Building Enclosure Hygrothermal Performance Study, Phase 1

Abstract

The moisture performance of three different classes of wall systems has been investigated in the context of the preliminary hygrothermal analysis of walls in Seattle. The results reported in this phase specifically address the moisture performance of walls designed with loads that have some unintentional water penetration. The results have been developed in a manner to present the relative performance of the walls in the same climate with similar water penetration effects. The analysis was performed with the best available input data. Several limitations should be recognized within the context of this study. Results showed that selection of wooden sheathing boards on interior vapor-tight assemblies does not significantly influence the performance of stucco-clad walls. A larger effect was observed when the interior vapor control is made vapor open. When continuous cavity ventilation is employed, the effect of the selection of the type of sheathing board on the hygrothermal performance of the wall was found to be negligible. When comparing oriented strand board sheathing performance against the performance of exterior grade gypsum, the differences are very significant in terms of the amount of moisture content present in the walls. Moisture content alone does not indicate their respective durability as durability is directly related to the combination of relative humidity and temperature, mechanical, chemical, and biological properties of the substrates. This study did not investigate the durability performance of either sheathing. In terms of interior vapor control, inhabitant behavior must be considered during the wall hygrothermal design stage. If interior relative humidity is maintained below 60%, then a latex primer and paint may perform better than the use of PVA or even a polyethylene sheet. When the interior environment is maintained at a higher relative humidity, then stricter vapor control is needed. Multilayered building paper was experimentally shown to enhance the drainage capability of the stucco walls in a set of preliminary drainage tests. Water entry was found to be present in either dual- or single-layered systems, but single layers allowed substantially more water penetration. The effectiveness of the building paper was found to depend on the type of vapor control strategy used on the interior. This connectivity is important to recognize. Results have shown that two layers of a 60-min paper system performed better than a single layer of No.15 felt paper. In general, weather-resistive building papers play a very important role in a stucco-clad wall system. Vapor diffusion control is only one element of control these membranes offer. Continuous venting of stucco walls provides a beneficial drying performance, which is not present in walls that are not vented. Continuous ventilating of stucco walls provides a further improvement in the drying performance of the stucco walls as compared to vented walls. The effectiveness of the weather-resistive barrier vapor diffusion control was found to be significantly smaller in the presence of ventilation, thus the choice of the weather-resistive membrane should be made for reasons other than vapor control. Increasing the insulation levels from 89 mm to 140 mm or even 280 mm resulted in slightly higher moisture accumulation. Walls that have been designed with a more vapor-open interior showed negligible effects on moisture content accumulation caused by increased amounts of insulation applied to the walls. The limiting factor was establishing an acceptable interior environment, limiting the interior to 60% relative humidity. The indoor environment plays a critical role for the performance of all wall systems. Increasing interior ventilation rates decreases the interior vapor pressure and relative humidity and enhances the performance of the stucco wall during different times of the year. The amount of ventilation required to reduce the interior relative humidity may be prohibitively large, and other means of dilution by dehumidification may be beneficial when high interior moisture loads are present.