Spraying Polyurethane Foam Over an Existing Roof
Spray foam isn’t just used for insulation — it can also be used for roofing
When Taya and Stephen Shoup's old tar-and-gravel roof began leaking, the couple hoped to find a replacement roofing that would be energy-conserving, leakproof, cost-competitive, and reasonably green. As the house's roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. doubled as a finished ceiling, the old insulation was scant. They broiled in the summer and hemorrhaged money during the heating season.
The Shoups live in a 1958 Eichler-designed house in Emeryville, California. Taya is a landscape architect and Stephen is the principal of BuildingLab, a green design/build firm whose reincarnated shipping containers were chronicled in a GBAGreenBuildingAdvisor.com article titled “Creating an Urban Oasis.”
Ultimately, the Shoups chose to install a closed-cell sprayed polyurethane foam (SPF) roof — sprayed directly over the old tar-and-gravel roof — because:
Insulating inside was out. The exposed underside of the 2x6 T&G roof deck was classic. Besides, the ceilings were just a shade above 8 ft. and the couple didn't want to lose 4 in. to 6 in. of headroom to insulation.
Closed-cell SPF has an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of 6.5 per inch, making it the perfect antidote to the old roof's dismal energy profile. The old roof assembly, with two layers of 1/2-in. fiberboard, achieved (maybe) R-3 or R-4. Four inches of SPF would bring it up to at least R-26; six inches would bring it to R-39. Spray foam was also particularly well suited to this roof because it had parapets, which defined the edges of foam. (Exterior spray foam only works on flat or low-pitch roofs; for pitches greater than 4/12, rigid foam panels are the over-roof-deck choice.)
A spray-foam roof could be applied directly to the old roofing once it was prepped and cleaned up a bit. Not stripping the old roof and sending the debris to the landfill was also a big green plus. Besides, spraying-over would save the labor costs of stripping the old roof.
Four inches of closed-cell polyurethane is a Class II water retarder, a semi-impermeable membrane that makes leaks all but impossible. As the foam expands and adheres aggressively, it would seal the many pipes and conduits running across the roof of a house with no cavity under the sheathing in which to hide pipes.
Compared to the obvious alternatives, an SPF roof was actually less expensive than installing and then insulating either a new torch-down roof or a PVC membrane. And that didn't even include the cost of stripping the old roof. There were energy-efficiency rebates available for installing an SPF roof but the couple was so busy that, as Stephen put it, "We couldn't afford the time to save money."
Is spray foam roofing green?
So was this spray polyurethane foam a green choice? Yes and no, but mostly yes. Since the early days of sprayed foam insulation (the earliest foams were urea-formaldehydeChemical found in many building products; most binders used for manufactured wood products are formaldehyde compounds. Reclassified by the United Nations International Agency for Research on Cancer (IARC) in 2004 as a “known human carcinogen." based), the insulation industry has cleaned up its act a lot. Although the two chemical components of SPF are largely petroleum-based, once the polyurethane mixture foams and cures, it is chemically inert and doesn't off-gas. (Manufacturing PVC, in comparison, involves some particularly toxic chemicals.)
Not sending old roofing to the dump is a plus, as is the excellent R-value of SPF foam. (Sometimes it takes oil to save oil.) An SPF roof can also be quite durable when the polyurethane is sprayed with a special acrylic top coating that protects the foam against UV degradation. Lastly, the foam's light color will reflect sunlight so, all in all, it should be a relative cool roof.
This posting was excerpted from Renovation 4th Edition, newly published by Taunton Press. The book contains the collective wisdom of hundreds of master builders and tradespeople whom I began interviewing when I helped launch Fine Homebuilding. The book's 614 pages contain more than 250 technical illustrations, roughly 1,000 photos (of the 40,000 I've taken over the years) and thousands of field-tested tips and techniques.
- Mike Litchfield, Renovation 4th Edition
- Taunton Press
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