building sciences Photo 8: Interior Wetting Apparatus. Note blue capillary active mat used to distribute water injected into assembly over interior surface of exterior OSB sheathing. Photo 9: Exterior Wetting Apparatus. Note that the capillary active mat used to distribute water injected behind cladding is located behind the water control layer directly over the exterior surface of the OSB sheathing. turns out to be a very good idea for the stucco over OSB wall. We already knew this and shared this in “Mind the Gap, Eh?”. The folks in Vancouver have known this for a very long time. The walls were wetted both internally (Photo 8) and externally (Photo 9). The external wetting was done behind the “water control layer”—directly into the OSB sheathing to be representative of a flaw or a true leak or construction error or defect. The internal wetting was done in a similar fashion. Water was injected directly against the cavity side of the exterior sheathing—again to be representative of a flaw or construction error or defect. Wetting events were a week in length and consisted of adding approximately 2 ounces (57 grams) in the morning and 2 ounces (57 grams) in the evening each day. This was our best estimate of a persistent leak resulting in wetting in a concentrated area.9 Four external and one internal wetting events were done. Why more from the outside? Easy, it was Vancouver. Duh! The results are presented in Figure 2 for the north elevation and Figure 3 for the south elevation. Drying turns out to be faster on both the east and west elevations than the north elevation, and fastest on the south elevation. No surprise. The OSB moisture content for Wall 7 (the stucco over foam over OSB 9 Photo 10: Fiber-cement siding. If you install a polyethylene vapor barrier on the inside of your wall and you sheath it with OSB you had better back ventilate your external cladding. It makes a big difference. Note the “blue” spacer strips that create a 3/8 in. (10 mm) ventilation and drainage gap between the siding and the water control layer. One of our “scribes” Master’s thesis provided the basis for this estimate. Thank you, Jonathan Smegal. 84 ASHRAE Journal ashrae.org March 2012