| EXECUTIVE SUMMARY
Phase I of the City of Seattle Hygrothermal Building Envelope Performance Preliminary Analysis has been completed. Within Phase I, 35 walls were evaluated for their hygrothermal response to environmental loads present in the greater Seattle area. These wall systems were classified in three sets¡ª a set of conventional stucco-clad walls, a set of pre-1984 stucco and cedar-siding walls, and a set of current nonstucco-clad walls. While all three classes were investigated, emphasis was placed on stuccoclad syste ms.
The selected walls were analyzed in terms of their hygrothermal performance and capability to handle moisture loads that are more in line with real conditions rather than ideal loads used in today's designs. For the first time ever, the concept of incidental water penetration was incorporated into the performance analysis of stucco-clad wall systems and nonstucco-clad wall systems for climatic conditions of Seattle. Several key moisture-design performance issues were investigated. In this report, pre-1984 walls are compared to post-1984 walls, and vented walls were compared to ventilated walls and walls with no ventilation. Walls were compared with different exterior sheathings and with different weather resistive barriers. This report examines the effect of increasing insulation levels, compares several different interior vapor control strategies, examines the effect of mechanical ventilation "negative pressures," and investigates the effects of interior humidity control in building envelopes.
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. The following findings are noted for the performance of the selected wall systems.
1. 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 sheathing board on the hygrothermal performance of the wall was found to be negli gible.
2. 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.
3. 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 polyvinyl acetate (PVA) or even a polyethylene sheet. When the interior environment is maintained at a higher relative humidity, then stricter vapor control is needed.
4. 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 l ayers allowed substantially more water penetration.
5. 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 diffu sion control is only one element of control these membranes offer.
6. 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.
7. 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.
8. 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 dilunition by dehumidification may be beneficial when high interior moisture loads are present.
9. Operation of the mechanical system to provide .35 air changes per hour for at least 8 hours per day as required by the 1997 Washington State Ventilation and Indoor Air Quality Code caused increased moisture accumulation of the stucco-clad wall system. These code requirements may have contributed to increased moisture contents in the stucco wall systems, but it appears that mechanical ventilation merely acts to augment the existing significant air pressure differences caused by wind and stack effects. More testing is warranted to better understand the effects of mechanical ventilation on wall perfor mance and durability. 10. Water penetration is the most critical influence on moisture management of wall systems. Neither vapor diffusion nor air leakage is comparable in magnitude. Water penetration may be several orders of magnitude greater than allowed by simple vapor or even liquid diffusion because of wind-driven rain. Water penetration at even a small fraction of the total rain load (1 to 2%) must be included in designs to represent realistic boundary conditions. The presence of windows and joints amplifies the local hygrothermal influences, as water loads become many times greater that in walls without window s and joints.
11. Exterior gypsum provides an additional resistance to both thermal and moisture flow that was found beneficial for a 2 ¡Á 4 old stucco wall arrangement. In this wall arrangement, concepts of "flowthrough moisture control" seem to have very positive hygrothermal performances. At different times of the year, the conventional stucco wall displays higher amounts of moisture accumulation, while at the same time, allows more rapid drying. Cedar siding was found to permit a synchronized moisture performance with distinct moisture drying and wetting distributed during the year.
12. In all of the nonstucco claddings, the capability of the cladding systems to store water is limited in comparison to the stucco-clad cases. In most of these nonstucco-clad walls, a strong ventilation drying capability was present in the wall system. This ventilation factor reduced the available moisture in the wall components and limited moisture storage in the exterior cladding. This moisture performance resulted in wall s that had less water stored in the critical elements of the wall.
 Seattle Report
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