Add foam during reroof
Kenny78
| Posted in General Questions on
It is finally time to put a new metal roof on our house. Location is on the north end of climate zone 3a, zip 74301. The roof stack up is currently; Sheetrock, 2×6 with a mix of fiberglass batts and cellulose, 2×4 on side for purlins and then corrugated tin.
Awful I know but the roof didn’t leak when we bought the house and we are planning to build a new house soon further into the property. We are wanting to make the best of a bad situation with the existing house as it is going to be retained for aging parents etc.
I would like to boost the r value of the roof assembly during reroof. The new roof material will be screw through metal. I have a correspondence opened with the manufacturer as to acceptable structure/support on their end.
I would assume the proper(compromise) approach would be to; remove purlins, place osb, foam(31% or above) then follow roof panel manufacturer guidelines. Such as another layer of osb or roof membrane etc
The interior has very low headroom and intact Sheetrock so I need to build up instead of fixing from the inside. I would also like to take the opportunity to make the structure more efficient. Any suggestions or critiques are appreciated
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Replies
Ken,
You suggested: "Remove purlins, place OSB, foam (31% or above), then follow roof panel manufacturer guidelines. "
In your climate zone, the rigid foam layer only needs to have an R-value (at a minimum) equal to 13% of the total R-value of the roof assembly, not 31%. For more information, see:
https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation
Above the rigid foam, you'll need either (a) roofing underlayment followed by 1x4 or 2x4 purlins, 24 inches on center, or (b) OSB or plywood, followed by roofing underlayment.
Martin, thank you for the reply and also this great site. I am tracking down reclaimed foam which I was educated about from this site. It’s mind blowing how inexpensive it is compared to new. Hopefully I can make some real and durable improvements to this house.