How do you control outside air condensing on cold building members within a ventilated cathedral ceiling chase?
I am constructing a detached garage with an insulated second story area enclosed within the roof framing. There is a 1-1/2 inch vent chase framed within 2 x 10 roof rafters spaced 24 inches OC. on a 10 in 12 pitched roof. The roof 5/8 inch plywood with composition shingle. Two layers of 2 inch thick foil face, closed cell insulation encloses the lower side of the chase resulting in an R 28 section. I plan to add an additional layer of R 15 rockwool beneath the closed cell insulation. There is a continuous eave vent and ridge vent. After the chase was constructed we experienced 12 days of freezing weather with temperatures in the 20's. The roof was covered with an inch of frost and snow the whole time. The day the freeze ended I noticed moisture dripping at the base of the chases where the rafters meet the floor. The building is unheated and unoccupied. There was also considerable condensate moisture forming on the underside of an attached porch cover on the same building. Since I am doing the work myself as a homeowner the work has progressed slowly so the builidng material has had several months to dry out; no rain wetted the framed structure before it was closed in. The structure is located within the Cascade range foothills at about 1100 feet. Annual rain fall is approximately 50 inches. A tree covered ridge eliminates any direct sunshine on the structure for 4 months during winter. I believe the moisture is being caused by outside air being drawn into the vent chase and then condensing on the framing. If this is the cause can be done to prevent it. During other seasons it seems the venting is necessary. Any suggestions would be appreciated.
Posted Jan 25, 2013 2:07 PM ET
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