Typical Seattle, WA Light Frame Wall Assembly

[Hayden Robinson]

I've just returned to Seattle from the San Diego WUFI Pro workshop, and am using WUFI to take a look at typical wall assemblies for this area. As a base case, I've chosen a common, code-minimum assembly that has not been optimized for either thermal or moisture performance, but which has been in common use here for decades without evidence of wide-spread, moisture-related failure. However, in my attempts to model it, I find abject failure: the assembly accumulates water over time, with the moisture content of the siding hitting 35% in year 3, and the moisture content of the plywood sheathing reaching 75% in the same time period. The failure of my modeled assemblies is at odds with the observed performance of these assemblies in practice. I tried removing the exterior SBP membrane (paint) and setting the exterior permeance to 5 perms, but the water accumulation became worse. Can anyone help me fix my model so that it accurately reflects observed performance?

From outside to inside: spun-bonded polyolefin membrane (to simulate a vapor-open, but absorption-limiting coating), on 3/4" western red cedar (siding) with a 1% ASHRAE 160 leak at the interior cell of the siding (One Element, Fraction of Driving Rain, 1%, 0.75" depth), on spun-bonded polyolefin membrane (WRB), on 1/2" plywood USA (sheathing), on 5 1/2 fiberglass (cavity insulation), on kraft paper membrane (vapor retarder), on 1/2" interior gypsum board (interior cladding). The interior surface of the GWB has its permeance set to 10 perms under the Surface Transfer Coeff. tab.

Under the Assembly/Monitor Positions tab, Edit Assembly by: is set to Graph. The Grid is set to Automatic Grid, Medium.

Orientation is due south, to agree with Seattle Wet Year wind-driven rain exposure. Inclination is 90. Rain load calculation according to ASHRAE 160 is checked. FE is 1.0. FD is 0.5.

Under the Surface Transfer Coeff. tab, Heat Transfer Coefficient for the external wall is left blank, wind-dependent is checked. Permeance is set to No coating (because of the exterior layer of spun-bonded polyolefin). Absorptivity is 0.4. Emissivity is 0.9. Explicit Radiation Balance is unchecked. Ground Reflectivity is 0.2. Adhering Fraction of Rain is 0.7. Permeance of Interior Surface is set to 10.

Initial Conditions are constant across components at 0.8 RH and 68F.

The Calculation Period is 10/1/12 - 10/1/15, Time Steps 1. No changes have been made to the Numerics.

Outdoor climate is Seattle Cold Year. Indoor Climate is ASHRAE 160 derived from Seattle Cold Year.

Any help would be much appreciated.

Hayden

[Manfred Kehrer]

Hello Hayden,

I do not know what went wrong in your calculation, but I think you should not have any moisture accumulation in the simulation.