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The Effect of Indoor Humidity on Water Vapor Release in Homes

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The Effect of Indoor Humidity on Water Vapor Release in Homes The Effect of Indoor Humidity on Water Vapor Release in Homes Anton TenWolde Crystal L. Pilon Member ASHRAE ABSTRACT With the introduction of moisture engineering and new design approaches for moisture control in buildings, it has become important to formulate a realistic design value for indoor humidity. The design value for indoor humidity is one of the most impor- tant parameters when determining the need for vapor retarders and other building envelope design features, especially in colder climates. Seasonal indoor humidity is primarily determined by a balance between moisture production rates and removal rates (by ventilation or dehumidification). However, experience has shown that a simple mass balance calculation tends to produce indoor humidity results that are too high for humid cool (coastal) climates and too low for dry climates. In these calculations, moisture sources are assumed to be constant and not a function of the ambient indoor humidity. In this paper we examine the most common sources of water vapor in homes and how they might vary with indoor humidity. Our review indicates that most of the sources, such as contributions from inhabitants and their activities, are virtually independent of humidity. However, moisture contributions from potted plants and from a wet foundation vary with indoor humidity levels. Both types of sources contribute less when the humidity is high and more when the humidity is low. This behavior is especially important because moisture from wet foundations overwhelms all other contributions. We show in this paper how taking the variability with humidity into account can lead to substantially lower estimates of indoor humidity, especially in airtight homes with low ventilation rates. Given the importance of moisture from foundations, we believe much more measured data are needed, both on the quantity of water vapor contributed by foundations as well as on its variability with indoor humidity and temperatures, including the temperature of the foundation itself. INTRODUCTION primarily determined by a balance between moisture produc- tion rates and removal rates, either by ventilation or dehumid- With the introduction of moisture engineering, the ification, and ignores the effect of moisture storage in advance of hygrothermal computer models for buildings, and hygroscopic materials as well as the effect humidity may have new design approaches for moisture control in buildings, it has on the moisture production rate. Field measurements by become important to formulate a realistic design value for TenWolde (1988, 1994) and others have shown that moisture indoor humidity. Indoor humidity is one of the most important storage in wood-frame residences stabilizes the indoor humid- parameters when determining the need for vapor retarders and ity and that daily or even weekly averages can be used for the other building envelope design features, especially in colder purpose of building moisture analysis and design. Over the last climates. Proposed ASHRAE Standard 160, Design Criteria few years, the International Energy Agency (IEA) Annex 41 for Moisture Control in Buildings1 (ASHRAE n.d.), contains a methodology to calculate design indoor humidity. The basis 1. As a result of public review comments, the title of this standard is for the methodology is described by TenWolde and Walker likely to change to Criteria for Moisture-Control Design Analysis (2001). The method assumes that seasonal indoor humidity is in Buildings. Anton TenWolde is a research physicist and Crystal L. Pilon is a general engineer at the USDA Forest Service, Forest Products Laboratory, Madison, WI. © 2007 ASHRAE. has focused extensively on the moisture storage phenomenon, vapor added by human or animal respiration should diminish and a number of papers will be devoted to that at the 2007 with increasing humidity, as the air that is taken into the lungs Thermal Performance of the Exterior Envelopes of Whole already has a higher vapor content. Searching the building Buildings X conference. This is one reason why this paper science literature did not yield articles discussing this effect. focuses exclusively on the effect of indoor humidity on mois- We therefore also searched for this information in the fields of ture production. Another reason is that while moisture buffer- human, animal, and plant physiology. ing can explain much of the shorter-term behavior of indoor humidity in wood-frame construction, it cannot explain some SOURCES OF WATER VAPOR of the discrepancies in seasonal behavior that have been expe- We tried to determine which sources of indoor humidity rienced. From anecdotal evidence we have often found that might be affected by indoor humidity and which most likely calculated seasonal indoor humidity at the extreme low and are not. We assumed a constant indoor temperature of 21.1°C high ends deviates considerably from the measured data: (70°F). We did not consider the effects of humidifiers, as they calculated high humidity often exceeds measured humidity at usually have some sort of humidistat control. Any humidifi- the high end (e.g., in wet coastal climates) and is often too low cation or dehumidification device with a humidistat control at the low end (e.g., in cold winter climates). Moisture buff- renders a humidity mass balance calculation unnecessary ering is an unlikely explanation for this because the effect because the indoor humidity will likely be at or near the tends to be too short-term in wood-frame buildings to affect humidity setpoint. Air conditioning was also not considered. seasonal humidity. The effect of thermostat-controlled air conditioning on indoor humidity is complicated and depends, among other things, on BACKGROUND the length of runtime cycles and, thus, on the proper sizing of Data on residential moisture generation reported in the the equipment (Shirey and Henderson 2004). literature vary widely. Measurements were taken under differ- Many activities, such as showering and cooking, release ent conditions and climates and in different building construc- bursts of water vapor in relatively short periods of time. tion types, and the studies may or may not include the effects However, it has been well demonstrated that this moisture is of seasonal moisture storage. Hite and Bray (1949) conducted temporarily stored on surfaces and in hygroscopic materials a study of common moisture sources in houses, and the data and then is released soon after the event. Therefore, the have been widely cited ever since (e.g., Angell and Olson amount released, not the momentary rate of release, is the most 1988). Unfortunately, over time, errors due to unit conversions important for long-term indoor humidity calculations. crept in, and some of the data cited (e.g., in Angell and Olson [1988] and Christian [1993]) are about 10% higher than the People original data reported by Hite and Bray (1949). The most People and other animals add water vapor to the indoor air complete summary of data was published in a report of IEA by respiration and transpiration. Hite and Bray (1949) report Annex 14 (IEA 1991). The reported average total moisture that a family of four contributes 0.20 kg/h (0.45 lb/h) at night production rate for one to two adults, based on multiple refer- and 0.21 kg/h (0.46 lb/h) during the day (three persons at ences, is on the order of 8.2 L/day (0.75 lb/h). These data can home, a higher activity level) based on the ASHVE Heating, be augmented with data published by TenWolde (1988, 1994), Ventilating, Air-Conditioning Guide (ASHVE 1946). Angell who reported a measured average of 6.8 L/day (0.62 lb/h) for and Olson (1988) cite the exact same numbers, and the IEA seven households without children. Based on these data, Annex 14 Sourcebook (IEA 1991) quotes moisture release TenWolde and Walker (2001) proposed design residential rates from 30 to 300 g/h (0.07 to 0.7 lb/h) per person, depend- moisture production rates based on the number of expected ing on level of activity. Sanders (1996) lists generally accepted occupants, as shown in Table 1. The data in this table were rates for “respiration” as 0.9 to 1.25 kg/day (2 to 2.8 lb/day). adopted as default design values in the public review draft of Standard 160 (ASHRAE n.d.). The design values represent an Table 1. Design Residential Moisture Generation estimated 32% exceedance level, i.e., 32% of homes in each Rates (TenWolde and Walker 2001) category can be expected to have higher moisture production rates. This value is then combined with a 32nd percentile low Number of Number of Moisture Generation Rate ventilation rate to arrive at approximate 10th percentile indoor Bedrooms Occupants L/day kg/s lb/h humidity values. –4 In all indoor humidity calculations, the water vapor 1 2 8 0.9 × 10 0.7 lb/h production rates are thought to be constant, regardless of 2 3 12 1.4 × 10–4 1.1 indoor humidity level. However, it is reasonable to assume that 3 4 14 1.6 × 10–4 1.3 if the temperature remains unchanged, evaporation into the air –4 slows down at higher humidity levels, as the vapor pressure 4 5 15 1.7 × 10 1.4 difference between the evaporating surface and the air +1 per Additional +1 +0.1 × 10–4 +0.1 becomes smaller. It is also reasonable to assume that water bedroom 2 Buildings X The 1961 ASHRAE Guide and Data Book: Fundamentals and Transpiration. Ferguson and Martin (1991) provide Equipment (ASHRAE 1961) provides data for “approximate measured transpiration rates from burn wounds, but, for loss in body weight by perspiration” for men at rest and at comparison reasons, also provide transpiration rates from work when exposed to heat. According to this source, at healthy skin, in terms of skin diffusion resistance (the diffusion 21.1°C (70°F), a man at rest loses about 90 g/h (0.2 lb/h) and resistance was defined in terms of absolute humidity gradient at work loses 270 g/h (0.6 lb/h).
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