Spray foam over shingles?
I am renovating a ca. 1840 timber frame house, with odd-spaced rafters, one layer of board sheathing, one layer of plywood sheathing, one layer of composition shingles, minimal overhangs, and ridges that have settled a few inches. I am planning to install new rafters over the existing roof, with larger overhangs, new zip decking, synthetic underlayment, and a snap-together standing seam roof. This approach gives me a straight roof, allows me to leave the ceilings below intact, allows me to match the rafter tails on the addition I built recently, and allows insulation to be installed on top of the existing roofing.
I had planned to use EPS for insulation in between the rafters, however I’m thinking I’ll get too many gaps and it will be labor intensive. Has anyone here used closed-cell spray foam in a similar situation? It would have the advantage of being continuous plus it would get under the rafters where they clear the existing roof. It would also be quick.
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Replies
Joel,
What climate zone are you in? How complex is your roof line? How deep are your rafters? These details are helpful to folks who might want to offer advice.
Installing reclaimed (much cheaper) rigid foam is often recommended on GBA. Closed-cell is not particularly green, but it is sometimes the best solution.
I'm in zone 5, fairly simple roofline with only one short valley, 5/12 and 7/12 roofs, rafters are rough cut 2x6. I'm demolishing the little low-roof add-on and making roof plane continuous.
Joel:
Here is a good article: https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing
It looks like this would be a good fit for exterior insulation--especially if you can locate some reclaimed roofing foam. If you are worried about the aesthetics, you could combine exterior and interior insulation. See here for more info: https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation