The Effect of Indoor Humidity on Water Vapor Release in Homes ✓ Most Recent

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 important 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.

Presented at Thermal Performance of Exterior Envelopes of Whole Buildings X – December 2007

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