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Community and Q&A

Spray foam & sealing off soffits from attic

Bdgray | Posted in Energy Efficiency and Durability on

I had a spray foam contractor to my house last week to discuss spray foaming the underside of our roof deck. We did everything in renovating our home right except for the ~30 can lights we had put in. I don’t notice heat loss as much but the heat gain in the summer is obvious. I’ve given up trying to insulate and air seal so I want to move the envelope up. I’ve researched this a bit and know that the recommended approach to seal off the soffit from the attic is to insert a square of rigid foam and spray foam over it. That will be a pain in our house because our roof has a shallow pitch. Just getting to the soffits requires some dexterity. Doing this several hundred times at each opening would be very labor intensive.

The contractor said they solve this problem by shoving a piece of batt insulation into this crevice and then spray foam over it. The batt insulation works as a backstop for the sprayfoam. I wasn’t sold on this approach as it seems like a shoddy short cut. At the same time, it didn’t seem crazy and certainly sounds easier. Any thoughts?

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  1. charlie_sullivan | | #1

    Sorry you found out how bad can lights are a little too late. It is possible to patch up those leaks but it sounds like you've already decided you don't want to do that.

    You want to be sure the spray foam covers and seals the top plate of the exterior wall below the soffit. If they are stuffing the insulation past that point, into the soffit, it sounds OK. If they are putting it over the top plate, so that the spray foam doesn't get out that far, you can leave air leaks between your walls and the soffit, which might cause worse problems than those leaks have produced previously.

    Note that standard closed-cell spray foam has blowing agents that are far worse global warming agents than CO2. But Lapolla is now offering a "4G product that avoids this problem. If you care about climate impact, that's the way to go.

  2. user-2310254 | | #2

    Brian. It sounds like you are planning to use a flash and batt strategy. You need to install a specific thickness of closed cell foam to avoid moisture issues. The balance of the insulation can be air permeable, but it has to be installed against the foam. Depending where you live, you need R-38 or R-49 in total (at least ideally).

  3. GBA Editor
    Martin Holladay | | #3

    I agree with Charlie's analysis and advice.

    I disagree with Steve. It doesn't sound like you are adopting the flash-and-batt approach; the bunches of fiberglass are simply being used as a substrate for the spray foam.

    The basic problem with the type of tight spaces you describe -- the tight spaces at the perimeter of an attic under a roof with a shallow pitch -- is that there isn't enough room for the R-value required by code. That's why the best solution in cases like yours is to install one or more layers of rigid foam above the roof sheathing, followed by another layer of roof sheathing and new roofing. That's an expensive approach, of course, but it's the best approach from a building science standpoint.

  4. user-2310254 | | #4

    Brian. I agree with Martin that placing the insulation on the exterior would be best and a flash and batt approach would be challenging if you have a cramped attic space.

    If exterior foam might be an option, consider using reclaimed material. It is much more affordable and environmentally friendly.

  5. GBA Editor
    Martin Holladay | | #5

    This question has nothing to do with flash and batt!

  6. user-2310254 | | #6

    Martin. I was responding to this sentence. "I've given up trying to insulate and air seal so I want to move the envelope up." Admittedly, that's a separate issue from sealing at the soffits, and maybe I was reading too much into Brian's description.

  7. Bdgray | | #7

    Thank you Steve, Martin, and Charlie. To Martin, I've thought a lot about insulating over the roof deck (mostly because I'd love an excuse to have a standing seam metal roof). I just don't think I could justify the cost. In addition to the insulation & new roof, I'd need to replace the fascia and gutters.

    As a poor man's and more environmentally friendly alternative to spray foam, what if I just buy used sheets of polyiso and secure it to the bottom of the rafters? I'm not talking cut and cobble. These would go up whole: two layers of 4" polyiso that get installed and taped? 8" of polyiso would be >= R48. The rafters would stay vented and I could add a ridge vent along the length of the roof so each rafter bay had proper circulation. It would be tight but getting the sheets into the attic is possible. The big concern I have is how to seal all the seems? It seems it would be impossible to verify. There would be thousands of linear feet of seems that all might have little air leaks. And of course - I still have to insulate and air seal where the rafters meet the top plate (which is going to be a pain no matter what I do).

    BTW, Martin - to your point on properly insulating the perimeter. The pitch is shallow but the rafters & soffit extend 2' beyond the exterior walls (kind of a FLWright style). Hence, even going underneath I think I can get a full 8" of foam all the way to the wall's top plate.


  8. user-2310254 | | #8

    Are you planning to cover all your can lights?

    In any case, polyiso experiences thermal drift, and you probably can't get to r-48 with 8 inches in any climate zone.

    But if you have the headroom, you can simply go with a thicker layer or layers of well sealed foam. Or maybe you are close enough from a prescriptive standpoint. Will be curious to see what Dana suggests.

  9. Bdgray | | #9

    Steve, I'm not a GBA prime member (just basic GBA) and cannot read the file. I think I know the issue though - I've read that polyisio loses some of its R-value over time. I'm in a Chicago suburb that requires R-39. At 8 inches, I think I'd be fine as long as the polyiso still has R5/inch or more.

    On can lights, I've already done what I can. I crawled through the attic for days adding those fibrous top hats to all the cans. I spray-foamed where the conduit punches through the framing and I also rolled out an extra R30 over everything. I haven't mentioned this but part of my motivation is we lack central AC (heat is hydronic on a 95% ef boiler). My wife likes minisplits but she has vetoed the indoor wall units (on aesthetics). So beyond addressing the dozens of holes in our ceiling, I'm also looking for some conditioned space to install ducting. AC probably isn't a 2017 project but moving the envelope up to the roof deck at least puts me in a position to solve that problem.


  10. user-2310254 | | #10

    Brian. The article suggests r-4.5 per inch for your climate zone. So you want to create a conditioned attic and then add AC down the road. Is that correct?

    Consider buying a month of access. It will more than pay for itself by helping you to save money and avoid mistakes.

  11. greenleopard | | #11

    What about replacing the normal cans with these?

    They have foam rings you can buy and are air tight, then you can insulate normally above them and be done with it.

  12. GBA Editor
    Martin Holladay | | #12

    Q. "What if I just buy used sheets of polyiso and secure it to the bottom of the rafters? I'm not talking cut and cobble. These would go up whole: two layers of 4-inch polyiso that get installed and taped?"

    A. Yes, that approach can work. Remember that rigid foam needs to be protected on the interior with 1/2-inch drywall for fire safety.

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