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Community and Q&A

Spray Foam & Mineral Wool in Wall

Mgrinde | Posted in General Questions on

Hello, we are building a house in Zone 6, southwest Wisconsin. Our plan for insulation is to do 2-3 inches of closed cell spray foam on attic floor and then blow in cellulose to get up to an R-value of 60-70. On the exterior we are using the Zip R-3 system to wrap the house. Our plan is to use closed cell spray foam on exterior walls and then fill the rest of the cavity with 3.5″ mineral wool batt. We have two questions:

1. Should we do 2.5″ of closed cell spray foam to fully fill the cavity but may have slight compression of the mineral wool depending on the spray foam contractor’s ability to get exactly 2.5″, or should we do 2″ of spray foam to leave some space?
2. Our building inspector wondered whether we would get any moisture from the cold side of the mineral wool touching the warm side of the closed cell foam? He said there is no evidence that he knows of with this yet, but in his mind he could see this as a potential issue but wasn’t sure. 

Thanks in advance to any advice!

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  1. Andrew_C | | #1

    In the attic, you don't need spray foam on the floor. Just do normal air sealing and then blow in cellulose to the desired depth/R-value. Aside: avoid canned lights in the ceiling of the top floor and this will be easier. There are a lot of articles/Q&A threads on canned light alternatives here on GBA.

  2. Expert Member
    BILL WICHERS | | #2

    In the attic, assuming this is a new build, you can do just as well air sealing the "old fashioned" way, detailing the ceiling drywall air tight, then skipping spray foam on the attic floor. This will be cheaper, and will perform just as well.

    Regarding the wall, a few concerns here:

    1- How thick is the wall? 2x6? 2x8? From your mention of your planned insulating materials, I'm kinda-sorta guessing 2x6? If that's the case, you ideally want AT LEAST R11.25 of exterior continuous insulation (the "R" part of the Zip-R you're planning to use). R3 is not enough. Using a fully adhered interior insulation layer of closed cell spray foam could deal with that issue, but you would be MUCH MUCH better off, both for performance and cost reasons, putting more exterior insulation up and skipping the spray foam layer in the wall. An assembly build this way would use Zip-R 12 on the exterior and 5.5" of mineral wool on the interior, which would give you a total R value of R35 -- pretty good! Also, pretty robust in terms of moisture resistance. You'd seal the Zip-R panel around the perimeter to make it air tight, and ideally would detail the interior drywall airtight too.

    If you have no ability to upgrade the exterior R value, which may be the case depending on where you are in your project, you're probably stuck with that interior layer of closed cell spray foam to deal with any potential moisture issues. 2" would get you up to R12, which should be sufficient when also allowing for the R3 on the exterior. That would leave you 3.5" of space in a 2x6 wall BUT spray foam never goes in that evenly, so you'll end up with issues getting the batts to fit properly. You could use fiberglass batts instead of mineral wool, since fiberglass batts are easier to compress, but I'd just have the spray foam contractor finish the wall off with open cell spray foam over the first layer of closed cell and call it done. This avoids any issues with batts not fitting well, and since the spray foam contractor will already be on site, cost shouldn't be too bad.

    You won't have any issue with condensation on the interior surface of the spray foam as long as you stick with the R value ratios mentioned. You won't have any issues if you go with thicker exterior rigid foam either. My own preference here would be to use either Zip-R12 on the exterior, or plywood+polyiso with at least 2" polyiso (R13), then mineral wool batts in the walls. For the attic, I'd detail the upper level ceiling air tight, eliminating the need for an air sealing layer of spray foam on the attic floor, then go with ~R60 of loose fill cellulose on the attic floor as you were planning.

    BTW, with very thick layers of attic insulation, you may want to consider beefing up the attic floor / upper level ceiling a bit to avoid future issues with drywall sagging.


  3. walta100 | | #3

    The way I see it spray foam industry has pulled of the greats feat in marketing history by making the most expensive riskiest least green product be seen as a highly desirable product.

    To my eye large amounts of spray foam are huge red flags in new construction plans as there is almost always a way to do the job without the spray foam that will give you a higher R value at a lower cost using greener materials with less risk of failure if you are willing to put a few moments of thought into planning.

    What is the point of spray foaming the attic floor? There are lower costs ways to air seal the ceiling.

    To my ear Zip+R3 seems like a silly option to select. Why only R3 zip go big and get all the foam you want or skip the +R Zip. I see no point in spray foaming a zip wall as you are not getting any air sealing value from the spray foam.

    Consider using regular Zip and adding the correct amount of sheet foam for your zone to the exterior of the wall.

    Have you read this article?


  4. Expert Member
    Akos | | #4

    "1. Should we do 2.5″ of closed cell spray foam"

    If the insulation is too bulgy the drywallers will be not be happy. I've had no issues compressing MW 0.5" but some might disagree. The problem is that it is pretty hard to get a consistent thickness of SPF, 2" would be easier for everybody and your budget.

    2."inspector wondered whether we would get any moisture from the cold side"

    You want the mineral wool batts always pushed right against the foam. Any gaps should be on the drywall side. I'm not sure what your inspector means as the warm side of the SPF and the cold side of the MW are exactly at the same temperature. The combination of ZipR3+SPF puts you well outside of condensation range in zone6 so the surface of the SPF will never be cold enough to create moisture issues.

    I think overall, your dollars would be better spent by skipping the SPF and bumping up the Zip to R6 or R9 and insulate with 2x6 batts. In zone 6 this would require a warm side vapor retarder (ie smart vapor retarder or faced batts). Even though the center of cavity R value is less, the assembly R value is higher. Since ZIP needs to be taped anyways, you don't need the air sealing property of SPF. Either can be nailed up with a standard framing nailer although the R9 needs nails that are harder to source.

    You also only need an 1" of foam in the attic to air seal. Paying for more gets you nothing extra. Even there, you only really need foam over wall plates and fixtures. Spraying over flat drywall is waste of expensive material.

  5. Mgrinde | | #5

    Thanks for all the information so far. For the attic then, I have attached a picture of what we want to do. 2" of closed cell on the top side of ceiling drywall and enough cellulose on top to get a minimum of R-60 in a vented attic.

    With having the 2" of closed cell spray foam, will we have any issues with drywall sagging in the future or does the spray foam provide more rigidity to support the weight? Would we need any strapping to support this?

    We also have a vaulted ceiling (pictured) in the living room that does not perfectly follow the truss line on one side since the kitchen/dining has a flat ceiling. What would be the best way to insulate this area? We were told to baffle the roof and spray foam the baffles going toward the front side (which is the bottom of the picture), then spray foam the opposite side of the vault a little thicker until it got closer to the flat ceiling over kitchen/dining where the cellulose can stack. Does this seem effective or are there better alternatives?

  6. Expert Member
    BILL WICHERS | | #6

    You really aren’t gaining anything with that 2” layer of spray foam in the attic. It may add some rigidity, but there are cheaper ways to do that. If you want spray foam for air sealing, use in only in targeted areas like top plates as Akos mentioned. Applying the spray foam over drywall, which itself is already an air barrier, gains you nothing but extra cost for your project.

    How much depth do you have in the framing over that vaulted ceiling area? I agree that venting that is the way to go, but sometimes there isn’t enough space available for the vent channel and at least code minimum levels of insulation. You don’t need to spray foam in those vent baffles, either, you just need to make sure the ceiling below (probably drywall) is well air sealed. If you don’t have enough space to fit enough insulation, then you might need to go all-spray foam in the vaulted ceiling areas and build an unvented assembly.

    BTW, is there any particular reason you’re wanting to use spray foam in so many areas?


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