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Community and Q&A

Low slope roof and what type of insulation?

randynavarro | Posted in GBA Pro Help on

I live in Seattle – Marine climate – in a single story home built in 1962. I have a low-slope roof (2/12). The “roof rafters” are 4 x 8 beams at 48″ o.c.. The roof decking is 2×6 T&G planks. The entire ceiling is vaulted. In other words, what I see from the inside are the 4 x 8 beams and T&G decking. That’s it!

Right now, there is a layer of built up roofing with a mod-bit cover. I’d like to add as much insulation on top of the deck that I can afford and re-roof with TPO or white granular coated mod-bit.

Concerned that if I add that insulation , I’ll now be trapping interior moisture traveling through the T&G planks and cause problems.

How would you add insulation to the roof? Adding from the inside is not an option.

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  1. GBA Editor
    Martin Holladay | | #1

    This type of roof is common in commercial construction. Such a roof is easily insulated above the roof sheathing with polyisocyanurate insulation, a type of rigid foam. I suggest that you install at least 6 inches of polyiso, followed by new roofing.

    Any commercial roofer should be happy to give you an estimate for this work.

  2. randynavarro | | #2

    Thanks for the quick response, Martin. I probably can't afford poly-iso but have to go with XPS instead. I know I'll sacrifice R-value but I figure going from zero to something is a vast improvement.

    So. . . no worries about trapping moisture under the foam or the roof coating?

  3. GBA Editor
    Martin Holladay | | #3

    Any of the rigid foams will work well -- XPS, EPS, or polyiso -- although polyiso is more environmentally friendly than the other two types of foam. In many parts of the country, polyiso costs less than XPS. (If you are counting your pennies, you might consider using reclaimed or recycled polyiso sold by an outfit like Insulation Depot.)

    Once you install insulation above your roof sheathing, your roof sheathing will be warm and dry. It will basically be at indoor conditions. Even in winter, it will never get cold enough to allow moisture accumulation or condensation. So your moisture worries are groundless.

  4. randynavarro | | #4

    Excellent, Martin. Thank you.

    Prices for EPS and iso are about the same here on the west coast. I've contacted Insulation Depot and they don't have much presence here on the west coast except for a project in Portland, but they may be able to help out.

    Thank you!

  5. Expert Member
    Dana Dorsett | | #5

    Commercial roofing contractors will often have reclaimed roofing foam stock on hand available at a discount. It may be worth calling around. Insulation Depot will ship in quantity almost anywhere, but it's not free, and if it has to move 500+ miles it adds up.

    On a price/performance basis (not per board-foot) fiber-faced roofing polyiso is usually about 9-11 cents/R-foot, to XPS at 12-14 cents/R-foot.

    If you've been looking only at box-store pricing on foil-faced polyiso, try getting quotes from building goods distributors who cater to commercial contractors for some 3" roofing iso (a double-layer of which would hit ~R38). In my neighborhood 3" roofing iso runs ~$55-58 for a 4x8 sheet, f.o.b. the distributor's yard, and is rated R18-R19. That's about a buck-eighty a foot for R18-min at the high end. If you can't figure out who is carrying the stuff in Portland, call some of these folks:

  6. heidner | | #6

    Randy, I'm in the Seattle area also... you probably should have a structural engineer take a quick look also. Many of the Seattle area houses with roofs you describe -- built in the early 60's are marginal for wet snow loads and do not have seismic connections. If you are going to peal the old roof off to the roof deck - now is the time to fix anything that might need it.

    Martin, if he develops a roof leak with polyiso - it would absorb water correct? If so with all the rain we get - he'd really want something that doesn't gain weight.

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