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Thoughts on Open Cell Insulation on the Underside of the Roof Deck

Gvandrew5 said:
No venting is required by current code? Shouldn't conditioned air be supplied in that area for moisture control?
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It looks to me like Climate Zone 4 and colder, you can have an unvented attic with an air impermeable spray foamed roof deck, and no conditioned air is required for that space.

I think it's a horrible idea, but that is how R806.5 seems to read.
 
BN4537 said:
It looks to me like Climate Zone 4 and colder, you can have an unvented attic with an air impermeable spray foamed roof deck, and no conditioned air is required for that space.

I think it's a horrible idea, but that is how R806.5 seems to read.
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It does apply to the ceiling being attached to the rafters and requires "The unvented attic space is completely within the building thermal envelope." Seems to me if it's within the thermal envelope it's somewhat conditioned. I guess you could have an insulated attic floor, and the attic would get colder (in my zone 6 climate) or warmer, but why add an extra layer of insulation and if the attic floor is insulated, is the attic really within the thermal envelope?
 
bill1952 said:
It does apply to the ceiling being attached to the rafters and requires "The unvented attic space is completely within the building thermal envelope." Seems to me if it's within the thermal envelope it's somewhat conditioned. I guess you could have an insulated attic floor, and the attic would get colder (in my zone 6 climate) or warmer, but why add an extra layer of insulation and if the attic floor is insulated, is the attic really within the thermal envelope?
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Yeah, the part that scares me is the lack of airflow.
 
BN4537 said:
Yeah, the part that scares me is the lack of airflow.
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I'm not crazy about "hot roofs" but they can work. And much commercial construction is same thing - metal deck with foam on top and a ceiling under it - and that seems to work. If you can prevent condensing surfaces, it should be ok. Joseph Lstiburek has some good thoughts on this.
 
All my experience is in climate zone 1A and 2A and I try to avoid spray foaming the underside of decks if possible. But if the client is pushy then I go open cell with a small supply up there to (try to) control moisture.

I wish vented nailboards were more price competitive.
 
I had a builder-developer who was going all-in on spray foam on the underside of the roof sheathing. Then they had 2 workers hospitalized who had inhaled the fumes during the application. They have long term damage. For them, the safety risks were too high (even though the workers should have known better).
 
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