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Roofing/cold flat roof condensation


QUESTION: hi john
i had a contractor finish out a carport to make it a garage.  it's a cold flat roof, and now i'm getting major condensation in the underside of the garage ceiling.  no opportunity for soffit vents, as their is no eave.  will spray foam insulation in between the beams on the inside of the now garage fix this?

ANSWER: Condensation is probably occurring because of heat losses through the walls, windows, doors from the adjoining home. Also if the kitchen is nearby, it is a place of high humidity generation and this would also end up in the garage as condensation.

Spray foam is a possible approach however you better check with your local building officials since this carport is now a garage you may need a fire resistant interior finish such as drywall. Also the code may denote the doors to the house have to be sealed so no exhaust fumes will enter the home.

All the best.

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QUESTION: hi john
thank you for your response.
you were right to assume there is a common wall between the house and the, now, garage.  there is a window to the kitchen in the common wall, and also stove vent ducting on the wall that exhausts to the flat roof top.  i did have permits from the city, and did have the window covered with a framed drywall piece that has since been removed, for aesthetics, after the permit was signed off on.  there are no doors directly from the garage to the house.  i've researched online re differences between cold and warm flat roof construction, and would prefer, from a cost basis, to try and stay with a cold roof.  would spray foam in between the beams on the underside of the garage and spray foam over the ducting help resolve my condensation issues?

The spray foam would insulate the ducting and the underside of the roof deck so condensation should be minimal if not reduced. However check to see if a flammable foam can be exposed in the garage between the joists. As previously mentioned drywall may be the only option to conform to the local building code.


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D. John Henderson


My 33 years in the building science business has involved air barriers, waterproofing, and flat & steep slope roofing systems.


My roofing encompasses commercial systems (BUR, mod-bit, single-ply) and residential applications (asphalt shingles, cedar shingles, slate and metal. I am cognizant of code stipulations in regards to FM and UL for commercial roofs.

Attended numerous courses offerd by Roof Consultants Institute, Roofing Industry Educational Institute, Factory Mutual Research Corporation, University of Wisconsin, and BURSI. Lectured for RCI and mentored at a local university for a Building Science Certificate.

Past/Present Clients
Government, commercial businesses such as The GAP, hospitals, school boards as a roof designer.

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