Flash and batt insulation at roof
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| Posted in Energy Efficiency and Durability on
I am designing an EPDM hot roof with tapered rigid insulation over a 3/4″ plywood deck on wood trusses in zone 4-5.
In order to create a flash-and-batt roof and have no condensation within the assembly, do I need to have a minimum of R-20 above the deck or can I have R-6.5 (1″) above and R-13.5 below before I put in the dense-packed cellulose throughout the truss?
Finally, do I also need 10-mil poly between the rigid insulation and the str. decking?
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Replies
William,
Be sure to discuss this issue with your local code official before proceeding. The interpretation of your local code official determines how you can proceed.
The code requirements found in section R806.4 of the 2009 IRC are summarized in my article, Calculating the Minimum Thickness of Rigid Foam Sheathing. That code section requires that a certain amount (R-20 in your case) of air-impermeable insulation be “applied in direct contact with the underside of the structural roof sheathing.” The code does not include credit for insulation installed above the roof sheathing.
This code provision is illogical. In a situation like yours, with some rigid foam above the sheathing and some closed-cell spray foam below, both types of foam work together to reduce the condensation risk.
However, why are you going this route? It would be a lot simpler just to put as much rigid foam as you need on top of your roof sheathing -- assuming, of course, that this approach is acceptable to your local code official. Polyiso is better for the planet than closed-cell spray foam.
Concerning your last question: I don't know what you mean by "str. decking." However, it's safe to say that you don't want a polyethylene vapor barrier in this type of roof assembly.
This assembly makes little sense to me. A flat roof is going to need repair at some point, and the deck will probably need repair too. With foam stuck to the bottom of it, you will have a nasty, major repair. I would put all the foam on top and glue nothing to the deck.