Helpful? 0

Best way to air seal an attic?

Just had a BPI Energy Audit and was told that they would use 1 inch of CC spray foam to air seal the attic floors and then blow in 15 inches of cellulose. Is this the best way to air seal and attic floor? Is it the most cost effective? There are ducts and an air handler in one attic space and just ducts in the other. The house is in Zone 4. Thank you.

Asked by Woody McMahon
Posted Thu, 03/27/2014 - 09:34
Edited Thu, 03/27/2014 - 10:06


4 Answers

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Their suggested method is likely to work, as long as the installer is conscientious.

In some cases (especially if the cost of labor is low or free, as is the case when homeowners do the work themselves), it may make sense to use the spray foam to seal certain areas rather than the entire ceiling. However, if one has to pay for labor, it may make sense to simply spray the entire ceiling.

For more information on this issue, see Air Sealing an Attic.

There's another issue embedded in your question, however: if ducts are located in the attic, does the insulation belong on the attic floor, or should the insulation follow the sloped rafters? I tend to support those who argue in favor of creating an unvented conditioned attic by insulating the roof plane.

For more information on this issue, see Creating a Conditioned Attic.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 03/27/2014 - 10:05

Helpful? 0

I would agree that insulating the roof plane makes more sense but will be more expensive with less insulation value and come with the downside of making attic roof leaks more difficult to spot.

Answered by Woody McMahon
Posted Thu, 03/27/2014 - 14:51

Helpful? 0

An inch of cc foam over the entire attic floor makes sense if it's planks, not so much if you're air sealing sheet gypsum that's tight to the joists.

Ducts in unconditioned attics a pretty much an energy-disaster to be mitigated in any climate. (If I ruled the world it would be explicitly dis-allowed in building codes ! :-) ) Whether & where it's cost effective to retrofit to a conditioned attic varies, but on simple gabled roof lines it can be easier and more cost-effective than you might initially think.

Answered by Dana Dorsett
Posted Thu, 03/27/2014 - 17:50

Helpful? 0

I agree with Dana. I tried this assembly a few times, probably almost 10 years ago. Subcontracting the spray foam lead to much worse results than what we get now with pretty standard air sealing. Expect a lot of unsealed eaves, dropped soffits, and a just general bad job. Now I have my own spray foam truck and I still don't use this technique for air sealing. Too much expense, and very hard to drag the gun around and hit everything effectively. Save the spray foam for the ducts.

Answered by Jesse Smith
Posted Thu, 03/27/2014 - 18:57

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