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Community and Q&A

Air seal exterior board sheathing from inside?

soyelmoreno | Posted in Energy Efficiency and Durability on
I’m a homeowner in Seattle, WA, in Climate Zone 4C. House was built in 1954. I’m renovating a family room (new roof, windows, electrical, insulation, walls, floor) to create a music and dance studio. The room is part of an addition built around 1960. The exterior walls are, from outside-in: two layers of cedar shake shingles | roofing paper/felt | 1×8 board sheathing (not even ship lap…just boards) | 2×4 studs. On one wall the sheathing is 3/8″ or 1/2″ plywood. I will be adding: R15 Rockwool insulation | Membrain smart vapor retarder (maybe) | two layers of drywall with Green Glue (for soundproofing). 
1. Should I attempt to air seal the board sheathing from the inside? I’m pretty sure that all of those gaps between the boards are “air leaky”…when we took down the old insulation I saw lots of black, some of which looked like water infiltration (where there were roof leaks) and some appeared in nice straight lines, indicating air infiltration around the boards (see attached photos). I can see myself patiently spray foaming all the gaps between the boards…but only if you folks think that would be beneficial. Any other ideas for air sealing from the inside?
2. Should I add the Membrain smart vapor retarder over the insulation, before drywall? From my reading of the residential building code, my climate zone is considered a cold climate, so I’m supposed to have a vapor barrier, but I don’t want to install 6 mil polyethylene based on what I’ve read on this site. Would rather simply omit the vapor barrier…but I’ve seen the Membrain product recommended here numerous times. Install it or skip it? In case it makes a difference: during our occasional dance parties there might be higher-than-normal interior humidity due to sweaty dancers; the room will have a ductless mini-split heat pump for heating/air conditioning, so hopefully this will keep the temperature and humidity well-regulated.
Note: The wall is sitting on a short (~6″) concrete stem wall. There is no sill gasket, so air also leaks in under the sill plate…I’m planning on caulking that gap.
Also, I’ve read a lot about wall construction techniques on this site, but I don’t want to do a complete redo! I.e. tearing down the exterior siding and sheathing, then installing plywood sheathing, house wrap, exterior foam insulation, and new siding. Nor do I want to fur out the existing studs to 2×6 in order to add R21 insulation, although I know that is the current standard. With everything else we are doing, these would just be too much. Thanks!

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  1. walta100 | | #1
  2. GBA Editor
    Martin Holladay | | #2

    I disagree with Walta/Walter. Your house has sheathing and a WRB, so the article titled "Insulating Walls in an Old House With No Sheathing" is irrelevant to this discussion. That article does not apply to your case.

    If you are working from the interior to reduce air leakage in a house with board sheathing, the best approach is the flash-and-batt approach. For more information, see "Flash-and-Batt Insulation."

  3. soyelmoreno | | #3

    Thanks for the responses, Walter and Martin. Very interesting idea to do flash-and-batt. I know that spray foam is an expensive product, but I might consider it for my small job (~400 sq ft, only ~1 inch of thickness). I'll get a quote from a spray foam contractor. Alternatively, I'm considering a DIY spray foam kit...I've heard those are a little problematic, but maybe just fine for what I need.

    If I do spray foam, can I/they apply only a 1/2" layer? Would that create a proper air seal...if not, how many inches would I need?

    A perhaps less expensive option: Could I instead install 1/2" polyiso rigid foam against the exterior board sheathing, leave a gap around the perimeters, and fill the gap with canned foam? I've done this at the rim joists (with 2" polyiso)...would be basically the same process.

    Either way, if I add 1/2" to 1" of foam insulation to air seal, then my R15 Rockwool 3 1/2" batts will be a little too thick. I could trim off a small layer, maybe 1/2" to 1" (tedious but doable). Or I could just compress the batts. I've read that fiberglass batts can be compressed, reducing total R value but increasing R per inch. How about Rockwool? Should I trim or compress?

    Does adding the foam air seal against the exterior board affect my question about the smart vapor retarder?

  4. Expert Member
    RICHARD EVANS | | #4


    The "cut and cobble" method that Martin described (and with which you have experience) necessitates spray foaming the edges, just as you described.

    Closed cell cell spray foam should provide a nice air barrier at 1/2" and is sufficiently thick enough (for marine CZ 4) to avoid any dew point issues within the wall. You could combine this with Roxul and compress the batts or use something like the Owens corning mineral wool batts which is only 3" thick and readily available at Home Depot.

    Open cell foam might work too (depending upon which brand was available) but I would fill up the entire cavity to decrease air permeance and also eliminate the need for batts. I would probably add felt or a peel and stick over the board sheathing first if I went the direction though.

    If you use cut and cobble or closed cell spray foam, then you won't need a vapor retarder. Building code/inspector may require one if you fill entire cavities with open cell though...

    Love looking at the old boards- especially from a home in the Northwest. Enjoy while you have it exposed!

  5. GBA Editor
    Martin Holladay | | #5

    Q. "If I do spray foam, can I/they apply only a 1/2 inch layer? Would that create a proper air seal...if not, how many inches would I need?"

    A. Once again, I urge you to read the following article: "Flash-and-Batt Insulation." That article has the answer to your question -- and probably has answers to other questions in your mind as well. (You need at least 1 inch of spray foam for walls in your climate zone. One inch of closed-cell spray foam will provide an adequate air barrier.)

    As Rick suggested, the second approach you suggest is called the cut-and-cobble approach. If that's the way you want to proceed, you should read this article: "Cut-and-Cobble Insulation."

    If you follow either approach, it's OK to compress the batt insulation.

  6. martinamw | | #6


    I have a very similar build I’m working on with the same questions - considering rigid foam & canned, or spray foam and fiberglass batts.

    What did you end up going with ?

    Thanks in advance

  7. soyelmoreno | | #7

    Oops, I forgot to follow-up with what I ended up doing! Thanks for the nudge, martinamw.

    First of all, thanks for your reply, Rick. Yeah, it's a lovely, old home...very sturdy, charming, with a neat double bonus room in the attic. The addition created a whole bunch of additional living space, but back then they just didn't do much sealing or add much insulation. Oh, and that charming attic needs air-sealing and insulation! Oh well, another project for another day (or year).

    Back to my questions here: 1. No one thought I should air seal by simply filling the gaps between the boards with canned foam or caulk! Good thing I asked, I guess.

    So, I learned about the flash-and-batt approach that Martin recommended. I spoke with a spray foam contractor and he said the thinnest layer they would do is 2", which would be more than I wanted.

    So I researched the DIY two-component spray foam kit, and decided against it (hard to use, and I envisioned myself making a complete mess trying to learn it).

    That left me with cut-and-cobble (defintely read the Cut And Cobble article). I thought I could do just 1/2" polyiso, but according the Minimum Thickness article (, I needed at least 1".

    That left me with: Compress my 3 1/2" R15 Rockwool batts or trim 1" off? I emailed Rockwool, and at first the representative was very concerned that I would be reducing the R-value of my batt, until I explained that I would be replacing it with R-value from polyiso. Once they understood that, they said "Trimming to size would be better option. It’s a very springy material and if you compress it will want to spring back out which may put undue pressure on the drywall and pop the screws." Not sure if that extra 1" would really put much pressure on my drywall. But when I tried compressing...I didn't like it. The batts really bulged out. So I grabbed a very long handsaw and got to work trimming approximately 1" off of each R15 batt to make them 2 1/2" thick. At first I used a little wooden frame that was 2 1/2" thick, set the batt inside it, and rode the saw sideways on the wood. But eventually I tossed the frame, and just eyeballed it, and with some practice, I got good enough at trimming.

    Attached photos are:
    1. the cut and cobbled 1" polyiso chunks, foamed (I trimmed the foam off)
    2. an untrimmed batt where you can see it bulging out of the stud bay...the other batts are trimmed to 2 1/2"
    3. my set up to trim the batts with a handsaw
    4. a stack of 1" trimmed pieces. I ended up using all of these because I had a ton of odd-sized stud bays and gaps

    For my question 2. about the Membrain Smart vapor retarder: Rick suggested that if I use closed-cell foam that I wouldn't need a vapor retarder. I bought all the Membrain, but I ended up returning it, and I think I will just go without it. I haven't put up the drywall yet (busty with a different remodel), so I still could if someone objects.

    1. Jon_R | | #8

      Consider a blower door test to find leaks. And some unfaced foam on the interior side to reduce thermal bridging.

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