Spray polyurethane foam on flat roof
FrankFulton
| Posted in General Questions on
What are the pros/cons of SPF on top side of a flat roof, relative to exterior rigid foam insulation? We have a large (600sqft) flat roof in CZ4, and need to 1) increase R value, and 2) increase slope of the overly flat roof that is beginning to mildly pond. Of course, we’d like things to look aesthetically pleasing when complete. Thanks.
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Replies
Frank,
For a full discussion of the use of spray polyurethane foam as roofing, see this article: "Roofing with Foam."
Frank,
What the plan for increasing the roof pitch and how much space with there be for the spray foam insulation?
Martin great article, thank you. Do you have any first or second hand experience?
Brian, the roof has several dormers and a fairly complex shape (12ft in some places, 18ft in others). One potential appeal of SPF is no carpentry - the slope could be made w the foam.
To increase R and add slope, our options seem to be:
1) tear down ceilings and foam underside of roof deck, which would not address roof slope;
2) external foam, which would require carpentry; or
3) SPF, which we’ve just learned about - which would be least attractive but perhaps “easiest” to execute and most effective solution. We would still need to address air sealing from beneath - perhaps by dense packing the rafter bays?
4) Because of a breezeway, we could also do one large section from the inside, and another large section from the outside, but this seems aesthetically least appealing and perhaps overly complex.
As a quick follow-up:
Could we also "dense pack" cellulose in the 2x8 rafters, to air seal and add R value? Then, we could sum that interior R value (7.25in * 3.5 = 25) with the R value provided by the SPF roof (6.5/inch - say either 6>3 inch taper, or 4>1 inch taper)?
Thanks.