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If we use spray foam on the inside of our attic, should we also use rigid foam on the roof?

If we use spray foam on the inside of our attic, should we also use rigid foam on the roof? This is being built in Upper MI.

We had an estimate for spray foam and along with spray foam. The company states it would then put 1" rigid foam on the outside of the roof. Is this normal practice, as I thought you insulate one side, not both? It is an A-frame with cathedral ceilings.

Must be vented by code, insulated to r-49 on roof, R21 in walls. spray foam was to 39 with 1 inch rigid foam on exterior roof. We are planning on putting 4 inches of rigid foam on outside walls.

Asked by gary mccaslin
Posted Jan 1, 2013 9:07 AM ET
Edited Jan 1, 2013 1:39 PM ET


4 Answers

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It’s neither that simple nor a rule of thumb. If you are building an Unvented Attic Assembly in Climate Zone 6 or 7, is very important that you read the 2009 IRC R806.4. There are three ways to insulate your roof depending on the type of insulation (open or closed cell) you are using, but also other specifications on vapor retarders, permeability and roofing type.
Here is a good read: http://www.greenbuildingadvisor.com/blogs/dept/musings/creating-conditio....
While you are at it, read IRC R601.3.1 to know what you need to do in your wall assemblies; same concept.

Answered by Armando Cobo
Posted Jan 1, 2013 10:50 AM ET
Edited Jan 1, 2013 10:54 AM ET.


Your post is confusing. You wrote that the roof "must be vented by code."

If you are planning to vent your roof, and there will be rigid foam on top of the roof sheathing, then make sure that the vent channel is above the rigid foam -- not under the roof sheathing.

Another confusing part of your post: R-39 spray foam plus 1 inch of rigid foam does not equal R-49.

Answered by Martin Holladay
Posted Jan 1, 2013 1:35 PM ET


Hi Martin,
Sorry for the confusion...as that was meant that the contractor was "cheating" the Rvalue in the estimate. He also stated we did not need to vent if we used spray foam inside the roof and 1 inch ridgid foam on the outside of the roof. Even tho we confirmed with the building inspector we did have to have a vented roof. I already have decided this contractor is not for us... What I need to know is what is the best way to insulate an A-frame in Upper Mi that is already framed with roof (2 x 6's) and tarped for the winter. Walls are (2x4's) covered with 6 mil plastic on outside. In the spring we are closing it in with asphalt shingles and vinyl siding. Total sq. footage is 1250, 2 levels. Tot
al roof 2050 sq. ft. We would like to put 2 inch foam on the outside, but we are open to ideas.

Answered by gary mccaslin
Posted Jan 1, 2013 1:52 PM ET


To read about the many ways to create a vented cathedral ceiling, see this article: How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay
Posted Jan 1, 2013 5:08 PM ET

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