I'm trying to find out if I can spray-foam fill a gap in my ceiling. I have an almost-flat rubber roof, 1" foam, wood planks, then a gap created by 2x6s, then the sheetrock ceiling. I'm in PA, climate zone 5/6. There are soffits at two edges of the almost-flat roof and no ridge vents.

I'd like to pull off the sheetrock ceiling, add 6 inch of closed-cell spray foam (filling in the 2x6 area), and then put sheetrock back.

Concerns: No air gap, just rubber, foam board, wood, then sprayfoam. Would this introduce a problem? Specifically, is there a possibility for moisture problems with the wood in this setup?

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  • 6" of closed-cell spray foam is a lot. From what I understand 3-4" is more typical for getting a high R value. Though the rating is ~R6-7 per inch, the pro spray foam guys I've talked to all seem to think it's effectively a much higher R value presumably because of zero air and vapour movement. I've also been told that beyond 4", it really doesn't add much (other than cost and install time -- apparently you can only spray 1-2" before letting it cure for an hour or so, then putting the next coat on).
    – gregmac
    Commented Nov 12, 2014 at 14:41
  • But even 4*R7 = R28 (only). I'd like R50-R60...
    – mankoff
    Commented Nov 12, 2014 at 15:16
  • Yes foam has higher up-front costs due to materials and labor, but I don't think it is competitive over the long run to have R4/in (R20 total since I need an air gap) v. R6.5/in. (R39, no air gap needed when foamed).
    – mankoff
    Commented Nov 13, 2014 at 14:02

3 Answers 3


Ignore people who tell you that you don't need as much spray foam because it is better air sealing or whatnot. That just comes from lazy installers who don't like working with the material and want to prevent sticker shock. R-value is R-value. 4" will be twice as insulating as 2".

But to get to the heart of the matter, there's one and only one reason why you even need spray foam there instead of batt insulation: to create an air barrier to prevent any moisture-laden air that gets past the drywall from hitting the cold roof sheathing, condensing, and starting to rot the wood, which as you can imagine is very bad.

You don't need a full 6" of spray foam up there. You only need enough to fully encapsulate the underside of the roof sheathing to prevent interior air from hitting it. So 2" of spray foam would be fine, and then you could fill the remainder of the cavity with lower-cost open cell spray foam, or typical batt insulation, or blown-in cellulose, or anything really; it's all safe because you've created that crucial air barrier.

Alternatively, you could avoid this whole rigamarole by increasing the thickness of the foam insulation above the roof deck, thereby keeping it warmer and above the dew point, which would make any moist air that gets up there not condense in the first place. You'd want to increase it by at least 5". If you're in a predominately cold climate, don't use polyiso, which has worse performance in the cold--stick to XPS or EPS. In this case, you could stick whatever you wanted in those cavities. I would recommend blown-in dense-packed cellulose, because you wouldn't even need to take off the drywall.

If you want R-50 or 60 in your roof, you don't have a lot of great options without significant modifications. Entirely filling the bays with spray foam will only reach R-42, plus R5 or so from the foam above the sheathing. It requires tearing off and replacing the ceiling drywall and only gets you to R-47. Don't forget that thermal bridging of those 2x6s reduces the effective R-value. To go farther and correct some of these issues, you could additionally put more foam boards between the underside of the rafters and the new drywall, which could boost you up to R-57 with 2" foam boards. You'd only lose two inches of ceiling height.

Packing more foam above the roof deck will let you reach whatever R-value you want and fill the bays with whatever you want. It's the cheaper preferred approach, but requires removing the rubber roofing membrane and may undesirably increase the height of the roof above parapets or whatnot. If the roof is in bad shape and needs replacement soon anyway, that would be a perfect opportunity to substantially increase the foam thickness underneath the new roof.

This is the approach recommended by the Building Science Corporation:


So, what should we do with these flat roofs and cathedral ceilings that can’t be vented and insulated in the “typical” way? Simple, follow the building code. Add insulation on the top of the deck to elevate the roof deck temperature above the dew point of the interior air vapor mix (Figure 1) before dense packing or take the interior ceiling down and install an air impermeable insulation. Spray foam on the underside of the roof deck (Figure 2). The thermal resistance of the insulation needed to be added above the roof deck or in the form of spray foam on the underside of the roof deck is dependent on climate and the interior moisture load.

Good luck!

  • Thanks for detailed reply. I'm trying to maximize R while minimizing cost w/ the low cost introducing problems (aren't we all? That is a generic statement). So... Just filling with foam gets better R for not too much cost than the foam boards or blown cellulose (not as much R) or removing the roof (too much $), etc. But I'm still concerned about 1 thing you wrote: "Crucial air barrier" created by 2 inch of foam, but the horizontal wood boards are above/inside that. Yes they are air-sealed, but could the moisture still form inside those boards and eat them away?
    – mankoff
    Commented Nov 12, 2014 at 16:53
  • You can leave the rubber roof membrane in place, insulate over it, and add another rubber roof membrane on top. Saves work, might save trouble in the event of a leak on the new roof membrane.
    – Ecnerwal
    Commented Nov 12, 2014 at 16:57
  • @Ecnerwal The roof is good, and a new roof is probably waaay more expensive than foam-filling that air gap. When I need a new roof in a decade, yes I'll leave the old one and just add on top. I think rubber makes that easy to do.
    – mankoff
    Commented Nov 12, 2014 at 17:00
  • @mamkoff The joists should be fine; they won't get below the dew point because wood itself is a decent insulator, and 6 inches of it should be enough to keep it above the dew point, especially with an inch of foam above the roof deck. It's the wood of the roof deck itself that you need to protect. Best bang for buck is probably going to be 2" closed cell spray foam on top and 4" mineral wool batts filling the rest of the cavities (combined R-30). Way cheaper than 6" of spray foam, which will be better (R-42), but you'll sure pay for it.
    – iLikeDirt
    Commented Nov 12, 2014 at 17:39

The spray foam will help with your insulation problem for sure. It will potentially introduce three issues though;

  1. A lot of spray foams expand as they dry out. If this happens, it will push your ceiling down and either break it or push it right off the rafters!
  2. This will potentially remove any room for new wiring or anything else, so if your wires ever go bad, you will have a heck of a time replacing them
  3. Make sure that any foam you use is non-flamable and meets flame retardant requirements. Need I say more?
  • Would the wood boards be subject to moisture problems with this configuration?
    – mankoff
    Commented Nov 12, 2014 at 15:16
  • 1) I'd take the ceiling down, spray-foam, then re-attach. 2) No wiring now. I'd add some below the spray foam before re-attaching the ceiling. 3) Yes of course.
    – mankoff
    Commented Nov 12, 2014 at 15:18
  • No foam is non-flammable. They're all petroleum products. Non-flammable insulation would be mineral wool or unfaced fiberglass; basically anything made out of rocks rather than gasoline.
    – iLikeDirt
    Commented Nov 12, 2014 at 15:34
  • @mankoff (Re comment 1): Assuming your roof is water tight (If not, you would have water leaking into your ceiling), this shouldn't be an issue. I have never known expanding foam to "damage" wood. 1) Do you have a way of making sure the foam keeps a neat shape to fit into the ceiling? I tried to fill part of a huge hole in a outdoor building wall, but the foam went everywhere. Commented Nov 12, 2014 at 15:47

The issue here comes down to moisture management.

In general, you never want two vapor barriers with a space between. Your design leaves the 2x6 rafters as the only component that can transport moisture that condenses below the foam board.

Now this may all work out... search for "unvented roof assemblies" for both success and horror stories.

But if you just want something that will just work without hassle, blown in insulation. It will "breathe" with the seasons attaining a humidity similar to the average of the room below. And it will do a perfectly good job of changing the dew point (the boundary between warm room air and the freezing roof) to somewhere below the roof level. Blown in fiberglass lets air circulate more freely than blown in cellulose.

For more a place where moisture management geeks hang out, head over to http://www.greenbuildingadvisor.com/ . For some technical reading check out http://www.tlpca.org/images/articles_condensation.pdf

  • I don't believe this is a good plan. Blown in insulation will permit humid interior air to condense on the underside of the roof sheathing, as that insulation will make it even colder but not block air movement. The risk of condensation in this roof assembly will be from inside, not outside. The foam board and rubber roof above it are sufficient air barriers to prevent that.
    – iLikeDirt
    Commented Nov 13, 2014 at 14:28
  • Joe L from BSC warns against what you are suggesting at buildingscience.com/documents/insights/bsi-043-dont-be-dense The suggestion proffered there is to add rigid foam above the roof desk and spray foam the underside.
    – iLikeDirt
    Commented Nov 13, 2014 at 14:30

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