How to Insulate a Low-Slope Roof
The owner of a 1920s Baltimore row-house looks for the best insulation strategy
Eric Dymond lives in a 1920s Baltimore row house that needs a new roof. He plans on replacing the low-slope, built-up roof with an EPDM membrane, and the question is how to insulate it correctly.
Currently, the built-up asphalt roof is installed over Homasote (or something similar) and a roof deck made of wood planks. Although there’s some “sparsely distributed” insulation in the space between the ceilings and the roof deck, it won’t meet current recommendations for Dymond’s Climate Zone 4 house.
A roofing contractor has recommended that the EPDM membrane be installed over 2 in. of rigid polyisocyanurate insulation. “Does EPDM over foam board make sense?” Dymond asks in this Q&A post. “It doesn’t make sense to me to have 7 in. of foam board to meet R-38, and I’d be concerned about walking on the roof with that setup.”
In addition, Dymond is concerned that 2 in. of insulation is woefully inadequate. Would it make more sense, he asks, to eliminate the foam board recommended by the roofer and insulate beneath the roof deck with fiberglass batts or open-cell foam?
GREEN PRODUCT GUIDE
Dymond’s request for feedback and suggestions on other options is the topic for this week’s Q&A Spotlight.
EPDM over foam will work
Despite Dymond’s trepidation, a roof consisting of an EPDM membrane over a thick layer of polyiso foam is common, Jesse Thompson replies.
He adds a link for Firestone roofing products, which include a number of the synthetic rubber products made from ethylene propylene diene mononer (EPDM). The company’s web site includes a number of drawings showing how the roofing should be installed.
Insulation can be installed in the attic or on top of the roof deck, says GBAGreenBuildingAdvisor.com senior editor Martin Holladay. The key is making sure the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the insulation meets minimum code requirements. (Maryland has adopted the 2009 International Energy Conservation Code, whose requirements for insulation can be found here).
Holladay adds that it's important for Dymond to find a way to deal with energy losses through the attic's masonry walls.
But the added complexity is a drawback
To Dymond, the idea of adding 7 in. of insulation above the roof deck is “probably a non-starter” because it makes adding gutters and other details more complicated. For that reason, he leans toward blowing in cellulose above the interior ceiling.
“I have 5 in. of rigid foam under EPDM and it is easy,” says Keith Gustafson. He calls it “by far the simplest solution” to Dymond’s insulation problem.
“They make tapered foam for just the kind of edge issues you are speaking of,” Gustafson adds. “Worst case, do you think redoing gutters and drip edge is going to be more complex than trying to insulate above the ceiling? I don't.
“A new roof needs all new flashing, drip-edge, etc.” he writes. “Look at it this way: you are either getting a cheap roof to go with your insulation or cheap insulation to go with your roof... Make sure they are doing a ‘fully adhered’ installation, and that they stagger the foam seams in both directions.”
(Tapered polyiso insulation is available from ModulR TS .)
“Well, there’s ‘cheap’ as in inexpensive and then there’s cheap as in shoddy,” Dymond replies. “One doesn’t necessarily mean the other. The very big benefit to batts or blown-in cellulose is that I can DIY it, which I’m less comfortable with doing for an EPDM roof.”
Another option is spray foam
Yet another possibility is spray polyurethane roofing, an option that Kevin Dickson especially likes for retrofits.
Dickson writes that Dymond probably will be able to avoid tearing off the old roof. In addition, he says, roof penetrations are “flashed for free;” polyiso sheets can be added as needed under the spray foam top layer; the roof is white, not black, which will reduce cooling loads; and the job should cost about half as much as any of the other options Dymond is considering.
The caveats are that the roof will need an inspection once a year (“true for ALL low-slope roofs, but mandatory for foam”), it will need recoating every five years, and it’s not the best approach in areas where there’s a threat of hail greater than 3/8 in. in diameter.
What about adding a rooftop deck?
One other potential complication: a rooftop deck. “Since rooftop decks are reasonably popular in our neighborhood, would using exterior foam (either board or spray) preclude putting up a deck that bears on the roof surface?” Dymond asks. “To be clear, I'm not actually installing a deck, but could conceivably do so in the future. Floating deck on sleepers is pretty popular around here. The ability to handle foot traffic without a deck would also be useful.
If you’re planning on adding a deck, Dickson says, the question is what’s going to hold it down? “Now you need an engineer,” he says.
But if that’s what Dymond is planning, the spray polyurethane roofing makes the most sense, because it is “the only easy way to flash in the short columns needed to support a deck.”
A deck is not a problem, Gustafson adds. “There are numerous solutions to putting a deck on an EPDM roof,” he writes. “An EPDM roof can handle some foot traffic, and they make walking pads for anticipated traffic.”
Our expert's opinion
We asked GBA advisor Mike Guertin for his take:
In general, we just remove the existing roof, apply layers of fiberglass faced polyiso (tapered for flat roofs or uniform thickness for sloped roofs) with long screws and screw-plates, then install new roofing on top of that. On commercial jobs there usually isn't insulation below in the roof framing but if cavities exist that make sense to insulate. They can be filled with spray foam or netted cellulose.
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