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Q&A Spotlight

How to Insulate a Low-Slope Roof

The owner of a 1920s Baltimore row-house looks for the best insulation strategy

The new roofing will be EPDM. But what about insulation?
Image Credit: Fine Homebuilding

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?

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 GBA senior editor Martin Holladay. The key is making sure the R-value 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.


  1. Amanda Evans | | #1

    Thermal bypass
    But if you insulate on top of the existing roof, how are you going to deal with the thermal bypass that will occur around the walls in the attic cavity where the existing batt insulation is poorly installed?

  2. Joe Schmo | | #2

    Baltimore City Codes
    FYI, Baltimore City often lags in code adoption so they may be using an older code than what the state of MD has adopted.

  3. Keith Gustafson | | #3

    re: Amanda
    I would think insulating the walls/edges is much less complex than the whole ceiling

  4. Amanda Evans | | #4

    I think either way is
    I think either way is difificult - walls or ceiling - when you can't access easily from the inside and you don't know what it in the cavity.

  5. Joe Schmo | | #5

    Also be careful insulating in existing cavities
    Especially in older, existing building, insulating in wall and ceiling cavities can be a bit dangerous considering the state of some older wiring and connections. There is some talk brewing about on whether fires have occurred due to insulation around wires and insulation. If a wire is over insulated or the insulator is degraded, an arc can occur along the surface of the wire insulation, causing a fire. Note the difference between wall insulation and wire insulation in this context. Basically, if you spray or stuff insulation into a wall cavity in an existing building, pay close attention to the type and condition of the wiring that may be in the cavity. The insulation itself can burn or melt in some cases or can over insulate a wire or connection, causing a fire, which causes the structure itself to burn. Just an FYI.

  6. Amanda Evans | | #6

    Agreed - that's why it is so
    Agreed - that's why it is so difficult to do and why the thermal bypass ends up being an issue if you just insulate above the roof.

  7. Michael Blasnik | | #7

    Amanda is right
    If you want to add insulation on top of the roof then you must also deal with the entire perimeter of the attic/roof cavity -- which is quite challenging unless you rip open / trench the roof from above or else cut an access and send in people to crawl and seal. if you are going in to crawl and seal in the roof cavity to get the perimeter, it would actually be easier to just seal the bypasses and do some dense pack in the small section in the rear that is too small to crawl. You can then blow cellulose in the rest of the cavity. This approach can be done for about $1.50/sqft -- far less than any of the foam approaches. it also leaves you with a solid roof for adding a deck.

    You should perform voltage drop tests on all of the wiring that runs through the attic before and after the work and fix any problems found or created.

  8. Kim Shanahan | | #8

    Amanda ususually is
    If the joist space has been ventilated, as would have been proper and expected, then adding insulation above a ventilated space will provide no insulation value. Unscrupulous and/or ignorant spray foam roofing contractors in our market, of which there are many, will often tear off old built-up tar and gravel roofs and replace them with sprayed on polyiso systems. They make good roofs and provide good insulation, but not when the space below the polyiso is ventilated. These contractors make false claims to the homeowners on improved efficiency and lower energy bills. Incidentally, it is common for gravel to be placed on the foam roofs to protect them from UV degredation rather than elastomeric paint, which is only used around pipes, penetrations, flashings, etc, that the gravel does not cover. The skin of the polyiso is considered durable enough in and of itself. Most reputable roofing contractors offer 10 year warranties.

  9. Todd Oskin | | #9

    Gaco silicone liquid applied roof?
    I know this is a very old thread, but in case someone else checks it out...

    How about this :

    It's a silicone based liquid applied roofing 'coating.'

    You can DIY pretty easily..

    I've done it on a couple very small 'low-slope roofs'.. and so far so good (I have no long term experience with these roofs however).

    In terms of insulation, I've also done a lot of blown-in cellulose installations in pitches roofs, and low-slope roofs... in really old buildings (usually are 80-100 years old).

    I blew in about 10" of cellulose in my own cathedral ceiling...and has been doing well for at least the past 2 years.... I'm thinking about using Gaco coating for roof too... as I will need a new roof soon...

    Anyone else have experience with Gaco coating?


  10. Scott Raney | | #10

    Don't forget the coverboard!
    I note that "coverboard" (such as DensDeck) is not mentioned in this post. Putting coverboard between the foam and the EPDM (or TPO) membrane is recommended in the manufacturer's installation instructions, may be required by fire code, and is the only smart thing to do if there is any chance that the roof will be walked on (i.e. if there ever will be a roof deck or drains that need to be cleaned) because it will keep the foam from breaking down at the edges of the panels. It's just a bad idea to put a membrane that should last 40 years down on a substrate that will start failing decades earlier.

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