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Duct Insulation R Value in a Attic with a spray foam roof deck.

Trippank | Posted in Building Code Questions on

Definition of Conditioned Space – Spray foamed attic/roof deck
I am consulting on a new construction house for efficiency. We are spray foaming the entire house including the bottom of the roof deck. The HVAC installer finished his rough in. His duct work is wrapped with R6 insulation in the attic because he understood it was a “conditioned space”
The building inspector failed his rough in because he said the area was not directly conditioned with its own supply and return so the duct need to be R8.

What is your opinion and suggestions?
-Tripp

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Paul,
    I think that the HVAC installer was right. But as we all know, the building inspector has the final word.

  2. user-1118336 | | #2

    If the official is quoting Section 403.2.1, the exception is clear: "ducts or portions thereof located completely inside the building thermal envelope". See the 2009 IECC Code Commentary for the definition of "building thermal envelope". It supports that the duct is inside the thermal envelope: "A space is conditioned if it is heated or cooled directly or where a space is indirectly suppplied with heating or cooling through uninsulated walls or uninsulated ducts". This would support not requiring insulation at all. Take the insulation off and the attic is officially conditioned space. The unvented attic is inboard of the insulation and has an uninsulated separation to conditioned space. This exposure is better than a vented crawl space, which only requires R-6. Hopefully logic prevails and R-6 is an acceptable compromise.

  3. Expert Member
    ARMANDO COBO | | #3

    Paul,
    I don't know where you live or what code your municipality is in, but must states are now on the 2009 IRC, plus the 2009 IECC that Mark pointed out on comment #2. There you can look at R806.4 Unvented Attic Assemblies deffinition and N1103.2.1 (same as 2009 IECC 403.2.1). If you only sprayed closed cell foam, then you are in compliance, however, if you only sprayed open cell foam under the roof decking, you are not in compliance... it's a common mistake made in the foam application business.

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