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Michael Griffin Posted: May 15, 2008 02:43 PM	Air circulation under plywood roof decks - mold and rot ? My local foam contractor in NY is reccomending spraying closed cell foam direclty to the underside of the plywood roof deck which is topped with a standing seam metal roof. The application is complicated by the fact that the rafters are steel I beams and the house ( being contemporary ) has some very crazy angles, leaving lots of nooks and crannies. I am concrned that if any mositure gets between the foam and the plywood it can cause mold and/or rot. Any opinions
Rob Granger Posted: May 15, 2008 10:10 PM	cbuilder, As a spray foam contractor in NY I would recommend the following: Spray the roof deck as well as the steel rafters. Then install a thermal break to the bottom of the steel rafters such as plastic or some other high R-Value material. Wood has a R-Value of about 1 per inch therefore 5/8 plywood only has an R-value of .62. Not much R-value keeping the heat and cold from conducting from the steel roof through the plywood decking and along the steel rafter into your living space. Closed cell foam is a vapor barrier so no moisture would get between the deck and the foam. I would also recommend 5 inches of foam due to the higher temp. of the roof given that it is metal. I hope this helps!
Terry Shepperd Posted: May 27, 2008 11:21 AM	Is this in an attic space or can you look up and see the plywood? I would spray no more than 3" to the deck and maybe 1" on the I beams. No disrespect to the other guy but 5" of closed cell is good enough for a walk-in freezer. You won't have a lot of heat transfer through the plywood to the I beams, mostly through the fasteners.

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Michael Griffin

Posted: May 15, 2008 02:43 PM

Air circulation under plywood roof decks - mold and rot ?

My local foam contractor in NY is reccomending spraying closed cell foam direclty to the underside of the plywood roof deck which is topped with a standing seam metal roof. The application is complicated by the fact that the rafters are steel I beams and the house ( being contemporary ) has some very crazy angles, leaving lots of nooks and crannies. I am concrned that if any mositure gets between the foam and the plywood it can cause mold and/or rot.
Any opinions

Rob Granger

Posted: May 15, 2008 10:10 PM

cbuilder,
As a spray foam contractor in NY I would recommend the following: Spray the roof deck as well as the steel rafters. Then install a thermal break to the bottom of the steel rafters such as plastic or some other high R-Value material. Wood has a R-Value of about 1 per inch therefore 5/8 plywood only has an R-value of .62. Not much R-value keeping the heat and cold from conducting from the steel roof through the plywood decking and along the steel rafter into your living space. Closed cell foam is a vapor barrier so no moisture would get between the deck and the foam. I would also recommend 5 inches of foam due to the higher temp. of the roof given that it is metal. I hope this helps!

Terry Shepperd

Posted: May 27, 2008 11:21 AM

Is this in an attic space or can you look up and see the plywood? I would spray no more than 3" to the deck and maybe 1" on the I beams. No disrespect to the other guy but 5" of closed cell is good enough for a walk-in freezer. You won't have a lot of heat transfer through the plywood to the I beams, mostly through the fasteners.

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