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Another flat roof question

FrankFulton | Posted in General Questions on

We have several hundred sq. ft. of unvented flat EPDM roof above a single story addition. The rafters are 2×8. We recently cut access and found 2-3″ of very old fiberglass batts in the rafter bays – a total roof insulation of R11. When the roof needs to be replaced in >10 years, we will install external foam sheathing.

In the interim, what is the best way to increase R value without creating a moisture problem? And if every approach has risks, what are they?

Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "What is the best way to increase R-value without creating a moisture problem?"

    A. If you are working from the interior, the answer is simple: Install closed-cell spray foam on the underside of the roof sheathing. For more information, see this article: Insulating Low-Slope Residential Roofs.

    Q. "If every approach has risks, what are they?"

    A. The risk in this case -- assuming you get the details wrong -- is moisture accumulation in the sheathing, followed by rot.

  2. FrankFulton | | #2

    Thanks Martin, and for the link - I've reread the very helpful article. I'm clear that the best options would be continuous exterior foam or closed cell foam from underneath.

    Nonetheless: Is there a safe option to increase R value that doesn't involve brining down the ceiling? Could I blow loose fill cellulose (and settling would leave a gap beneath the roof deck) or blow open cell foam into the flat rafter bays through holes in the ceiling?

  3. GBA Editor
    Martin Holladay | | #3

    Every year that your roof sheathing faces the moist circumstances you describe (cellulose without exterior insulation) puts the sheathing at risk. The longer you wait before you install the exterior rigid foam (which solves the problem), the greater the risk to your sheathing.

  4. Expert Member
    Dana Dorsett | | #4

    If applied when the roof deck is dry, 2" of closed cell foam (R12-R13) is sufficient to protect the roof deck. That would leave 5.25" for cellulose (R19-ish). At 2" closed cell foam is a Class-II vapor retarder, which is vapor tight enough top protect the roof. See:

    With 2" of foam an 5.25" of cellulose it would be a ratio of 38-40% foam/fiber, which is plenty of dew point margin at the foam/fiber boundary in zone 4 or lower without interior side vapor retarders(other than latex ceiling paint), and comfortably close that it probably works for most zone 5 locations too (simulate with WUFI, if you're nervous- the cellulose helps.) With a smart vapor retarder such as 2-mil nylon or a class-II vapor retarder such vapor barrier latex primer on the ceiling gypsum it would probably be fine in zone 6 as well. (Again, use WUFI carefully, if unsure.)

  5. FrankFulton | | #5

    Thank you Martin and Dana.

    The flat roof is about 600sqft, so there is a lot of attic surface area with R11. But given the substantial cost and mess to pull the ceilings and insulate from below to achieve R30, it might be a better long-term investment to apply 4" of external foam and replace the otherwise fine 10yo EPDM roof. Or perhaps we do both approaches, interior now and exterior in 10 years.

    I've read Martin's article and several of the other resources linked and have a few questions:

    How much of a difference will increasing the insulation make in comfort and heating expenses? (I will have this modeled but am seeking a rule of thumb for a situation such as ours.)
    Can you speculate on the relative costs of insulating from the interior (3 rooms' ceilings) or exterior?
    Would we need to remove the current EPDM, or simply apply foam then new roof deck above it?
    Would the new roof support the weight of a roof deck, if we ever wanted to add one?

    Thank you.

  6. jberks | | #6

    I don't think there will be any waste way out of it. Both will suck.

    I think your best bet for cost and less hassle and insulation performance is to insulate on top vs new drywall from the underside. I think that's less of a hassle between the two, but ultimately it comes down what roofers would charge you vs drywallers.

    For going top down, you can leave the epdm in place. Fasten polyiso over top of it and reroof. This also means new parapets and new flashing and eaves troughs. But you get a new roof.

    From bottom up, you can simply take down all your ceiling drywall, do the insulation work and redrywall and paint.

  7. FrankFulton | | #7

    Thanks Jamie. Your assessment is the same as mine. I'll get bids but have no idea the costs. I think we have 7 or more squares of roof, and there will be lots of details - moving gutters and a skylight etc.

    How much of a difference will increasing the insulation make in comfort and heating expenses?

    If we add exterior foam, would the new roof support the weight of a roof deck?

  8. GBA Editor
    Martin Holladay | | #8

    Q. "How much of a difference will increasing the insulation make in comfort and heating expenses? (I will have this modeled but am seeking a rule of thumb for a situation such as ours.)"

    A. You haven't even told us where you live. In Fairbanks, Alaska, insufficient insulation is a huge problem. In San Diego, not so much.

    Q. "Can you speculate on the relative costs of insulating from the interior (3 rooms' ceilings) or exterior?"

    A. All construction prices are intensely local. Call up some contractors and get some bids.

    Q. "Would we need to remove the current EPDM, or simply apply foam then new roof deck above it?"

    A. You could add rigid foam above the existing EPDM. As long as the rigid foam meets the minimum R-values listed in the GBA articles you're read, you won't have moisture problems if the EPDM stays where it is.

    Q. "Would the new roof support the weight of a roof deck, if we ever wanted to add one?"

    A. "Roof deck" has many meanings. It can mean "roof sheathing" or "patio." If you are thinking of installing a patio above the roofing, you'll need to consult an engineer.

  9. FrankFulton | | #9


    We are in Maryland and have more heating days than cooling days in CZ4.

    Since my last post re: our flat roof, we've decided that insulating from the exterior is not an option right now. Instead, we are considering air sealing the perimeter of the flat roof. This will be necessary no matter what we eventually do with the roof and insulation.

    We met with an installer today and discussed options. The flat roof is not vented - so the rafter bays are not literally wide open. But, there are many seams and other sources of air infiltration. With an IR camera, there are clearly leaks. The perimeter totals about 100 linear feet.

    How would you solve this problem? How much would you expect this work to this tighten the home?

    We discussed using a "feedbag" approach and dense packing the perimeter of the rafter bays along the entire outside edge of the flat roof area. We would aim to target the wall top plate as well as the other seams there. Because there is older aluminum siding that would require replacement, we discussed working from the interior. But, we are open to suggestions. This would be one step beyond low hanging fruit, and we want a good result.

    Thank you.

  10. GBA Editor
    Martin Holladay | | #10

    In general, there is no downside to air sealing work. Any efforts you make that aim to limit air infiltration are good.

    That said -- and I've mentioned this before -- your poorly insulated flat roof is at risk of moisture accumulation, because it isn't insulated in any recognized manner. If you decide not to insulate this roof according to one of the recognized methods in my article (Insulating Low-Slope Residential Roofs), then you are deciding to accept the risk. It's hard to say how the roof will perform.

    So you can perform air sealing work and still end up with damp roof sheathing.

  11. FrankFulton | | #11

    Martin, Thank you. Point taken. I'll go ahead and get the formal quotes for insulating the roof, then decide. The roof has been this way for ten or more years - that's why I've figured "it must be OK."

  12. FrankFulton | | #12


    When exploring options with contractors, two have suggested open cell foam, rather than closed cell foam, on the bottom of the roof deck. Their rationale is that this will make leak detection easier, if the flat roof ever leaks.

    From reading your article and the comments, it seems open cell is not advisable - why is this the case?

  13. user-2310254 | | #13


    Martin published an article in 2014 on risks associated with installing open cell foam against roof sheathing. Your contractors may not be aware of this potential problem.

  14. FrankFulton | | #14

    That's very helpful.
    Thank you.

  15. FrankFulton | | #15

    OK, to refresh, this thread is about our flat roof EPDM, currently poorly insulated. We are considering doing nothing, insulating from the inside (remove ceiling), or insulating from the outside (exterior foam).

    1. How should I inspect roof deck for moisture from the inside, ie, (above the kitchen plaster/drywall)? Current setup in 2x8 bays is plaster/drywall, 4-5" fiberglass batts, air, underside of plywood roof deck.

    2. What are risks of insulating underside dry roof deck w/SPF and cellulse, as Dana suggests above? eg, what happens if there is a water leak in the future, or if the plywood somehow is damaged?

    3. Why is the "foam sandwich" ill-advised? ie, If we insulated from underside now and got to R30, why not add more exterior insulation in 10y when EPDM needs to be replaced?

    Thank you.

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