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I’m building a new home, with a flat roof system in Massachusetts. It is 2×10 and 2×12 joist construction, with 3/4” CDX Fir plywood, followed by 1” of polyiscocyanurate foam on the deck, then a fully adhered EPDM membrane. I am planning to spray polyicynene insulation into the joist bays. Since the foam completely fills all voids, and is impervious to moisture I have been assuming that I do not need to provide ventilation between the insulation and the plywood deck. The building inspector says that I have to provide ventilation, and my architect is not familiar enough with residential construction to help very much. (He designed the house for us 15 years ago, now he does commercial work, and spec’d fiberglass).
If I have to I can paint the underside of the deck with a vapor retarder paint, or another sealant, but the cost of retrofitting a ventilation system would be huge.
Help!
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There should be a ventilated airspace above the insualtion/joists. this is usually done by cross strapping with 2"x's then the roof deck then the roof membrane. The insulation above the wood deck then has no R value and ia not necessary. The plywood will absorb moisture and rot. Do it right before it is too late.
A commercial system with a steel deck would have a vapour retarder on top of the deck, then insulation, then the roof membrane. The underside of the steel deck would be open to the interior air
Good luck
*"Since the foam completely fills all voids, and is impervious to moisture I have been assuming that I do not need to provide ventilation between the insulation and the plywood deck"Maybe today, but what about 10, 20, 50 years from now?I don't know if it shrinks; just want to point out that you have to consider future events.Whereabouts in MA? Big mistake I made was putting on a flat roof addition on the Cape back in, oh, well, awhile ago (pre-epdm days)
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I’m building a new home, with a flat roof system in Massachusetts. It is 2x10 and 2x12 joist construction, with 3/4” CDX Fir plywood, followed by 1” of polyiscocyanurate foam on the deck, then a fully adhered EPDM membrane. I am planning to spray polyicynene insulation into the joist bays. Since the foam completely fills all voids, and is impervious to moisture I have been assuming that I do not need to provide ventilation between the insulation and the plywood deck. The building inspector says that I have to provide ventilation, and my architect is not familiar enough with residential construction to help very much. (He designed the house for us 15 years ago, now he does commercial work, and spec'd fiberglass).
If I have to I can paint the underside of the deck with a vapor retarder paint, or another sealant, but the cost of retrofitting a ventilation system would be huge.
Help!