The research presented in this paper shows that moisture transfer between indoor air and hygroscopic building structures can generally improve indoor humidity conditions. This is important b...

In this paper, measurements are presented which quantify the mass transfer of tracer gases and water vapor between indoor air and a permeable and hygroscopic building envelope. The transfer of tracer gases through the envelope requires the entire envelope to be permeable, while the transfer of moisture requires sufficient hygroscopic mass to be in contact with the indoor air. The results show that mass transfer can improve the indoor air quality and climate. The diffusion of gases through the building envelope significantly increases the effective ventilation rate for poorly ventilated rooms, but only moderately increases the effective ventilation for well-ventilated rooms. Moisture transfer, on the other hand, has a significant influence on the indoor humidity for both poorly and well-ventilated rooms.

Vapor-permeable building envelopes have received renewed interest because they can moderate indoor humidity levels and improve the drying of the envelope during summer condensation conditions. In this paper, the moisture performance of a vapor-permeable building envelope is presented with field measurements and numerical simulations. The results show that the diffusion resistance of the internal surface should be greater than that of the external surface (typically recommended ratio of 3: 1 or 5: 1), but that the vapor resistance of the vapor retarder can be significantly below that provided by polyethylene and still result in a safe structure, even in a cold climate.

The diffusion of water vapor through construction materials is driven by the gradient of partial water vapor pressure. Traditionally, water vapor transmission tests (WVT), the `dry cup', and the `wet cup' tests have been conducted with 0 to 50% RH, and from 50% to 100% RH, respectively. Often, desiccants, saturated salt solutions or distilled water are used to generate the required RH. The stability of vapor pressure in WVT tests is of paramount significance because these values are used in calculating permeance. Dry cup tests performed with high permeance materials (>25 Perms) indicated a transient behavior for the two types of adsorbing desiccants; calcium sulfate and silica gel. Monitoring RH inside the dry cup was performed to examine the effect of low and high WVT on the performance of moisture sinks. The results indicate that for highly permeable construction materials, the use of vapor pressures corresponding to 0% RH in calculation of permeance is not appropriate.