Thermal Technical Analysis of Lightweight Timber-Based External Wall Structures with Ventilated Air Gap

sustainability

Article Thermal Technical Analysis of Lightweight Timber-Based External Wall Structures with Ventilated Air Gap

Denisa Valachova * , Andrea Badurova and Iveta Skotnicova

Department of Building Environment and Building Services, Faculty of Civil Engineering, VSB—Technical University of Ostrava, Ludvíka Podéštˇe1875/17, Poruba, 70833 Ostrava, Czech Republic; [email protected] (A.B.); [email protected] (I.S.) * Correspondence: [email protected]

Abstract: Lightweight timber-based structures are an increasingly common part of envelopes of new buildings due to increasing requirements for their energy performance. In addition, due to the fact that wood is a sustainable material, it can be assumed that the share of these structures in civil engineering will continue to increase. The subject of this article is the thermal analysis of timber-based lightweight structures under winter conditions to expand information about thermal processes in these structures. This article deals with the lightweight timber-based external wall structures with a ventilated facade and a double-skin roof structure. Experimental temperature measurements inside the structures and ventilated air gaps are used to perform the thermal analysis. By comparing experimental and theoretical data obtained by performing numerical simulation, it was shown that for achieving an ideal match of numerical simulations and measured physical properties it is necessary to take into account not only external temperatures affecting these structures, but also other factors such as solar radiation and heat emission into the cold night sky. In the case of the external walls with ventilated facade, the benefit of a ventilated air gap has been demonstrated in relation to smaller temperature fluctuations that affect the structures. Keywords: ventilated air gap; lightweight external wall structure; lightweight timber-based cladding; Citation: Valachova, D.; Badurova, temperature profiles; numerical modelling A.; Skotnicova, I. Thermal Technical Analysis of Lightweight Timber-Based External Wall Structures with Ventilated Air Gap. Sustainability 2021, 13, 378. 1. Introduction https://doi.org/10.3390/su13010378 Today, the development of building materials and new compositions of building structures is closely connected to the demands for improving the energy efficiency of Received: 10 December 2020 buildings [1–4]. Another advantage of wood-based constructions is their sustainability. Accepted: 30 December 2020 This is mainly due to the fact that almost forty percent of all energy in Europe is consumed Published: 4 January 2021 by buildings [5,6]. It is for this reason that the share of timber-based structures has been growing in recent years as part of buildings envelope. Much research abroad deals with Publisher’s Note: MDPI stays neu- thermal technical parameters of these structures [7–10], as well as in the Czech Repub- tral with regard to jurisdictional clai- lic [1,11]. The timber-based building construction systems can be divided into light skeletal ms in published maps and institutio- structures, heavy skeletal structures, wooden panel structures, and timber and log struc- nal affiliations. tures [12]. Of these the skeletal structures or panel structures are the most commonly used types in the construction of new buildings. Hybrid structures that combine a reinforced concrete or steel load-bearing structure and prefabricated wood-based panels are also Copyright: © 2021 by the authors. Li- very interesting. The main advantage of these structures is their greater usability in the censee MDPI, Basel, Switzerland. area of multi-story buildings. The advantage of all these structures is usually their smaller This article is an open access article thickness while achieving better thermal technical parameters, which is due to the relatively distributed under the terms and con- light load-bearing structure of wood or wood-based materials [13–16]. However, despite ditions of the Creative Commons At- the many benefits that these structures offer, many investors are still leaning towards using tribution (CC BY) license (https:// traditional masonry structures. However, based on a previous study [17], it was shown that creativecommons.org/licenses/by/ 46.3% of the Czech population prefers wood as a building material. The trend of increasing 4.0/).

Sustainability 2021, 13, 378. https://doi.org/10.3390/su13010378 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 378 2 of 15

the share of wooden buildings has been known in the Czech Republic since 2005, but it can be assumed that the share of wooden buildings will still increase. This paper deals with lightweight double-skin envelopes falling under a category of panel structures with a skeleton formed by I-beams. Attention is focused on double-skin structures. The principle of double-skin structures is based on a ventilated air gap. The design of a ventilated air gap is important for the proper functioning of such structures so that they meet the above requirements. However, the Czech legislation does not pay close attention to the design of a ventilated air gap. There is currently no standard available for the design of ventilated facades. The closest usable is the CSN 73 1901 standard [18], but it is primarily intended for the design of ventilated air gaps in double-skinned roof structures. The paper monitors thermal and humidity parameters of double-skinned structures, both for a facade and a single pitch double-skinned roof. Many researchers have been investigating the influence of ventilated air gaps on thermal technical parameters of structures [19–22]. Suarez et al. studied the benefits of ventilated facade in the Continental Mediterranean climate with cold winter and hot summer. The analysis was performed using the numerical simulation uses a commercial Computational Fluid dynamic code (CFD). The created numerical model has been validated with infrared measurements of the exterior surface of the actual facade. The research points mainly to energy savings in the summer with higher solar radiation and surrounding air temperature [23]. Serra et al. present the results of an experimental campaign on a climate facade with mechanically ventilated air gap. The energy efficiency of the facade has been evaluated considering the ability to pre-heat the ventilation air in the winter season, and the ability to remove part of the solar load during the summer season [24]. When using common calculation methods of energy efficiency of structures, it is accepted that the temperature in the ventilated gaps of envelopes corresponds to the exterior temperature. Susanti et al. demonstrate that due to various heat exchange process the temperatures of gaps may not be equated to the exterior temperature, especially in the case of double-skin roof [25]. The results of experimental research [26] demonstrated that the absolute error in the calculations of the average daily temperature of the ventilated air gap and external surface of the thermal insulation layer does not exceed the limits of 1.21 ◦C and 2.71 ◦C. The influence of double- skinned structures on the energy performance of buildings is dealt with by experts all over the world, and some research is focused on climatic areas with temperate climates [27–29]. In this research, results of experimental heat measurements within the compositions of the monitored structures and the parameters of the internal and external environment were used in order to evaluate the influence of ventilated air gaps on the thermal technical behavior of lightweight timber-based cladding. A similar experimental study on heat transfer through a lightweight building envelope wall under real, atmospheric boundary conditions was performed in this work [30]. Attention is focused on envelopes and roof structures. A numerical simulation of varying temperatures occurring in an envelope was performed and then compared with the data obtained by measurements, thus proving that the temperature distribution in the lightweight envelope corresponds, with a slight variation, with the results of the numerical calculations. Many studies deal with the assessment of the suitability of the use of numerical calculation methods for the prediction of heat transfer through the structures [8,31,32].

2. Materials and Methods 2.1. Analyzed Lightweight Envelopes For the purposes of a temperature analysis, lightweight envelopes of a passive experimental house were chosen; see Figure1. This house is located in Ostrava, Czech Republic. Sustainability 2021, 13, 378 3 of 15 Sustainability 2021, 13, x FOR PEER REVIEW 3 of 15

Sustainability 2021, 13, x FOR PEER REVIEW 3 of 15

FigureFigure 1.1. ResearchResearch andand InnovationInnovation CentreCentre ofof thethe NationalNational WoodWood Processing Processing Cluster. Cluster. Figure 1. Research and Innovation Centre of the National Wood Processing Cluster. TheThe official ofﬁcial name name of of this this building building is the is “Res the “Researchearch and Innovation and Innovation Center Centerof the National of the NationalWoodThe Processing official Wood name Processing Cluster”. of this This Cluster”.building experimental is This the “Res experimental buildingearch and was Innovation buildingbuilt in 2012 was Center and built thanksof inthe 2012 National to its and ex- thanksWoodceptional Processing to itsnature, exceptional allowsCluster”. a nature, long-term This experimental allows monitoring a long-term building and monitoringevaluation was built of andin physical-technical2012 evaluation and thanks of physical- to proper- its ex- technicalceptionalties of building nature, properties structures allows of a building long-term and the structuresinternal monitoring environment and and the evaluation internal of the entire of environment physical-technical building under of the proper-real entire ex- buildingtiesternal of buildingconditions under structures real [11]. external The andtechnical the conditions internal equipment [en11vironment]. of The the technicalbuilding of the entireincludes equipment building a superior of under the regulation building real ex- includesternalof the conditionsdesigned a superior heat [11]. regulationsources The technical with of the equipment possibilit designed yof heatto the utilize building sources them withincludes for theresearch possibilitya superior and educational regulation to utilize themofpurposes. the fordesigned research The system heat and sources enables educational with to measure the purposes. possibilit all necessary They to systemutilize quantities, them enables for flows, toresearch measure powers, and all andeducational necessary thermal quantities,purposes.energy. The The ﬂows, experimental system powers, enables building and to thermal measure is designed energy. all necessary as The a timber experimental quantities, prefabricated flows, building powers, panel is designedstructure. and thermal as The a timberenergy.structural prefabricatedThe solution experimental of the panel buil building structure.ding is is evident designed The structural in asFigure a timber solution2. prefabricated of the building panel structure. is evident The in Figurestructural2. solution of the building is evident in Figure 2.

Figure 2. Cross-section of the experimental building. Figure 2. Cross-section of the experimental building.building. External wall structures in contact with the outside air are made of prefabricated panels,External which wall were structures manufactured in contact under with stable thethe climatic outsideoutside conditions airair areare mademade in a ofofproduction prefabricatedprefabricated hall. panels,panels,Prefabricated which werepanels manufacturedmanufactured are formed by under a load stable-bearing climatic structure conditions based in on a productionwood, between hall. Prefabricated panels are formed by a load-bearing structure based on wood, between Prefabricatedwhich thermal panels insulation are formedis inserted. by Thea load outer-bearing and inner structure surface based of the on panel wood, is thermally between which thermal insulation is inserted. The outer and inner surface of the panel is ther- whichinsulated thermal too. During insulation the isproduction inserted. The of thes outere panels, and inner hygrothermal surface of thesensors panel were is thermally inserted mally insulated too. During the production of these panels, hygrothermal sensors were insulatedinto their too.structure, During which the production permits long-ter of thesme panels, monitoring hygrothermal of these parameters.sensors were The inserted load- inserted into their structure, which permits long-term monitoring of these parameters. The intobearing their elements structure, of whichthe external permits walls long-ter and mroof monitoring are made ofof theseSteico parameters. wooden I-beams. The load- Ex- load-bearing elements of the external walls and roof are made of Steico wooden I-beams. bearingternal wall elements structures of the are external insulated walls with and Stei roofco wood are made fibre ofinsulation, Steico wooden placed I-beams. between Ex-the External wall structures are insulated with Steico wood fibre insulation, placed between ternalbeams wall as well structures as on the are structures insulated fromwith Steithe cooutside wood and fibre inside. insulation, The cladding placed between of the indi- the the beams as well as on the structures from the outside and inside. The cladding of the beamsvidual aslayers well ofas theon theexternal structures wall structuresfrom the outside is made and of inside.Fermacell The gypsum cladding fiber of the boards. indi- individual layers of the external wall structures is made of Fermacell gypsum fiber boards. vidualAll assessed layers structuresof the external meet wallthe values structures of the is heatmade transfer of Fermacell coefficient gypsum Upas fiberaccording boards. to All assessed structures meet the values of the heat transfer coefficient Upas according to AllČSN assessed 730540-2 structures [33] recommended meet the values for passive of the buildings. heat transfer coefficient Upas according to CSNˇ 730540-2 [33] recommended for passive buildings. ČSN Two730540-2 types [33] of double-skinrecommended structures for passive were buildings. chosen for the thermal analysis, namely: Two types of double-skin structures were chosen for the thermal analysis, namely: • TwoExternal types walls of double-skin with ventilated structures facade, were chosen for the thermal analysis, namely: • External walls with ventilated facade, •• ExternalDouble-skin walls roof with structure. ventilated facade, • Double-skin roof structure. Double-skinThere are two roof designs structure. of the external walls with ventilated facade. The first one con- There are two designs of the external walls with ventilated facade. The first one sists Thereof a 20-mm-thickare two designs air ofgap the and external exterior walls wood with cladding.ventilated Thefacade. cladding The first is onemade con- of consists of a 20-mm-thick air gap and exterior wood cladding. The cladding is made of sistswooden of a slats 20-mm-thick spaced 30 air mm gap apart, and soexterior the clad woodding doescladding. not form The acladding continuous is made surface. of wooden slats spaced 30 mm apart, so the cladding does not form a continuous surface. Sustainability 2021, 13, 378 4 of 15 Sustainability 2021, 13, x FOR PEER REVIEW 4 of 15

wooden slats spaced 30 mm apart, so the cladding does not form a continuous surface. The Thesecond second design design of theof the ventilated ventilated façade façade consists consists of aof 40-mm-thick a 40-mm-thick air air gap gap and and an an external exter- nalcladding cladding made made of of Cetris Cetris cement-bonded cement-bonded particleboard. particleboard. Again, Again, the the boards boards do do not not form form a acomplete complete continuous continuous surface surface as theyas they are spacedare spaced from from each othereach other by 9 mm by gaps.9 mm The gaps. designs The designsof the facade of the claddingfacade cladding is evident is evident in Figure in3 Figure. 3.

(a) (b)

Figure 3. Ventilated facade designs: (a) Timber cladding; (b) Cetris board cladding. Figure 3. Ventilated facade designs: (a) Timber cladding; (b) Cetris board cladding. The double-skinned roof structure design is similar to the external walls structure The double-skinned roof structure design is similar to the external walls structure design, as part of its composition consists of a 60-mm-thick ventilated air gap. The exterior design, as part of its composition consists of a 60-mm-thick ventilated air gap. The exterior side of the structure is ﬁnished by Steico Universal thermal insulation layer. In this case, sidethe outerof the surfacestructure of theis finished thermal by insulation Steico Un isiversal provided thermal with insulation a safety diffusion layer. In foil. this case, the outer surface of the thermal insulation is provided with a safety diffusion foil. 2.2. Experimental Measurements 2.2. Experimental Measurements The monitored building is equipped with a measuring system, thanks to which many physicalThe monitored quantities havebuilding been is continuously equipped with recorded a measuring since the system, building thanks was to put which into service.many physicalMeasuring quantities devices have of TR been instruments continuously spol. s.r.o. recorded are used since for the these building long-term was measurements put into service.incorporating Measuring type devices Rotronic of TR sensors. instruments The measuredspol. s.r.o. dataare used are recordedfor these inlong-term a time interval meas- urementsof 15 min, incorporating using a multi-channel type Rotronic dataTaker sensors. DT80G The measured Geologger data measuring are recorded control in apanel. time intervalFor measuring of 15 min, of hygrothermalusing a multi-channel parameters dataTaker inside of DT80G the structures Geologger ROTRONIC measuring HC2-CO4 control panel.RH/t sensorsFor measuring are used. of For hygrothermal measuring of parame thermalters parameters inside of onthe external structures and ROTRONIC inner surface HC2-CO4of structures RH/t there sensors are usedare used. ROTRONIC For measuring TG 68-60 of Pt1000thermal and parameters TG 7 Pt1000 on external sensors. and For innermeasuring surface of of internal structures and there external are used air temperatures ROTRONIC thereTG 68-60 are usedPt1000 ROTRONIC and TG 7 Pt1000 Pt1000 sensors.sensors. For Temperatures measuring and of relativeinternal humidity and exte measuringrnal air temperatures sensors were there built intoare theused exterior RO- TRONICwall structures Pt1000 alreadysensors. in Temperatures the prefabrication and relative phase. humidity Location ofmeasuring sensors for sensors continuous were builtmeasurements into the exterior inside wall the structures exterior walls already and in the the roof, prefabrication including thephase. compositions Location of of sen- the sorsindividual for continuous structures measurements are shown in Figuresinside 4th ande exterior5. For thewalls purpose and the of thisroof, research, including Arexx the compositionsdevices with of TSN-TH70E the individual and structures TSN-33MN are sensors shown werein Figures installed 4 and inside 5. For the the air purpose gaps of ofexternal this research, walls. The Arexx measured devices data with were TSN-TH70E recorded inand a timeTSN-33MN interval sensors of 45 s. Thewere accuracy installed of insidethe measuring the air gaps devices of external used is walls. given inThe Table meas1.ured Only data certain were time recorded periods in were a time selected interval for ofthe 45 purposes s. The accuracy of this analysis,of the measuring which are devices subsequently used is evaluated.given in Table 1. Only certain time periods were selected for the purposes of this analysis, which are subsequently evaluated. Sustainability 2021, 13, x FOR PEER REVIEW 5 of 15 Sustainability 2021 Sustainability, 13, x FOR PEER2021, REVIEW13, 378 5 of 15 5 of 15

Figure 4. Figure 4. Composition of the exterior wall and position of the built-in measuring sensors. Figure 4. CompositionComposition of the of theexterior exterior wall wall and and posi positiontion of the of thebuilt-in built-in measuring measuring sensors. sensors.

Figure 5. Composition of the double-skin roof and the position of the built-in measuring sensors. Figure 5. Composition of the double-skin roof and the position of the built-in measuring sensors. Figure 5. Composition of the double-skin roof and the position of the built-in measuring sensors. Table 1. Measuring equipment used—measurement accuracy. Table 1. Measuring equipment used—measurement accuracy. Table 1. Measuring equipment used—measurement accuracy. Equipment Sensor Measures Variables Accuracy Equipment Sensor Measures Variables Accuracy Equipment Sensor Measures Variables Accuracy Rotronic TG 7 Pt1000, ClassRotronic A TG 7 Pt1000, External Class Aand internal External andsurface internal temperatures surface temperatures ±0.3± 0.3K K Rotronic HC2-CO4 RH/T Temperatures within the constructions ±0.3 K Rotronic Rotronic TG 7 Pt1000, Class HC2-CO4 A RH/T External and internal Temperatures surface within temperatures the constructions ±0.3 K ±0.3 K Rotronic HC2-CO4 RH/T Rotronic Temperatures Pt1000, Class within A the constructions Air temperature ±0.3 K ±0.1 K Rotronic Pt1000, ClassArexx A TSN-TH70E Air temperature Air temperature ±0.1± 0.5K K Rotronic Arexx Pt1000, Class A TSN-TH70E Arexx TSN-33MN Air temperature Air temperature Air temperature ±0.1 K ±0.5± 0.5K K Arexx Arexx TSN-TH70E TSN-33MN Air temperature Air temperature ±0.5 K ±0.5 K Arexx TSN-33MN The measurements Airwere temperature performed in January, during which ±0.5 the K lowest outside tem- peraturesThe measurements were recorded. were Data performed obtained in by January, measurements during which on the the inner lowest and outeroutside surfaces tem- The measurementsperaturesof the structures, were were recorded. performed external Data and in obtainedJanuary, indoor climatic duringby measurements which data, andthe lowest dataon the obtained outside inner and tem- by outer measurements surfaces peratures wereofinside the recorded. structures, the air Data gaps external obtained are used and by for indoorme theasurements analysis. climatic on Thedata, the measurement andinner data and obtained outer results surfaces by were measurements used to ana- of the structures,insidelyze external thethe effectair gapsand of airindoorare gaps used climat on for the theic structuresdata, analysis. and data andThe alsomeasurementobtained to deﬁne by measurements theresults boundary were used temperature to ana- inside the airlyze conditionsgaps the are effect used for of thefor air purposethe gaps analysis. on of the numerical Thestructures measurement simulation and also results of to heat define were propagation the used boundary to in ana- the temperature exterior wall. lyze the effectconditions of air gaps for on the the purpose structures of numericaland also to simula definetion the ofboundary heat propagation temperature in the exterior conditions forwall.2.3. the Numerical purpose Simulationof numerical of Temperaturesimulation Curvesof heat in propagation Lightweight Externalin the exterior Wall Structures wall. Numerical calculations of thermal processes in structures were used in the research. In common building practice, analytical methods are usually used for thermal engineering Sustainability 2021, 13, 378 6 of 15

assessment of structures [34]. However, they are not suitable for detailed evaluation of temperatures in lightweight perimeter structures due to their design, where the structures include systematic thermal bridges. In this case, it is necessary to perform thermal engineering tasks using numerical calculation methods. The output of numerical calculations is a temperature field that describes the temperature distribution in the evaluated model of the construction. These are determined by converting the partial differential equations defining the heat conduction to the system of linear equations and their subsequent solu- tions [35]. Thus, as in the case of analytical methods, the numerical solution is based on the Law of Energy Conservation, which is applied to thermal engineering phenomena such as Fourier’s Law. Heat dissipation through conduction is characterized by a heat flux q that is directly proportional to the temperature gradient. This dependence on stationary heat conduction is expressed mathematically by the first Fourier’s law [35]:

δθ q = −λ· = −λ·gradθ (1) δx

where q is density of heat flow (W·m−1), λ is thermal conductivity coefficient (W·m−1·K−1), θ is the time temperature (◦C), and x is the coordinate of the point (m). If the temperature does not change over time, as in the case of steady (stationary) heat conduction, then for heat conduction, assuming the independence of the thermal conductivity coefficient λ on the temperature and direction of heat propagation, the so- called Fourier’s partial differential equation can be defined for three-dimensional heat conduction in the Equation (2), analogous to one-dimensional and two-dimensional heat conduction. Further, in the case of unstabilized (non-stationary) heat conduction, the fact that the temperature is a function of time θ = f (t), enters the relationships. Here, the so- called second Fourier’s law applies, analogously for one-dimensional and two-dimensional heat conduction: δθ δ2θ δ2θ δ2θ = a· + + (2) δt δx2 δy2 δz2 where a is thermal conductivity coefficient (m2·s−1), θ is the temperature (◦C), t is the time, and x, y, and z are the coordinate of the point (m). For the purposes of the research, the Finite Elements Method (FEM) was used. The Finite Elements Method is a variation method, the basic principle of which is the division of a continuous region into a set of separate sub-regions, the so-called finite elements [36,37]. The detected parameters are then determined at the individual nodal points of these elements. The principle of the method is also the conversion of the partial differential equation into a system of linear algebraic equations for the desired potential values at the nodes of the finite elements. Since this is an iterative method, first, an anticipated solution is selected at the beginning of the calculation, and then the solution to the equations is sought using successive iterations (refinements). The iteration process stops at the moment when a predetermined inaccuracy is reached, which is the difference between the observed temperatures of the given and the previous iteration round at all nodes [38]. When computers are used, it is possible to choose very low values of inaccuracies and for this reason these methods are sometimes incorrectly referred to as “exact”. In the case of manual calculation, it would not be possible to achieve such accuracy due to the time-consuming nature of the calculations. As part of the research, numerical calculations of the courses of heat variations in the structures were performed and these were then compared with the actual measured data. To be able to perform such comparison, it is necessary to carry out numerical calculations under non-stationary boundary conditions, i.e., under a state corresponding to the actual behavior of exterior and interior temperature boundary conditions, which are under normal circumstances never stationary but change over time [39]. Unlike stationary tasks, the temperature distribution in the design at each calculation step is dependent on the temperature distribution in the structure in the previous step. Sustainability 2021, 13, 378 7 of 15

In the case of non-stationary problems, according to Fourier’s Second Law, is the physical magnitude, which influences the temperature distribution in the construction under the influence of the variable boundary conditions, called the coefficient of thermal conductivity a. This quantity expresses the rate of balancing of the temperatures in the substances and depends on the coefficient of thermal conductivity λ, the density ρ, and the specific heat capacity of the substance. Therefore, in order to create a computational model for non-stationary heat conduction tasks, it is also necessary to specify the physical properties of the materials in addition to the geometry itself. For non-stationary calculations, it is therefore necessary to define these material characteristics, and variable boundary conditions, as a function of time and also the initial state of temperature distribution in the construction. This fundamentally affects the results of the calculation itself. The initial state can be set by determining the temperature field by means of a stationary calculation with the input of boundary conditions so that the resulting temperature field closes as closely as possible to the actual temperature distribution in the structure at the beginning of the non-stationary simulation.

2.4. Theoretical Analysis of Temperature Curves in Lightweight Exterior Wall Structures The theoretical analysis included only a selected detail of the external wall with a ventilated air gap. The numerical simulation was performed for a time interval of 48 h with a time step of 600 s. The ANSYS R16.0 program [40] was used for the simulation of the temperature conduction, which is capable of solving non-stationary thermal-technical tasks. The detail of the construction model was created in accordance with the actual dimen- sions of the structure composition. Individual materials were assigned thermal-technical properties declared by the manufacturer of the building materials used; for the purposes of simulation, these declared properties were adjusted to the so-called calculated values, which took into account the degree of moisture absorption of materials. After creating the model, it was necessary to define the boundary conditions. Boundary conditions used were those of Newton’s type II, which are defined by the temperature of the ambient air on the inside and outside of the structure and the heat transfer coefficient on the inside and outside of the structure. The indoor and outdoor air temperature was entered as a non-stationary boundary condition. These values were determined by experimental measurements. The heat transfer coefficients on the inside and outside of the structure were chosen as stationary according to CSNˇ 73 0540-3 [41], which may lead to a certain calculation error, as it is a variable. The calculation thus does not take into account the effect of air temperature and its flow at the surface of the structure on the outside and inside on the thermal resistance during heat transfer. The temperature distribution in the structure, which corresponds to the temperature field determined on the basis of the average boundary conditions of the previous 48 h was chosen as the initial state of the simulation.

3. Results 3.1. Temperature Measurements Inside the Structures and Their Comparison with Numerical Simulation As already mentioned, a numerical simulation of the temperature fields was performed for a characteristic section of the external wall with a ventilated air gap. The result of the numerical simulation is the distribution of temperatures in the structure under loading by the actual temperatures of the indoor and outdoor environment obtained by experimental measurements. Graphical outputs of temperature ﬁelds are shown in Figure6. These ﬁgures illustrate the change in temperature distribution in the evaluated detail during the simulation. Sustainability 2021, 13, x FOR PEER REVIEW 8 of 15

Sustainability 2021, 13, 378 These figures illustrate the change in temperature distribution in the evaluated detail8 dur- of 15 ing the simulation.

Figure 6. Temperature distribution in the evaluated detail for selected simulation times of non-stationary heat conduction. Figure 6. Temperature distribution in the evaluated detail for selected simulation times of non-stationary heat conduction. Based on the results of numerical simulations, a comparison of actually measured temperaturesBased on withinthe results the compositionof numerical ofsimulati the externalons, a comparison wall structure of actually and the measured results of temperaturesnumerical simulations within the of composition temperature of fields the ex usingternal non-stationary wall structure and boundary the results conditions of nu- mericalwas performed. simulations This of comparison temperature was fields made using for both non-stationary the location boundary of the thermal conditions bridge was and performed.for the axis betweenThis comparison the thermal was bridges. made for The both average the location differences of the of thermal resulting bridge temperatures and for thebased axis on between experimental the thermal measurements bridges. The and average numerical differences simulation of resulting of five positions temperatures in the basedevaluated on experimental details are then measurements shown in Table and2 .nume The resultsrical simulation are given of as five the averagepositions for in thethe evaluatedfirst 24 h ofdetails simulation, are then followed shown in by Table the average 2. The results for the are subsequent given as the 24 haverage of simulation for the firstand 24 finally h of simulation, by the average followed for the by entire the averag simulatione for the time, subsequent i.e., 48 h. 24 When h of simulation comparing and the finallyresults, by it the is also average necessary for the to entire take intosimulation account time, the accuracyi.e., 48 h. When of measuring comparing devices the results, and to itthis is also end necessary the measurement to take into deviations account are the given accuracy in Section of measuring 2.2, when devices these and measured to this dataend thewere measurement entered as external deviations and are internal given boundaryin Section 2.2, conditions when these and thenmeasured the measured data were data en- teredinside as structures external againand internal compared boundary with simulation conditions results. and then Larger the differences measured betweendata inside the structuresmeasured again and calculated compared values with simulation were found results. at the Larger site of differences the thermal between bridge, the meas- result uredof which and calculated is related to values the deformation were found ofat the si temperaturete of the thermal field bridge, at the thermal the result bridge of which site. isThe related established to the deformation smallest temperature of the temperatur differencee field of 0.3at the◦C thermal and the bridge highest site. temperature The estab- lisheddifference smallest of 1.3 temperature◦C can be considered difference as of very 0.3 °C good and results. the highest Larger temperature temperature difference differences of 1.3were °C foundcan be closer considered to the as outside very good of the result structure,s. Larger which temperature correspond differences to the actual were effectsfound closeracting to on the the outside structure of fromthe structure, the outside, which such correspond as sunlight, to airthe flow, actual etc. effects These acting effects on were the structurenot included from in the the outside, numerical such simulation, as sunlight, only air outdoor flow, etc. temperature These effects was were used asnot a included marginal outside condition. However, choosing a structure with a ventilated facade should eliminate these effects. A graphical display of the course of the temperature variations was created for greater clarity; see Figures7 and8. Sustainability 2021, 13, 378 9 of 15

Figures7 and8 show the results of measurements and numerical simulations over 48 h. The results show a difference between the experimental and theoretically determined Sustainability 2021, 13, x FOR PEER REVIEWtemperatures in the given cross-sections of the structure. It is clear from the ﬁgures9 thatof 15 the simulation did not take into account the effect of solar radiation, when signiﬁcantly higher temperatures were experimentally found in the afternoon in position 5, i.e., in the position on the outside of the structure (in this case in front of the ventilated air gap). This in the numerical simulation, only outdoor temperature was used as a marginal outside phenomenon also manifested itself in position 4 in the experimentally obtained data, but condition. However, choosing a structure with a ventilated facade should eliminate these with a certain time response, when the measured temperatures rose slightly in contrast to effects. A graphical display of the course of the temperature variations was created for the calculated temperatures. In other positions, this phenomenon is no longer apparent. greater clarity; see Figures 7 and 8. Table 2. Average differences of measured data results and numerical simulations. Table 2. Average differences of measured data results and numerical simulations. Average Difference (◦C) Position Average Difference (°C) Position Temperatures on the Axis Temperatures at the Site of Thermal Bridge (from Exterior) Temperatures on the Axis Temperatures at the Site of Thermal Bridge (from Exterior) TimeTime (Hours) (Hours) 0–24 0–24 24–48 0–48 0–48 0–24 0–24 24–48 24–48 0–48 0–48 11 0.50.5 1.1 0.8 0.8 0.7 0.7 1.3 1.3 0.9 0.9 22 0.40.4 0.3 0.3 0.3 1.4 1.4 1.2 1.2 1.3 1.3 3 0.6 0.6 0.6 1.0 1.0 1.0 3 0.6 0.6 0.6 1.0 1.0 1.0 4 1.1 1.0 1.1 0.9 1.0 1.0 54 1.11.1 0.81.0 1.1 1.0 0.9 0.1 1.0 0.1 1.0 0.1 5 1.1 0.8 1.0 0.1 0.1 0.1

25.0 20.0 15.0 10.0 5.0 0.0 -5.0

Temperature [°C] Temperature -10.0 -15.0 -20.0 0:00 6:00 0:00 6:00 0:00 12:00 18:00 12:00 18:00

exterior interior Time position 1 − measurement position 1 − numerical simulation position 2 − measurement position 2 − numerical simulation position 3 − measurement position 3 − numerical simulation position 4 − measurement position 4 − numerical simulation position 5 − measurement position 5 − numerical simulation

Figure 7. CourseCourse of temperature variations in axis between beams. Sustainability 2021, 13, 378 10 of 15 Sustainability 2021, 13, x FOR PEER REVIEW 10 of 15

25.0 20.0 15.0 10.0 5.0 0.0 -5.0 -10.0 Temperature [°C] Temperature -15.0 -20.0 0:00 6:00 0:00 6:00 0:00 12:00 18:00 12:00 18:00 exterior interior Time position 1 − measurement position 1 − numerical simulation position 2 − measurement position 2 − numerical simulation position 3 − measurement position 3 − numerical simulation position 4 − measurement position 4 − numerical simulation position 5 − measurement position 5 − numerical simulation

Figure 8. Course of temperature variations at the beam.

3.2. EvaluationFigures 7 and of Experimental 8 show the Measurementsresults of measur of Ventilatedements and Air numerical Gaps simulations over 48 h. The results show a difference between the experimental and theoretically determined In addition to the above-mentioned measurement of temperatures inside structures, temperatures in the given cross-sections of the structure. It is clear from the figures that the results of which were compared with the results of numerical simulation, other mea- the simulation did not take into account the effect of solar radiation, when significantly surements were performed to monitor the effect of ventilated air gaps in these structures. higher temperatures were experimentally found in the afternoon in position 5, i.e., in the These measurements were also performed in January, and included an external wall with position on the outside of the structure (in this case in front of the ventilated air gap). This a ventilated façade structures and a double-skinned roof structure; a description of the phenomenon also manifested itself in position 4 in the experimentally obtained data, but compositions and the specification of the location of the measuring devices is given in withSection a certain 2.2. time response, when the measured temperatures rose slightly in contrast to the calculatedFigure9 depicts temperatures. graphically In other the temperature positions, this profiles phenomenon on the external is no longer side of apparent. the structures. Since these are double-skinned structures that are being evaluated, the temperatures 3.2.in question Evaluation are of those Experimental found inside Measurements the air gap of thatVentilated directly Air affect Gaps these structures. InThe addition analysis to ofthe the above-mentioned data revealed thatmeasur in theement case of oftemperatures the roof structure, inside structures, there are thesignificant results of drops which in were temperature compared in with the ventilatedthe results airof numerical gap compared simulation, to the other outside meas- air urementstemperature. were The performed subcooling to ofmonitor the air insidethe effect the of air ventilated gap occurs air in gaps connection in these with structures. the cold Theseradiation measurements of the night were sky [ 22also,42 performed]. This phenomenon in January, has and a negativeincluded effectan external on the wall heat with loss aof ventilated the roof structure façade structures at night, whenand a thedouble-skinned structure is burdened roof structure; with a a larger description temperature of the compositionsgradient. The and maximum the specification difference of found the location between of the the outside measuring air temperature devices is given and the in Sectiontemperature 2.2. inside the air gap is 5.5 ◦C. Regarding the temperature balance within the observedFigure period, 9 depicts the graphically average air the temperature temperature inside profiles the on air the gap external was −2.2 side◦C, of whilethe struc- the tures.average Since outdoor these airare temperature double-skinned was −structur0.8 ◦C.es The that cold are air being inside evaluated, the air gap the then tempera- has a turesdirect in effect question on the are temperature those found of inside the outer the ai surfacer gap that of the directly lower affect skin of these theroof structures. structure. It is differentThe analysis in the of exterior the data double-skin revealed that walls in th whene case the of temperatures the roof structure, in the ventilatedthere are sig- air ◦ nificantgap are demonstrablydrops in temperature higher, onin the average ventilated by 0.5 airC gap in the compared case of the to Cetristhe outside cladding air tem- and ◦ perature.by 1.1 C The in the subcooling case of the of timber the air cladding. inside the This air factgap has,occurs among in connection other things, with a the positive cold radiationeffect on theof the energy night balance sky [22,42]. of the This external phenomen wall structures.on has a negative effect on the heat loss of the roof structure at night, when the structure is burdened with a larger temperature gradient. The maximum difference found between the outside air temperature and the temperature inside the air gap is 5.5 °C. Regarding the temperature balance within the observed period, the average air temperature inside the air gap was −2.2 °C, while the average outdoor air temperature was −0.8 °C. The cold air inside the air gap then has a Sustainability 2021, 13, x FOR PEER REVIEW 11 of 15

direct effect on the temperature of the outer surface of the lower skin of the roof structure. It is different in the exterior double-skin walls when the temperatures in the ventilated air Sustainability 2021, 13, 378 gap are demonstrably higher, on average by 0.5 °C in the case of the Cetris cladding11 ofand 15 by 1.1 °C in the case of the timber cladding. This fact has, among other things, a positive effect on the energy balance of the external wall structures.

10.0

5.0

0.0

-5.0 temperature [°C] temperature [°C] -10.0

-15.0 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 exterior temperature Time wooden cladding: ventilated air gap Cetris cladding: ventilated air gap roof: ventilated air gap

Figure 9. Course of temperature variations occurring on the externalexternal side of structures in the monitored winterwinter period.period.

Figure 1010 graphically graphically shows shows the the course course of oftemperature temperature variations variations on the on external the external sur- facesurface of structures. of structures. In Inthe the case case of ofthe the evalua evaluatedted double-skinned double-skinned structures, structures, it it is is therefore the temperature on on the outer surface of the stru structurecture in front of the air gap. In addition, given in in this this case case are are the the temperatures temperatures on onthe the outer outer surface surface of the of façade the façade containing containing con- tactcontact insulation. insulation.

10.0

5.0

0.0

-5.0

-10.0 temperature [°C] temperature temperature [°C] temperature

-15.0 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 0:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 12:00 Time exterior temperature Time wooden cladding: outer surface Cetris cladding: outer surface roof: outer surface

Figure 10. Course of temperature variations on the outer surface of structures in the observed winter period.

As in the case of Figure9, the effect of the radiation into the clear night sky [ 22,42] is noticeable. The most cooled part of the construction is the roof structure. Very low temperatures on the surface of the lower skin of the roof structure are affected by the Sustainability 2021, 13, 378 12 of 15

temperature of the air inside the air gap, which may fall below the ambient air temperature at night. This can be seen in the previous Figure9, when the temperatures inside the air gap at night and morning were signiﬁcantly lower than the outside air temperature. In the case of the external walls with a ventilated air gap, it is clear that the structures are effectively protected against external inﬂuences, and this can be stated on the basis of a more stable course of temperature variations.

4. Discussion The subject of this research was winter thermal analysis of the timber-based lightweight external wall structures. The aim of the paper is to expand information about thermal processes in these structures, whose main advantage is their sustainability. The evaluated structures are part of the experimental building constructed in the passive standard. This paper focuses on double-skinned structures, i.e., structures with a ventilated air gap. The attention is specifically focused on the external walls with a ventilated facade and a double- skin roof structures. The data obtained by experimental measurements of temperatures inside these structures including temperatures inside ventilated air gaps are used for the analysis, and the temperatures of the indoor and outdoor environment were also used for a comprehensive evaluation. As part of the analysis of temperatures inside the composition of the external wall structure with a ventilated façade, a comparison of the results of the experimental measurements and numerical simulations of heat diffusion in the building structure was performed. The numerical simulation was performed using ANSYS software for non-stationary boundary conditions. Temperatures determined by experimental measurements for indoor and outdoor environments were included in the calculation as non-stationary boundary conditions. To perform the numerical simulation, a characteristic section of the structure of the evaluated external wall was chosen. The model of the structure was made for thermal technical parameters of materials used in the composition of the evaluated structure. Nu- merical simulation was performed for an interval of 48 h. Subsequently, the temperatures were taken at the places of the characteristic sections, which correspond to the places where the measuring devices are located in the monitored structure. These results were compared with the outputs of the experimental measurements, where the average temperature deviations obtained by numerical simulation and measurement were determined; see Table2.A graphical comparison of the results is shown in Figures7 and8. The results show that the deviations between numerical simulation and the measurement are very small, especially in the place of the structure in the axis between the beams, where there is no deformation of the temperature field. The temperature differences for the 10 evaluated positions in the characteristic section of the structure range from 0.1 to 1.4 ◦C. With regard to possible inaccuracy of the experimental measurement it is noted that these values are very low and a method of using numerical simulation of temperature fields can be regarded as a suitable tool for the prediction of thermal processes in lightweight external wall structures. At the same time, however, when comparing the results, the error of numerical calculation is also noted, which arose by simplifying the boundary conditions that affect the structures, when only the temperature in the interior and exterior was entered without taking into account other influences such as solar radiation. Particularly the effect of solar radiation is evident in Figure7, when the thermal response of the structure to solar radiation at temperatures determined experimentally is apparent. Accordingly, it is necessary to take this effect into account to achieve greater numerical calculations accuracy—a point that will be the subject of further research. The second part of the paper deals with the influence of the air gaps on the thermal- technical parameters of external wall structures. This section interprets only the results of the experimental measurements. The results of the experimental measurements for the evaluated lightweight external wall structures are shown in Figures9 and 10. The results of these experimental measurements brought to light interesting findings. It has been shown that the external wall with a ventilated façade has a positive effect on the structure Sustainability 2021, 13, 378 13 of 15

itself, as it is not subjected to temperature fluctuations, as is the case with an external wall with contact insulation, which is directly exposed to the external environment. The frontal façade effectively protects the external wall structure from low temperatures in the winter, which at the same time allows enhanced removal of moisture from the structure due to the higher temperatures inside the air gap. This is not the case with the evaluated double-skin roof structure, when a fundamental phenomenon was recorded, namely the subcooling of the air inside the ventilated air gap in the winter as the result of the radiation of the roof surface into the clear night sky. At very low ambient temperatures, temperatures lower by more than 5 ◦C were found inside the air gap compared to the outside air temperature. Subcooling of the surface of the inner skin is taking place at the same time. A cause of this phenomenon is also an insufficient air flow inside the ventilated air gap due to its inadequate thickness, which was the case of the monitored structure. The behavior of temperatures inside ventilated air gaps is the subject of further research. Follow-up research should also include the use of the simulation numerical methods, which have been evaluated as suitable to be used for the simulation of thermal processes in structures, taking into account other recorded influences that in addition to outdoor temperatures affect structures noticeably as well.

5. Conclusions The subject of this research was the thermal analysis of timber-based lightweight external wall structures under winter conditions. The evaluated structures are part of the experimental building constructed in the passive standard, which is located in Ostrava, Czech Republic. The building is designed as a timber prefabricated panel structure and thanks to its exceptional nature, allows a long-term monitoring and evaluation of physical- technical properties of its structure. This paper focuses on its double-skinned structures, i.e., structures with a ventilated air gap. Attention is specifically focused on external walls with a ventilated facade and a double-skin roof structure. The data used by experimental measurements of the temperatures inside these structures and temperatures taken inside the ventilated air gaps are used for the analysis; to accomplish a comprehensive evaluation the temperatures of the indoor and outdoor environment have been taken into account as well. Numerical methods for calculating the course of temperature variations in structures under non-stationary boundary conditions are shown to be a suitable tool for predicting temperature processes in the lightweight external wall structures; however, all factors that affect these processes must be taken into account. In the case of structures with ventilated air gaps, the influence of heat emission into the cold night sky as well as the influence of solar radiation were monitored as part of the experimental measurements and should become part of further thermal technical analyzes using numerical simulations.

Author Contributions: Conceptualization, D.V. and I.S.; methodology, I.S. and D.V.; software, D.V.; validation, A.B.; formal analysis, A.B. and I.S.; investigation, D.V. and I.S.; resources, D.V. and A.B.; data curation, D.V. and I.S.; writing—original draft preparation, D.V.; writing—review and editing, A.B.; visualization, D.V. and A.B.; supervision, I.S.; project administration, D.V.; funding acquisition, D.V. All authors have read and agreed to the published version of the manuscript. Funding: The work was supported by funds for Conceptual Development of Science, Research and Innovation for 2020 allocated to VSB–Technical University of Ostrava by the Ministry of Education, Youth and Sports of the Czech Republic. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available on request due to restrictions eg privacy or ethical. Conﬂicts of Interest: The authors declare no conﬂict of interest. Sustainability 2021, 13, 378 14 of 15

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