potential easy/fast-to-install wide applications building in envelope the Tai- system. This building is representative of typical residential buildings in Taiwan, thereby wanese4. Case context. Study: The Retrofitting of a Typical Multistory Building 4.presenting Case Study: an exemplary The Retrofitting retrofitting case of a study Typical for potential Multistory wide applications Building in the Tai- 4.1.wanese ClimaticIn context. this and case Environmental study, the Conditions selected residential building for retrofitting is located in down- In this case study, the selected residential building for retrofitting is located in down- town Kaohsiung, an area with extremely high real estate values. Therefore, the retrofit town4.1. Climatic Kaohsiung, and Envi anronmental area with Conditions extremely high real estate values. Therefore, the retrofit interventions have to include only the compact and easy/fast-to-install building envelope interventions have to include only the compact and easy/fast-to-install building envelope system. This building is representative of typical residential buildings in Taiwan, thereby system. This building is representative of typical residential buildings in Taiwan, thereby presenting an exemplary retrofitting case study for potential wide applications in the Tai- presenting an exemplary retrofitting case study for potential wide applications in the wanese context. Taiwanese context.

4.1. Climatic and Environmental Conditions

Energies 2021, 14, 4518 12 of 19

4.1. Climatic and Environmental Conditions Retroﬁtting this residential building is challenged by its climate and site conditions, which are associated with dynamic and variable weather conditions in a humid subtropical climate, with abundant local rains, sunny days and brief periods of overcast skies. The location of this building lies above the 22.4◦ north latitude, a degree south of the Tropic of Cancer in Kaohsiung, Taiwan. This region experiences temperatures ranging from an average low of 10 ◦C (50 ◦F) in January to the average high of 33 ◦C (91 ◦F) in July, with a consistent daytime length throughout the year, approximately 13.5 h of daylight on the summer solstice and 10.5 h on the winter solstice. With more than 2210 h of bright sunshine, the city is one of the sunniest areas in Taiwan. The noon temperature reaches 35 ◦C in the summer and 25 ◦C in winter. The high summer sun angles emphasize the need to provide a sheltering roof to control light levels, solar gains and temperatures in the indoor environment. Despite about 225 bright sunny days per year, there remains a monthly rhythm of overcast or rainy days, including an average of 2–3 typhoons per month during summer time. The annual mean solar irradiation and precipitation are 3896 MJ/m2 and 1968.2 mm, respectively [29].

4.2. Case Study Building Multistory buildings are commonly seen in urban areas of Taiwan and they are usually built alongside streets to provide spaces for multiple uses. For instance, the ground floor is usually used for retail or a restaurant, with upper floors being used as residential units or offices. This typical building typology represents 90% of Taiwanese building stock and contributes to 15.3% of energy consumption [47]. Mostly built in the 90s, they have poor energy efficiency due to the absence of a stringent building code. Being built adjacently to other buildings in a long stretch, they also represent a great potential of widespread fire hazard. Therefore, a building of this type is selected in the case study to explore a retrofitting approach to improve its energy efficiency with improved fire safety. As shown in Figure8, this east facing building is a 13.45-m tall reinforced concrete building composed of four floors. The total building area is about 347.15 square meters, and the floor-to-ceiling height of each story is 3.3 m, with a 60% window-to-wall ratio on the glazed building façade. This large window-to-wall ratio design ensures an even distribution of natural light. However, it leads to a strong greenhouse effect throughout the day. This greenhouse effect accumulates a lot of solar heat during the summer and leads to an overheated indoor environment. Although there is a ceiling-level solar shading design on each floor, the solar heat gain via the glazing and wall still represents a significant cooling load and must be reduced. As shown in Figure9, this building has a rectangular floor plan with a length of 19.73 m and a width of 4.74 m. The building offers spaces for both residential and commercial uses. The first floor is rented out by the owner as a shop and the second to fourth floors are for residential use. Since the building faces east and the homeowner adheres the opaque film on the glass in order to reduce the solar radiation in the morning. Thus, there was insufficient penetration of natural light, leading to poorly lit indoor space before the retrofit. The results of measuring indoor illuminance by the illuminometer indicated that at 10:00 in the morning, the illuminance near the window can reach up to 1050 Lux. The daylight can only diffuse to the indoor depth of 2 m in the afternoon and the average illuminance at other measurement locations is below 250 Lux. The uniformity of illuminance level on the entire floor was rather poor.

4.3. Retroﬁtting Scenario: Building Envelope In order to reduce the heat gain of the building envelope and the electricity consumption of the air conditioning system in this case study, we reduced the window area by 35%, changed the glass from clear tempered glass to green tempered glass and installed a dry cladding system on the existing external wall with the aerogel porous board. After retroﬁtting this building, the SHGC of the glass was reduced from 0.83 to 0.60 and the visi- Energies 2021, 14, 4518 13 of 19

ble light transmission of the glass was changed from 0.88 to 0.74. Recladding signiﬁcantly 2 Energies 2021, 14, 4518 reduced the overall U value of the wall from 3.495 to 1.394 W/m /K because of its fabric13 of 19 construction change.

(a)

(b)

(c)

FigureFigure 9. (a)9. Existing(a) Existing floor ﬂoor plan plan for for typical typical floor, ﬂoor, ( (bb)) illuminance illuminance contour contour at at 10:00 10:00 and and (c) ( illuminancec) illuminance contour contour at 14:00. at 14:00.

4.3.4.4. Retrofitting Evaluation Scenario: on the Energy Building Efﬁciency Envelope Improvement In Inorder this to study, reduce the the building heat gain energy of the simulation building and envelope analysis and platform the electricity (BESTAI) consump- was tionadopted of the toair conduct conditioning simulations system of energyin this consumption.case study, we It isreduced the integrated the window graphical area by 35%,interface changed software the glass program from developed clear tempered for the purposeglass to ofgreen conducting tempered an EnergyPlus glass and build-installed a drying cladding energy consumptionsystem on the analysis existing as a external dynamic wall engine. with BESTAI the aerogel provides porous users withboard. the After modeling and design of an on-line real-time building simulation analysis to obtain optimal retrofitting this building, the SHGC of the glass was reduced from 0.83 to 0.60 and the visible light transmission of the glass was changed from 0.88 to 0.74. Recladding significantly reduced the overall U value of the wall from 3.495 to 1.394 W/m2/K because of its fabric construction change.

4.4. Evaluation on the Energy Efficiency Improvement In this study, the building energy simulation and analysis platform (BESTAI) was adopted to conduct simulations of energy consumption. It is the integrated graphical interface software program developed for the purpose of conducting an EnergyPlus building energy consumption analysis as a dynamic engine. BESTAI provides users with the

Energies 2021, 14, 4518 14 of 19

energy analysis data in Taiwan. In addition, it offers users a customized analysis with various reporting functions, such as: ROI (return on investment) assessment, the annual energy consumption analysis and various types of electricity price estimates from the Taiwan Power company and other analytical services. BESTAI integrates ESCO, BEMS, the construction materials and equipment industry to provide customized recommendations for different conditions and applications through functional modules. It received a 2017 R&D 100 Award [48]. For computer simulation and modeling, the physical parameters include the window, the external wall structure and the window-to-wall ratio. The numbers of occupants, the heat gain of appliances and lighting for each floor are listed in Table5. In general, the lighting equipment, computer and appliances on the first floor have identical operating periods corresponding to the staff’s work shifts from 7:30 am to 8:30 pm. A load of operating lighting, computers and appliances, according to the period, on the other floors, are set according to the survey by Lin et al. [47].

Table 5. Physical parameters for the building.

Items Before After U-value @ exterior 3.495 W/m2K 1.394 W/m2K wall U-value @ roof 1.0 W/m2K 1.0 W/m2K Window to Wall Ratio 0.9 (façade) 0.65 (façade) U-value @ window 5.5 W/m2K 5.5 W/m2K Clear glass Green glass U = 5.97 W/m2K U = 5.97 W/m2K Glass SHGC = 0.83 SHGC = 0.60 Visible Light Transmittance = 0.88 Visible Light Transmittance = 0.73 1st F: 2 staff + 20 Persons/h 1st F: 2 staff + 20 Persons/h No. of people 2nd–4th F: 4 Persons/ each story 2nd–4th F: 4 Persons/ each story 1st F: 660 W (10 W/m2) 1st F: 660 W (10 W/m2) Lighting 2nd–4th F: 200 W/each story 2nd–4th F: 200 W/each story 1st F: 1200 W 1st F: 1200 W Appliance 2nd–4th F: 500 W/each story 2nd–4th F: 500 W/each story

The installed air conditioning system at this building is a multisplit type air conditioning system. The total cooling capacity of the air conditioning system installed on the first floor was 33 kW and on the other feet was 6 kW and the specified coefficient of performance (COP) was 2.7. The start and stop of the air conditioners on the first floor were scheduled according to the different seasons. In the summertime, the shop on the first floor had a significant cooling demand from June to September, as shown in Table6. Conversely, the cooling load was low in winter, so the operating hours of air conditioner were shortened on regular working days. On other floors, the occupants’ air conditioning hours were from 8 pm to 2 am on weekdays and from 1 pm to 2 am on Sunday and holidays. The weather data was taken from the latest typical meteorological years (TMY3) for the building energy simulation of Taiwan proffered by the Architecture and Building Research Institute (ABRI) [49]. Basically, it sampled 15 standard years from 1998 to 2012 as standard weather data. The weather data can be generated to analyze the power consumption of an air conditioning system once the user selects the location of the building. Energies 2021, 14, 4518 15 of 19

Energies 2021, 14, 4518 15 of 19

Table 6. Monthly operation schedule for switching on-and-off air conditioner.

Month Periods (hh:mm) Rate of Operating Load (%) Table 6. Monthly operation schedule for switching on-and-off air conditioner. January 10:00–18:00 FebruaryMonth10:00–18:00 Periods (hh:mm) Rate of Operating Load (%) MarchJanuary10:00–19:00 10:00–18:00 April 09:00–19:00 February 10:00–18:00 May 09:00–20:00 March 10:00–19:00 June 08:00–20:00 100 JulyApril08:00–20:00 09:00–19:00 AugustMay08:00–20:00 09:00–20:00 SeptemberJune08:00–20:00 08:00–20:00 100 OctoberJuly09:00–20:00 08:00–20:00 November 10:00–19:00 August 08:00–20:00 December 10:00–19:00 September 08:00–20:00 TheOctober weather data was taken from 09:00–20:00 the latest typical meteorological years (TMY3) for the buildingNovember energy simulation of Taiwan 10:00–19:00 proffered by the Architecture and Building Re- search InstituteDecember (ABRI) [49]. Basically, 10:00–19:00 it sampled 15 standard years from 1998 to 2012 as standard weather data. The weather data can be generated to analyze the power consumption of an air conditioning system once the user selects the location of the building. FigureFigure 10 10shows shows the the breakdown breakdown of of electricity electricity consumptionconsumption by equipment equipment type type in in this this building. Before the renovation, about 41% electricity consumption was associated building. Before the renovation, about 41% electricity consumption was associated with with the air conditioning system. Lighting system and appliance accounted for 26.5% and the air conditioning system. Lighting system and appliance accounted for 26.5% and 32.0% of the total electricity consumption, respectively. With our proposed renovation 32.0% of the total electricity consumption, respectively. With our proposed renovation concept the total and air conditioning system’s electricity consumption shown in Figure 11 concept the total and air conditioning system’s electricity consumption shown in Figure would be reduced by 29% and 70%, respectively, resulting in the air conditioning system 11 would be reduced by 29% and 70%, respectively, resulting in the air conditioning sys- accounting for 16.69% of the total electricity consumption. Figure 12 shows the comparison tem accounting for 16.69% of the total electricity consumption. Figure 12 shows the com- between the simulations and the real monthly electricity consumption of this building. parison between the simulations and the real monthly electricity consumption of this The monthly simulation result gave a power consumption that was significantly higher building. The monthly simulation result gave a power consumption that was significantly than real electricity consumption in the first half of the year. The simulated results came higher than real electricity consumption in the first half of the year. The simulated results out with 9.5% and 4.0% more electricity consumption than the metered data before and aftercame renovation. out with This9.5% discrepancy and 4.0% more could electricity have occurred consumption because than the weather the metered data filedata used before in BESTAIand after was renovation. for the years This discrepancy between 1998 could and have 2012, occurred while the because actual the data weather gathered data in file thisused building in BESTAI was for was the for years the years 2019–2020. between This 1998 case and study 2012, result while implies the actual that data the proper gathered usein of this the building aerogel porouswas for boardthe years inbuildings 2019–2020. not This only case reduces study result the energy implies usage that butthe alsoproper downsizesuse of the the aerogel HVAC porous system board during in the buildings design. not only reduces the energy usage but also downsizes the HVAC system during the design.

Fan Fan 1% 1% Air Air Lighting Conditioning Conditioning 17% 27% 40% Lighting 37%

BEFORE AFTER

Appliance Appliance 45% 32%

(a) (b) Figure 10. Comparison of annual electricity use breakdown for a residential building case study before (a) and after (b) Figure 10. Comparison of annual electricity use breakdown for a residential building case study before (a) and after renovation. (b) renovation. Energies 2021, 14, 4518 16 of 19 Energies 2021, 14, 4518 16 of 19

Energies 2021, 14, 4518 16 of 19

Figure 11. Electricity consumptions for the proposed retrofitting scenario. Figure 11. Electricity consumptions for the proposed retroﬁtting scenario. Figure 11. Electricity consumptions for the proposed retrofitting scenario.

Figure 12. Comparing the monthly electricity consumption between simulation and metered. Figure 12. Comparing the monthly electricity consumption between simulation and metered. Figure 12. Comparing5. Conclusions the monthly electricity consumption between simulation and metered. 5. ConclusionsIn the study, a new aerogel porous board for application in Taiwan was investigated. 5. Conclusions It canIn be the used study, in athe new near aerogel future porous for redu boardcing for energy application consumption in Taiwan and was enhancing investigated. fire Itsafety canIn beperformance the used study, in the a newin near buildings aerogel future porous duefor reduto board itscing low for energy thermal application consumption conductivity in Taiwan and and was enhancing good investigated. fire firere- safetysistance.It can beperformance usedThe results in the nearinof buildingsthe future experiment for due reducing to are its summarized energylow thermal consumption as conductivity follows: and enhancing and good fire fire safety re- performance in buildings due to its low thermal conductivity and good fire resistance. The sistance. The results of the experiment are summarized as follows: (1)resultsA well-designed of the experiment method are summarized including emulsification as follows: condensation gelation and con- (1) Atrolled well-designed ambient drying method at including150 °C for emulsification24 h was developed condensation to fabricate gelation an aerogel and con- po- (1) A well-designed method including emulsification condensation gelation and con- trolledrous board ambient with dryinga uniform at 150 skeleton. °C for One 24 h of was advantages developed is thatto fabricate it was fabricated an aerogel under po- trolled ambient drying at 150 ◦C for 24 h was developed to fabricate an aerogel porous rousambient board pressure with a uniformdrying without skeleton. tedious One of advantagessurface modification is that it was and fabricated the solvent under ex- ambientboard with pressure a uniform drying skeleton. without One tedious of advantages surface modification is that it was fabricatedand the solvent under am-ex-

Energies 2021, 14, 4518 17 of 19

bient pressure drying without tedious surface modification and the solvent exchange process. The reproducibility of the aerogel porous board was also good, especially with respect to mesopore size distributions. This suggests a low fabrication cost is technically viable. (2) For the microstructural properties, it was possible to verify high values of porosity, low density and superhydrophobicity for the aerogel porous board, similar to that of the benchmarked aerogel particle. (3) For the thermal insulation property, adding 1 wt % of glass fiber to the aerogel porous board could increase its mechanical strength but lead to the thermal conductivity of the aerogel porous board being higher than that of the commercially available aerogel particle. Compared with the other typical foam boards, the thermal conductivity of the aerogel porous board was slightly higher than that of other ones, but the U values of the EWI system with the aerogel porous board should be similar to that with other foam boards. (4) For the fire resistance property, the PU and PS foam boards were flammable, even when the flame-retardants were added to these foam boards. Therefore, reducing the flammability of traditional EWI materials by using the flame retardant is hard to achieve. The proposed aerogel porous board can dramatically inhibit the combustion intensity and restrict the heat release during combustion without using additives. It could reach non-flammable class 1 in accordance with CNS 14705-1-2013. (5) Concerning the hygric performance, the water absorption of the aerogel porous board was low (0.186 g/100 cm2) and the surface was hydrophobic, so it is suitable to use under the typical Taiwan climate, i.e., high temperature and humidity climate. The aerogel porous board will not absorb too much water vapor and cause a decrease in thermal insulation performance. (6) From the energetic point of view, the pilot study in this paper suggests that the aerogel porous board could be applied feasibly in regions with hot and humid climates in general and particularly in Taiwan. The evaluation result of the case study reported in this paper indicated that the utilization of dry construction with stone cladding on the external wall with an additional aerogel porous board unit could reduce 26.1% electricity consumption of a four-story building.

Author Contributions: Conceptualization, S.-K.L. and X.-F.Z.; Methodology, K.-S.L. and S.-K.L.; Data curation, formal analysis, investigation and validation, K.-S.L. and C.-H.H.; Supervision, S.-K.L.; Writing—Original draft, S.-K.L.; Writing—Review and editing, S.-K.L. and X.-F.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Ministry of Science and Technology, Taiwan, grant number 109-2221-E-214-002, and Bureau of Energy, Ministry of Economic Affairs, Taiwan, grant number 110-E0209. The APC was partially funded by IOAP, University of Nottingham (UK). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data that support the findings of this study are available from the corresponding author, upon reasonable request. Acknowledgments: The authors wish to express their sincere appreciation for the support and project funding provided by the Ministry of Science and Technology, in Taiwan (109-2221-E-214-002) and Bureau of Energy, Ministry of Economic Affairs under the contracts of 110-E0209, so that the project could be carried out smoothly. Conflicts of Interest: The authors declare no conflict of interest.

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