GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Knee wall spray foam question

MNHouse | Posted in General Questions on

I have read alot on this site already, thank you to everyone. I have a specific knee wall question that I haven’t seen addressed.
> In Minnesota, Zone 7, 1 1/2 story “Cape Cod” with finished 2nd floor and 5′ knee walls, done 20+ years ago. Am now re-doing the front attic behind the knee wall due to condensation problems – attic is 30′ x 7′. Venting the attic  with continuous soffit ventilation, baffles and roof vents at top of attic next to knee wall.
> For the attic deck, plan to spray foam top side of 1st floor ceiling, install joist blocking under the kneewall and then blow in insulation to R-60.
> My question is the kneewall: I have removed the R-19 fiberglass batts from the back side of the knee wall and now am looking at the bare backside of the 1/2″ drywall. Do I either:
A: Spray foam 2″ directly on the drywall, then install R-13 or R-19 FG unfaced batts, then sheath with 1″ or 1.5″ Foamular XPS board? (Concern here I think is sandwiching the FG batts between two vapor barriers)
B: Install the FG batts in the knee wall, sheath with the Foamular board and then spray foam the out side of the Foamular?
Thank you for any guidance.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Brian Pontolilo | | #1

    Hi MNHouse.

    Of the two options, I think the second is a better choice.

    More insulation on the attic side of the kneewall will keep the framing warm and you could have a class III vapor retarder (painted drywall) on the inside, which would allow the wall to readily dry inward if it did get wet. (However, this is a wall in an attic under a roof, it's not likely to get wet from rain or snow unless your roof leaks into the wall and if you do a good job air sealing, it shouldn't get wet from vapor either)

    I'm sure you'll get some more thoughts on the insulation types you are choosing soon.

    If you do go with spray foam insulation, it will go a long way towards air sealing. If you don't, be diligent about air sealing the drywall from the backside while you have the cavity open, and detailing the rigid foam as an air barrier when you install it.

  2. Expert Member
    BILL WICHERS | | #2

    I agree with Brian (the second option is better) except that I don’t think you gain much with spray foam here. I would skip the spray foam step and instead do some air sealing of the drywall while the wall is open, then put in the batts (I prefer mineral wool, but fiberglass will work fine too), then apply a bead of sealant to the perimeter of the wall, put up the rigid foam (I’d use polyiso here), then tape the seams between sheets of rigid foam.

    The bead of sealant will help with air sealing. Polyiso is easier to tape, a bit more R per inch, and greener than the XPS.

    You don’t really need to spray foam the attic floor either, there are less expensive ways to air seal (get a few of the “pro” cans or great stuff and the gun, then seal all the usual places like wire and pipe penetrations, top plates, etc) the attic floor. Spray foam will help to ensure a good air seal, but it’s not the only way to do it. Spray foam isn’t doing much in terms of insulating in your situation either — it’s really only an air sealing step in your application.


  3. MNHouse | | #3

    Brian, Bill - Thank you for your detailed responses, very helpful. Yes, I planned to seal around outlet/switch boxes etc. on the drywall before enclosing either way - good callout.
    Bill (or anyone), do you have a recommendation of what brand/type of tape to use to seal the polyiso (I would probably be using Johns Manville foil-faced)? Also, is the typical method of attaching the polyiso sheets to the studs using a cap nail?

  4. Expert Member
    BILL WICHERS | | #4

    I prefer using screws to attach rigid foam, you can get a wide, conical plastic washer to spread the load so that the screws don’t sink into the Polyiso.

    I’ve used foil tape to tape polyiso since it gives a clean appearance and holds up well. Hopefully you’ll get some other responses about the tape since I’m curious what others have used myself :-)


  5. MNHouse | | #5

    Thanks Bill - using screws instead of nailing makes sense, less risk of damaging the insulation board, and easier than swinging a hammer in that tight space!
    Foil tape makes sense, I've used it for metal duct work. Home Depot also has this 3M tape that seems to be designed for taping insulation panels - I'm trying to attach a pic.

  6. Expert Member
    Peter Engle | | #6

    For taping foil-faced polyiso, I like to use a UL-listed foil tape. The cost of a few rolls is not too bad, and the adhesive stick like crazy. I've opened walls done 20 years ago and the tape is still completely intact.

  7. olddogtrainer | | #7

    I have had good luck with Nashua 330 for a foil tape in this situation, or 3M 8067 for a non-foil tape.

  8. MNHouse | | #8

    Peter, Jim - thanks for the feedback, very helpful.

  9. AndyKosick | | #9

    Minimizing labor is everything is these spaces but it's also a shame not to maximize R-value. I think the two options here are labor intensive for a mediocre R-value. The two methods I like use only one type of insulation each. One is more labor intensive but uses only cellulose (if that's important) the other is the easiest and uses open-cell foam. Sometimes the choice is based on the best contractor you have available, foam or cellulose.

    For cellulose, insulate the knee wall and attic floor. Air seal with can foam, caulk, and duct mastic actually works well for somethings. Seal off the floor joist cavities beneath the knee wall and install air ways at the soffit. (sounds like you done all this) Then use 1x2's (you could use 2x2's for more nailing surface) and 6" x 12" OSB gussets nailed to the sides of the kneewall studs to form and double stud wall 12" thick. Staple a vapor permeable building fabric over this assembly and use abother set of 1x2's nailed at each center to strengthen the connection. Tuck a piece FG batt (prefferably the ones you just took out) at the top and bottom of each cavity so some air can escape. I found a cordless 15ga nailer made this process palatable. Then dense-pack the wall, which will be at least R40, and blow the floor to R-60. Last but not least make an air tight and insulated access panel. I usually created one behind the existing opening if there was one to avoid altering the look of things.

    This is good for an industrious homeowner with access to a rental machine but I have given up on it professionally because of the labor intensity. Although I thought it easier than the methods your looking at because I hated wrangling big sheets of foam inside the triangle.

    The method I use now requires a good open-cell foam contractor and insulates the roof deck. Install air way baffles from soffit, up the roof deck, to the top of attic, hopefully connecting to venting heading up from there. Have a 2 to 3 lifts of open-cell foam applied to underside of the air ways and the two triangular end walls, for an R-value of 50 to 60. Make sure it overlaps all the way to the ceiling drywall below and the knee wall drywall to ensure a good air seal. You're done and have a conditioned attic. Although I don't recommend storing things in this space, the other advantages are most electrical remains easily accessible and there's no need for an air tight insulated access panel.

    It may not be to late for number two. Also, creates possible location for future ducted mini-split. Even for the kneewall alone I'd consider 12" of open-cell. The process will be complete in the time it takes you to air seal it, let alone install FG, Rigid foam, and tape, and with near twice the R-value.

    Best of luck.

  10. MNHouse | | #10

    Andy, this is very interesting. My rafters are only 2x4 (1950 house) - would the open cell foaming on the underside of the roof deck you describe still result in that high an R value in my situation?
    Thank you for your thoughts.

    1. AndyKosick | | #11

      Brad, yes it will. It's going to exceed the depth of the rafters and in fact be more continuous than if the rafters were deeper. There will be an inevitable "pinch point" at the bottom if the heal height in limited. I actually just complete a job with 2x4 rafters. The ceiling will look like a big blob when you're done. Make sure the foam used has a Class 1 Fire rating and it can be left exposed as long as "The space is entered only for purposes of repairs or maintenance". Also, make sure it is applied in multiple lifts no more than 5" thick. Check with your local code official if necessary to clarify what is required in your area. Both these items are also a good foam contractor "check" because they should know all about it.

      1. vap0rtranz | | #13

        Andy & Brad,

        We have a similar type of house and facing similar challenges.

        > There will be an inevitable "pinch point" at the bottom if the heal height in limited. I actually just complete a job with 2x4 rafters.

        And what about the sloped ceiling of the knee walls? One contractor recommended we drop those ceilings down to allow for the thicker foam to fill the 2x4 rafters, but anyone who knows these type of homes realizes that head space is already at a premium: Cap Code & Farmhouse from before mid-century are almost ENTIRELY sloped ceiling on the 2nd floor. In our case, we have windows, mini-split head units, and other obstacles to dropping the sloped ceilings lower. So there's no way to go around the tiny spaces between the drywall and the roof deck.

        >make sure it is applied in multiple lifts no more than 5" thick.

        Another contractor wants to apply ours in 4" increments so this is good to read.

        >Minimizing labor is everything is these spaces but it's also a shame not to maximize R-value.

        Totally agree. We got 2 quotes after talking with several contractors. I've opted to DIY the basement and exterior walls and have the professionals do our roof in order to cut down costs. Roofs seem rather risky for a DIY homeowner ... but that's just my opinion.

        >Install air way baffles from soffit, up the roof deck, to the top of attic, hopefully connecting to venting heading up from there. Have a 2 to 3 lifts of open-cell foam applied to underside of the air ways and the two triangular end walls, for an R-value of 50 to 60.

        This is basically the approach that I was thinking of when researching DIY for our roof. The contractors here don't appear to be doing it that way.

        How do the air channels work for tight 2x4 rafters in our types of homes? See my sloped ceiling comment. All of our vented roof's air channels must go through the sloped ceiling portion, aka. there is no attic space that is deep enough to fit both 1-2" air channels + 5" thick spray foam. (1"+5" > 4" rafter depth). When I brought this up to a contractor, he switched to considering an unvented roof. I'm concerned about switching a vented roof that has worked for almost 100 YEARS to an unvented roof ... moisture problems, etc.

  11. MNHouse | | #12

    Andy, thanks, very helpful, I can visualize it now.
    I have some things to think about - thanks to everyone for their input!!

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |