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Flash & batt of conditioned attic

Coop Mag | Posted in Green Building Techniques on

Hey Folks.

I’m building a new home in Zone 4 (Nashville, tn). It has a conditioned attic with mechanicals. I’m looking for the most cost effective, yet durable, way to insulate it r-38 or better.

My plan is to spray foam then follow up with a batt after furring 2×10 rafters with 2×4’s. I can spray 3 inches of open cell for less than half the cost of 2 inches of closed cell. I’m thinking the OC is the way to go. OC is about $1,800 and CC is about $4,000.

I’ve read through the articles about insulating a cathedral ceiling and is OSB and open cell a bad idea threads. I know the recommendation is to flash with CC, but the cost difference is compelling. Am I asking for trouble by using OC of only 3 inches instead?

My roof assembly is as follows:
Metal roof
peel & stick underlayment
OSB
Spray foam
Batt
Drywall

Thanks!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    C. Maglio,
    You can use open-cell spray foam for this type of roof assembly, but the foam layer has to have an R-value of at least R-15 in your climate zone (Zone 4A). So you need at least 4 inches, not 3 inches, of open-cell spray foam to meet minimum code requirements (and to keep your sheathing from getting damp).

    For more information on all of the different ways to insulate this type of roof assembly, see How to Build an Insulated Cathedral Ceiling.

    If you want to save some money, you could install asphalt felt underlayment instead of peel-and-stick underlayment. Peel-and-stick underlayment is expensive and unnecessary.

  2. Coop Mag | | #2

    OK. I've kind of painted myself into a corner here. (First mistake was going with unvented attic assembly with forced air in attic, but that's a discussion for another day). So I spray foamed by attic roof. They put in about 5 to 6 inches of OC foam in 2x10 rafter. However, it's lumpier than I expected and it covers nearly the whole side of each rafter. So I can't really get a wool batt to fit in the cavity nicely.

    I could sister some 2x4's perpendicular, but that will leave a void of 2 to 3 inches between the batt and the foam in lots of places. I assume this is greatly inhibits the batt's ability to insulate or will the difference be minimal?

    I really liked the idea of the rock wool batt as added insulation with the bonus of fire protection. Is netting and blowing in rock wool an option? I don't see much, if any, discussion about that product.

  3. GBA Editor
    Martin Holladay | | #3

    Coop,
    If you are going to net the area, why not net it with InsulWeb and blow in some cellulose insulation? That will take care of the "bumpy foam" problem.

  4. Expert Member
    Dana Dorsett | | #4

    Batts over open cell foam almost never works, but it's pretty easy to blow cellulose, rock wool or fiberglass in netting works fine. Finding a local source for blown rock wool could be a problem, but most box stores carry some type of fiberglass blowing wools (usually only goods designed for open blown attics, but that's OK), or cellulose. If you can't find a local source for purpose-made blowing mesh, landscaping fabric works just fine.

    With open cell foam on the roof deck it's probably worth using "vapor barrier latex" primer on the drywall. It has about the same vapor retardency as 2" of closed cell foam, and would mitigate against the daily moisture cycling seen in some attics insulated with open cell foam.

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