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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.

Asked by Bob Bishop
Posted Fri, 01/25/2013 - 14:07

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4 Answers

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1.
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Sounds like you are describing typical eave and ridge venting. Is the rigid foam in between the rafters, and if so, is it foamed in place along the edges and ends? Is there drywall installed under the rafters yet? I'm guessing no to the second question since you mention the rock wool still to install. Can you get an accurate reading of temperature and humidity indoors and out to compare? Has there been any condensation on windows? What is the roof construction (slope, materials/layers) of the porch cover?

Answered by David Meiland
Posted Fri, 01/25/2013 - 14:35
Edited Fri, 01/25/2013 - 14:36.

2.
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Bob,
Under some weather conditions (when the outdoor air temperature rises after a period of snow, and with snow still on the roof) you can get condensation of moisture in outdoor air that is flowing through ventilation channels. But it is more common for the source of the moisture to be interior air that reaches the sheathing through air leaks in the roof assembly.

Did you do a thorough job of air sealing around the perimeter of the polyiso pieces? If there is any air leakage from the interior to the vent channels, that's bad.

Eventually, your airtight ceiling -- you are planning to install one of those, right? -- will solve the problem.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 01/25/2013 - 14:53
Edited Fri, 01/25/2013 - 14:54.

3.
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Polyiso foam is placed between rafters and has not been sealed around the edges as of yet. No drywall has been hung. The building is open to exterior via garage door openings and stairwell, so the measured inside and outside temperature is nearly identical. I do plan on sealing all edges of the polyiso, I understand the importance of sealing the vent space from the conditioned interior air. The attached covered porch has a roof slope of 4 in 12 and the underside plywood roof covering, rafters and supporting beam are exposed and painted. Moisture is condensing on the porch members too, leading me to believe that this is being caused by moisture content of the outside air condensing on cooler roof framing members both inside and out. Thanks for the input. I have not encountered this before having moved here from a warmer, dryer climate.

Answered by Bob Bishop
Posted Fri, 01/25/2013 - 18:52

4.
Helpful? 0

Perhaps it is condensing on the cold foil face in the chase as well. I made the foil board extend past the plane of the wall, and into my sofet area just in case this happens. You might do the same with your top 2in layer. since it's not all sealed up yet. The foil makes a nice slick surface for condensation to drip from. Just look at your car hood in the morning..

Answered by stephen edge
Posted Sat, 01/26/2013 - 04:15

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