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Moisture Content of Indoor Air and Structures in Buildings with Vapor-Permeable Envelopes

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Moisture Content of Indoor Air and Structures in Buildings with Vapor-Permeable Envelopes Moisture Content of Indoor Air and Structures in Buildings with Vapor-Permeable Envelopes Carey J. Simonson, Ph.D. Mikael H. Salonvaara Tuomo Ojanen Associate Member ASHRAE Member ASHRAE ABSTRACT It is known that a vapor-resistant layer on the inside of an insulated envelope in cold climates is needed to prevent excessive diffusion of water vapor from indoor air into the building envelope. However, the required magnitude of this vapor resistance has been debated in recent years. In this paper, the moisture transfer between indoor air and the building envelope and the mois- ture performance of a building envelope that has no plastic vapor retarder is analyzed with field measurements and numerical simulations. The results show that the diffusion resistance of the internal surface should be greater than the diffusion resistance of the external surface for a structure safe from moisture but that the vapor resistance of the vapor retarder can be significantly below that provided by polyethylene, even in cold climates. Meanwhile, the moisture transfer between indoor air and the building envelope can moderate the indoor humidity, which improves indoor climate and comfort. INTRODUCTION • energy consumption (Besant and Simonson 2000; Harri- Conditioning indoor air is very important because man et al. 1997). research has shown that both the indoor climate and indoor air Well-designed heating, ventilating, and air-conditioning quality (IAQ) can influence comfort, health, and productivity (HVAC) systems add or remove heat and moisture from the (Wargocki et al. 1999; Seppänen et al. 1999; Wyon 1996). Therefore, buildings with a good indoor environment are occupied spaces of buildings and provide an acceptable indoor necessary for a healthy, productive, and prosperous society environment in many climates. However, in many hot and because people spend 90% of their time indoors. An impor- humid climates, conventional air-conditioning units are tant, but often neglected, indoor environmental parameter is unable to meet the latent load and the indoor relative humidity humidity, and often indoor humidity is considered to be of exceeds the often recommended value of 60% to 70% RH small importance for a successful design because temperature (ASTM 1994; ASHRAE 1992; ASHRAE 1989). This has led is easier to sense, quantify, and control. Nevertheless, research to the growing application of heat and moisture transfer has shown that the indoor relative humidity can significantly devices that can reduce the latent load on air-conditioning affect units (Besant and Simonson 2000; Harriman et al. 1997; Rengarajan et al. 1996). With these devices, it is possible to • thermal comfort (Toftum et al. 1998a, 1998b; Berglund provide an acceptable indoor climate even in hot and humid 1998; ASHRAE 1992; Fanger 1982), climates. Nevertheless, there is a desire to develop more • the perception of IAQ (Fang et al. 1998a, 1998b), passive and less energy-intensive methods of moderating the • occupant health (Clausen et al. 1999; Cooper-Arnold et indoor environment. The passive method investigated in this al. 1997; Dales et al. 1991; Green 1985), research uses the moisture storage capacity of building struc- • the durability of building materials (Viitanen 1996; tures to damp occupant-induced moisture. The main focus will Ojanen and Kumaran 1996; ASTM 1994), and be on moisture transfer between indoor air and building struc- Carey J. Simonson and Tuomo Ojanen are senior research scientists and Mikael H. Salonvaara is a research scientist at VTT Building and Transport, Espoo, Finland. Buildings VIII/Moisture Model Validation—Principles 1 tures and the resulting effect on indoor climate, air quality to keep water vapor diffusion from causing a moisture prob- (IAQ), and building durability. lem in a cold climate such as Finland’s. Passive methods of moderating the indoor environment are gaining popularity because they are energy conscious and FIELD EXPERIMENTS environmentally friendly. In moderate climates, where air CONSTRUCTION AND INSTRUMENTATION conditioning is seldom or never used, passive methods may make it possible to provide an acceptable indoor climate The house monitored in this study is a two-story wood- during hot periods without the need of air conditioning. In cold frame house located in Helsinki, Finland, with a gross floor climates, such as Finland, passive methods could help control area of 237 m2 and an internal volume of living space of 470 the occupant-induced diurnal variations in indoor humidities, m3. The insulation material is wood-fiber insulation and the which are often moderated by providing outdoor ventilation thickness is 250 mm in walls and 425 mm in the roof, giving air. By appropriately utilizing moisture transfer between a wall U-factor of 0.16 W/(m2⋅K) and a roof U-factor of 0.10 indoor air and building structures, the needed ventilation rate W/(m2⋅K). To permit the diffusion of water vapor between could possibly be reduced because the perception of IAQ is indoor air and the envelope, diffusion permeable paints are closely linked to the humidity of indoor air (Toftum and applied and no plastic vapor retarder is used. The ratio of the Fanger 1999; Fang et al. 1998a, 1998b). Furthermore, the abil- internal to external vapor resistance is about 3:1 or 4:1 (1.5 - ity of buildings to damp changes in temperature is much 3 x 109 to 0.5 - 1.0 x 109 m2⋅Pa⋅s/kg depending on the outdoor greater than their ability to damp changes in humidity relative humidity). District heat and a wood-burning fireplace (Padfield 1998) even though humidity control can be provide heating, while a natural ventilation system provides extremely important. These factors indicate that there is a outdoor ventilation. The thick insulation is intended to keep great need for research and development before buildings with energy consumption low, whereas the porous envelope and greater hygroscopic mass will be realized, even though many natural ventilation systems are examples of passive methods materials have the potential for exploitation (Simonson et al. of controlling the indoor climate and IAQ. 2001a; Virtanen et al. 2000). Moisture Transfer Test Moisture transfer between indoor air and building mate- rials can improve the moisture performance of indoor air, but The moisture transfer between the building structure and the moisture performance of the envelope is important and the indoor air was studied in a second floor bedroom shown in must be addressed as well. Moisture accumulation in building Figure 1. The bedroom is nearly square and has two windows envelopes due to the convection and diffusion of water vapor and two doors and a volume of 29 m3. The north and west walls from indoor air is an important issue, especially in cold are exterior walls (250 mm wood-fiber insulation), and the climates (ASTM 1994). Moisture accumulation can degrade south (100 mm wood-fiber insulation) and east walls are inte- building materials through mold growth, rotting, corrosion, rior walls. The east wall and the small support wall are made and other physical or aesthetic damage. To minimize convec- of 140-mm-thick brick and coated with plaster and primer. All tive moisture transfer, the building envelope should be made other walls and the ceiling (425 mm wood-fiber insulation) are airtight and any exfiltration airflow should be very small of wood frame construction and, at the time of the test, were (Ojanen and Kumaran 1996). An airtight layer (often called air finished with 13 mm of gypsum board that was plastered and barrier) reduces air leakage through the building envelope, coated with a single coat of primer. The floor has 125 mm of thereby improving the moisture performance, energy wood-fiber insulation and the wooden floor board is 32 mm consumption, ventilation performance, and thermal comfort. thick. The door leading to the interior of the house was not in Even with an airtight building envelope, the diffusion of water place at the time of the test but was covered with 5-mm-thick vapor may be significant and, therefore, it is important to have wood fiberboard. As a comparative test, the ceiling, floor, and a layer that is resistant to vapor diffusion on the warm side of walls of the test room were covered with 0.2 mm polyethylene an insulated envelope in cool climates. The purpose of this vapor retarder (except for the external windows and door) to layer (often called vapor barrier or vapor retarder) is to reduce represent a room with vapor-tight sealing and to directly show the diffusion of moisture from indoor air into the building the influence of diffusion moisture transfer between the indoor envelope to such a level that it does not cause problems. Natu- air and the porous building envelope. rally, in cold climates, a very high vapor resistance is safer than During the test, water vapor was generated and monitored a very low resistance and often polyethylene vapor retarders with an electric humidifier located on an electronic balance. are recommended and applied in practice. Polyethylene also Ventilation airflow was provided with a variable speed fan that has a very low air permeance and therefore functions as both drew air out of the room through the interior door (Figure 1). an air and vapor barrier. Because of its dual function, polyeth- To replace this air, outdoor ventilation air entered the room ylene is often specified and the safety of envelopes with air and through a ventilation channel located above the balcony vapor barriers other than polyethylene is often questioned. window. The ventilation flow rate was measured with a cali- Therefore, one of the purposes of this paper is to present brated orifice plate and a digital pressure transducer with a research that illustrates the level of vapor resistance required precision of 0.1 Pa. The uncertainty in the measured ventila- 2 Buildings VIII/Moisture Model Validation—Principles Figure 1 Schematic diagram of the test room showing dimensions, instrumentation, and its location in the test house.
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