Spray foam attic sheathing
bigde2001
| Posted in Energy Efficiency and Durability on
I have built a house that is ready for insulating. I want spray foam the sheathing or underside of the decking. I have a large open attic with minimum low space attic. I live in north georgia below chattanooga tn. I feel closed cell is the way go. I was not planning on insulating above the sheet rock ceiling which would be the attic floor. I was looking to encapsulating the attic. I have researched and still sure what direction to go.
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Replies
You should have an air channel under the roof from soffit to peak to allow air to circulate and keep your roof cool, your roofing will last longer. This is outside the insulated envelope. The key is figuring out where the boundary is between insulated and uninsulated and enforcing that boundary.
Check out this link for a way to make your own baffles:
https://www.finehomebuilding.com/2016/06/29/rigid-foam-vent-baffles
Do that and then foam over it, the polyiso is about half the cost of spray.
Do you have HVAC in the attic? That would be a good reason to insulate the roof line.
This article should apply to your: https://www.greenbuildingadvisor.com/article/does-this-roof-need-more-foam
A mix of closed cell spray foam and open cell or fluffy insulation makes the most sense from a cost, performance, and environment standpoints. You are likely in climate zone 4a, but confirm that based on your specific location.
You need enough closed cell foam to prevent moisture from condensing on it. In 4a, this is 30% of R49, so about 2.1" of HFO blown ccSPF.
https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies#t02
How deep are the rafters?
Why would you want to insulate the roof?
The roof has 50% more surface area than the attic floor and the insulation on the roof will cost twice as much for half the R value as the attic floor.
The only time I recommend insulating a roof is when trying live with poor choices people are unwilling to correct, like cathedral ceilings, HVAC equipment and duct work in the attic or convert the attic to living space.
Walta
Another vote for NOT using spray foam here unless you NEED to. “Need to” means you have mechanical (HVAC, plumbing, etc) up there, which ideally you don’t have.
Much better all around (cheaper, less potential problems, greener material) is to do a conventional air sealing job on the attic floor — which is pretty easy on a new build — and then putting in blown cellulose insulation on the attic floor. Use some rigid foam on the attic hatch and secure it with draw latches to seal it against weatherstripping and you’re good to go.
Sprayfoam is good in cathedral ceilings where there are issues with insulation and venting. In a conventional attic, it’s really not an ideal material.
Bill