metal roof - condensation question


Old 01-25-14, 07:04 AM
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metal roof - condensation question


I have an existing pole barn that has been enclosed with walls. Also a plywood and shingle roof has been put over the existing sheet metal roof. I am installing 3/4" rigid insulation on the ceiling inside to stop the condensation. My question is this (please see the attached photo).

Would it be better to install the insulation on the bottom of the rafters and create an air gap between the insulation and the underside of the roof or cut the insulation and install it between the rafters, directly to the underside of the metal?

Thanks in advance for your help!
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Old 01-26-14, 05:39 PM
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metal roof condensation issue

Installing sheets of foam will probably not stop your condensation issue. You really need to have the foam directly bound to the metal with no air space between them. By no air space, I mean NO air space.
Old 01-26-14, 06:29 PM
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I would agree with calvert. Applying spray foam to the bottom of the metal roofing would be the only way to eliminate the air space underneath it. Condensation occurs on any cold surface that is below the dew point, so the fact that there will still be an air space and the fact that 3/4" of foam will only be R-4.... the foam would probably not help much.

Everything gets dew on it in the morning when the surface temperatures and dewpoint cross paths. So even if you insulate the ceiling, and your shed is not heated, the insulation will probably still get dew on it. It would be no different than laying a piece of r-19 fiberglass out in the yard overnight. In the morning, the grass will have dew on it and so will the insulation.
Old 01-27-14, 03:58 AM
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Unless you were to frame up and ventilate an attic space, I'd learn to live with it. Both my house and barn/shop have a metal roof. At times my shop ceiling [no insulation or covering] sweats big time but I've never noticed condensation in my attic. I believe the ventilation is the biggest factor.
Old 01-27-14, 05:07 AM
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I'm curious why plywood and a shingle roof was added on top of what looks like a good metal roof? Was it made into a pitched roof or is the new roof directly on top of the metal one?

My hangar had rainy seasons every spring and fall. So much moisture would condense on the metal roof that it reliably had rainy seasons indoors. Very annoying when trying to keep things clean and prevent tools from rusting. I think it's exasperated by our temperature swings and humidity. Heat (I didn't have AC) was the only way to reliably stop the rain but with no insulation I only ran it when I was in there working.

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