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Community and Q&A

How to air seal from interior with pine siding?

KBoothe | Posted in Green Building Techniques on

We just gutted the interior of our ~1950 cape on Long Island (NY). The Bottom layer of siding appears to be Pine 1x in good shape with cedar shingles on top of that. I want to air seal the siding from the interior, what’s the best way to do this? I assume caulk, but which type? or could I caulk strips of tyvek into the bays?

Question #2:
I plan to insulate with rock wool batts for 2×6 cavities. Its framed with 2×4. I was going to build out the wall using 2×2, but that still comes up 1/2″ short. I was going to use 1/2″ ply strips to finish it, but would it make more sense to cut strips of 1/2″ rigid foam and put it between the 2×4 and 2×2? Would this decrease a little of the thermal bridging from the studs?

Any recommendations or suggestions on better ways to insulate would be appreciated. In the future (~5 years) we plan to re-side with rigid foam underneath.

Thanks in advance for the help

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "I want to air seal the siding from the interior. What's the best way to do this?"

    A. Spray polyurethane foam.

    Q. "I was going to use 1/2" ply strips to finish it, but would it make more sense to cut strips of 1/2" rigid foam and put it between the 2x4 and 2x2?"

    A. That would work, but it is fussy and time-consuming. Why not just install your 2x2s horizontally? The type of insulation you end up using depends on whether you take my advice on using spray foam -- but if you want to install an air-permeable insulation, blown-in cellulose would work well with the horizontal 2x2s.

  2. GBA Editor
    Martin Holladay | | #2

    You mentioned pine boards and cedar shingles; I assume that when you wrote "pine siding," you meant "pine sheathing" -- and that the cedar shingles are the siding.

    However, maybe the first floor of your house has pine siding nailed directly to the studs, and the second floor has cedar shingles. If that's so -- if the photo is showing siding, not sheathing -- then spray foam is a bad idea.

  3. user-4524083 | | #3

    Christian - If you're planning down the road to put foam over the sheathing on the outside, I like your idea of Roxul or Martin's idea of dense pack cellulose, more than Martin's spray foam idea. With foam on the outside, the sheathing will only be able to dry to the inside, so you want the insulation in the cavity to be very permeable.If you are going to do this yourself, I like the idea of the foam layer on the studs then the wood screwed or nailed through the foam.It is fussy, but it's just your time.( Not that I'm saying your time isn't worth much,you just won't be paying for it.) But if you are serious about adding foam outside, later, it's probably not worth it. I'd go with 2x2's added now, then 2" foam outside with a rain-screen later..Since a full 2" add inside on the studs would be preferable, you might want to rip down 2x6's. You'd be wasting some wood, but wouldn't have to fuss with the 1/2 " plywood or foam.A third option is the foam strip add,say 2"then 2x2's screwed through that, and use roux's r-30 batts for 2x8 walls. This would leave all the extra insulation work inside, and you wouldn't have to extend your windows and doors on the outside.You wouldn't have a rain screen, but the wall can still dry to the inside and the outside.Good luck to you on the project

  4. KBoothe | | #4

    Martin- its 1" (actual dim) pine sheathing, a layer of tar paper, then cedar shingle on the outside. We had spray foam in our last house, loved the performance but I didn't like the difficulty getting back into the walls later on if needed. I was also a little concerned about all of the chemicals (unwarranted?). Maybe a solution would be 1" spray foam to air seal then batt insulation in the remaining cavity, is this a bad idea? would this setup be a problem later on if we re-sided with rigid foam?

    Kevin- I don't mind the fussy aspect at the moment, though I'm sure I'll get sick of it half way through and curse myself. I'm trying to set this up so it performs well if we end up not adding foam on the outside, but we don't screw ourselves if we do. I like the 2x8 idea, but I dont want to lose that much floorspace.

    what kind of caulk does one use to air seal at the top plate?

    What certifications should I look for from a spray foam company? the contractor that sprayed our first house had no clue what he was doing and we didn't realize it until after the fact.

  5. GBA Editor
    Martin Holladay | | #5

    You asked about the best way to air seal, and I answered "spray foam." But it's not the only way.

    The usual way to create an air barrier in this situation is with the Airtight Drywall Approach (ADA), which can be used with air-permeable insulation if you want.

    Another option is flash-and-batt, which you mentioned.

    If you decide to use spray foam, you might specify open-cell spray foam -- that way the sheathing can still dry to the interior, even when you add rigid foam to the exterior in the future.

  6. charlie_sullivan | | #6

    I'm liking some of Martin's ideas, particularly this combination: add 2x2s horizontally, fill with cellulose, and then do airtight drywall. Not having the outside well sealed would be OK if the inside is your main air barrier, particularly since dense-packed cellulose inhibits air flow to some extent.

    If you don't mind losing the space inside the house, you could make the cavity even thicker, e.g. with 2x3s, which could then be vertical, staggered relative to the original 2x4s. You could make the wall as thick as seems tolerable as far as losing space, just based on where you set the 2x3s. Then you wouldn't need to go back and put foam under the siding.

    Personally, my biggest concern about spray foam, aside from the cost, is the high global warming impact of the blowing agents used to make the bubbles-->1000X worse than CO2.

  7. LucyF | | #7


    I plan to add 2" of insulation to my interior walls so I am going to use an approach similar to Kevin's answer in comment 3 and Charlie's answer in comment 6. It's sometimes called a "Mooney wall" described pretty well with some nice photos on the Build It Solar website.

    The very first real project I did on my house was install 2" polyiso on the exterior when I installed new siding so like you I have to allow my house to dry to the interior. I will use some form of vapor permeable insulation - technically I like Roxul best, I just hate having to suit up to install it.

  8. user-1072251 | | #8

    There are two methods that work - a non permeable wall insulated with foam, and a slightly lower R value permeable wall insulated with something like Roxul. Since you are not doing the whole wall at once, I prefer the Roxul method as that leaves the wall breathable and is therefore somewhat less subject to moisture issues. So that leaves the infiltration issue; which in Martin's view can be solved by airtight drywall - assuming you can eliminate the infiltration around electrical boxes and other penetrations. My suggestion would be to try to minimize the infiltration by installing a permeable fabric membrane* on the interior side of the wall cavity against the sheathing, turning the edges and fastening them against the studs, perhaps with a bead of urethane caulk against the stud. Yes it will be time consuming, but keeping moving air out of the wall cavity will have definite benefits to your insulation. *It does not have to be a fabric, as long as it is very permeable. Should you decide to use spray foam in the cavity the fabric will prevent the foam from sticking to the sheathing and siding.

  9. Dana1 | | #9

    If you're going to air seal the siding with broadsheet goods like housewrap, don't cut it into strips & caulk them. Instead, start the edge by stapling the sheet to the interior face of a stud, then tuck it to the sheathing and side-staple it to the side of the stud, move across the bay to the other sheating/stud corner and staple it there, then WRAP the stud, side-stapling it to the to other side of the stud, etc. What you end up with is wrapped studs & sheathing, with only top & bottom edges to caulk.

    It's OK if the housewrap loops over the interior edges have a bit of slop to them- it'll fold & compress just fine when you add the next layer, be it sheet-foam or wallboard.

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