| The design and construction of building envelopes must be based on the assumption that some moisture will accumulate in the wall assembly, during construction and during the life of the building. Construction practices for multi-unit wood frame residential buildings in the coastal area of British Columbia, Canada are changing in response to a large number of envelope failures experienced in the period from 1985 to 1999. The new design approach includes the use of enhanced deflection and a drained cavity. While this approach will manage a large portion of the exterior moisture load, designs may also need to incorporate enhanced drying capabilities. A research program conducted at Forintek Canada Corp.'s western lab in Vancouver, Canada has evaluated the relative drying rates of wall assemblies under controlled laboratory conditions. The research ranks test wall panels in terms of their relative drying capacities, identifies potential wall locations at greater risk of slow drying (thus requiring enhanced material durability) and derives baseline data which can be used to improve parametric models of wall performance.
Results from the first group of 12 wall panels tested indicate that all the panels dried. There was a substantial range in drying rates, with ratios up to 3 times for comparable wall panels with and without a cavity. The major influences on drying rates were:
???The presence of a wide cavity (the 19 mm cavity performed better that either the 10 mm cavity or the 0 mm cavity)
???The choice of venting (top and bottom venting had a marginal improvement over venting at the bottom only).
???The choice of sheathing (plywood sheathed panels dried faster than OSB sheathed panels, partly because the plywood started out at a higher moisture content),
There was no clear indication that the choice of moisture barrier material had a substantial influence on drying.
The results of the test were compared to predictions made by CMHC's WALLDRY model. The WALLDRY parametric model demonstrated good predictive capabilities in terms of overall drying trends.
The first EDRA test has set a "benchmark" drying rate of 1600 ng/Pa.sec.m2 for the effective permeance of the 'best drying' panel in the test group. | 