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Air sealing

Mill_house | Posted in General Questions on

Climate zone 7, Canadian Prairies
I’ll be insulating the walls from the inside using mineral wool.  The house was built in the 1940s.  Having exposed the stud bays in one room on the main floor, I see the house has horizontal sheathing boards, felt and then stucco.  Do the horizontal gaps between the sheathing boards require sealing?  Does the inside perimeter where the studs make contact require sealing prior to adding the mineral wool?  If so, what type of caulking and or foam do you recommend?  Thanks

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  1. Expert Member
    Dana Dorsett | | #1

    Caulking the seams of planks isn't reliable. The seasonal expansion/contraction with moisture will cause just about any caulk to fail.

    If you can find half-inch fiberboard asphalted on one side, installing it with the asphalt side toward the sheathing and caulking the untreated side to the framing with polyurethane caulk makes a fairly robust air barrier. If horizontal seams in the fiberboard can't be avoided, nail the fiberboard to the sheathing near the seams for rigidity and use houswrap tape over the seam, followed up with 1/8" of duct mastic over the edges for long term air tightness.

    Cheaper than that would be housewrap laid completely over the studs then tucked-in and side stapled to the studs as close as practical to the sheathing, taping over the staples with housewrap tape. The housewrap can be left somewhat loose where it wraps over the studs, but not too loopy.

    Similarly, #15 roofing felt cut into strips wide enough for a stapling flange to the studs snugged up the sealed to the studs with a fiber reinforced liquid roofing patch material would work (if a bit messy.)

    I assume you'll be installing poly sheeting on the interior?

    BTW: The performance of the house can be improved substantially using 3/4" polyiso + 1x furring Bonfiglioli strips on the framing and using rock wool batts designed for 2x6 studs:

    1. Mill_house | | #8

      Hi Dana,
      I am going with the housewrap approach you recommend. After I wrap a stud, run the housewrap in the bay, and wrap the next stud, etc etc, I would then insulate with mineral wool, smart vapor, followed by drywall.
      Does your housewrapping of the studs and stud bays raise any concerns re: condensation issues given the following assembly? (From outside working in stucco, gap, felt, sheathing, housewrap, mineral wool, smart vapor, drywall.)
      Is there a particular way to install the housewrap - Tyvek logo facing in or out?
      (Note: Tyvek site does not recommend the use of its product on the inside.)

  2. Jon_R | | #2

    I'd create an interior side air barrier (consider drywall) and test the whole house with a blower door.

    Without specific data indicating otherwise, I wouldn't use any of the "not air barriers" (like felt and fiberboard) listed below. Do keep it vapor permeable to the exterior.

  3. CMObuilds | | #3

    I would flash coat it with 2 part spray foam, or loose fit foam sheets and perimter seal with a 1 part foam.

  4. Expert Member
    Dana Dorsett | | #4

    In Canada the NBC requires an air tight vapor barrier on the interior side of this type stackup, which would also be a primary air barrier. Felt or fiberboard is "tight enough" to prevent convection or wind washing of the rock wool, unlike caulked seams in plank sheathing.

    The housewrap approach could of course be detailed to meet the specifications of a true air barrier.

    Cut'n'cobbled rigid foam is risky. If going with foam, an inch or more of closed cell polyurethane would be needed for dew point control on a 2x4 wall in a climate zone 7 location, or 2" if furred out with Bonfiglioli strips to 5.25" of cavity depth.

  5. CMObuilds | | #5

    @Dana, flash and fill is risky? How about a real world failure case study/example to back that up?

    Fiberboard or WRB is the same idea as a flash fill except if you ever worked with fiberboard, such as a party wall scenario, you would know tape isnt going to reliably seal to fiberboard ever, except on a plan sheet. Fitting tyvek and then taping edges isnt field practical either.

    You said its a 40's cape or whatever, maybe full width 4" studs, also covered in a fine dirt same as the thin batt fiberglass you likely pulled out and tape isnt reliably sticking to that, ever. You can try all kinds of over complicated scenarios for simple problems, this site is absolutely full of that, but use common sense and you will be absolutely fine, even ignoring the foam/fiber rule. In fact caulk would help a lot even though the seal is going to fail for the simple fact that you are going to drastically shrink the "effective leakage area" by filling open spaces. Caulk might fail at the perimeter but it wont disappear in mass.

    I tried an airtight drywall approach ignoring plank sheathing in a flip house I lived in for a few years in WI climate 6 that I gutted and I can tell you with absolute certainty that air seal at interior plane in this case is not effective, you need to shut off the plank sheathing.

    1. Expert Member
      Dana Dorsett | | #9

      Flash'n'fill isn't risky as long as there is sufficient R for dew point control at the foam/fiber boundary to create a mold problem.

      Cut'n'cobble is prone to convective moisture transfer of humid cavity air around the foam layer, since it isn't fully adhered (unlike spray polyurethane, which is well bonded), and the air tightness can't be presumed to be even close to perfect, not initially, and certainly not forever. Moisture will accumulate in the sheathing at the leak points. Is that a disaster? Probably not, but it's a less robust approach the either flash'n'fill or using moisture tolerant more vapor permeable materials as the (not quite meeting the definition) air barrier.

      Fiberboard isn't anywhere near same as cut'n'cobbled foam board. Unlike foam board it has a fairly high vapor permeance, and it's "smart" class-III vapor retarder when dry, north of 15 perms when the moisture levels are high enough to be a problem for the studs/fiber insulation/ plank sheathing. Moisture that gets into the stud cavity can dry quickly through the layer, even when relatively air tight. Also unlike rigid foam fiberboard will share the moisture burden with the plank sheathing, and unlike the plank sheathing it is highly moisture tolerant. It has a lower R/inch than foam board but roughly twice the R-value per inch of plywood or OSB.

      1. CMObuilds | | #10

        The so called cut and cobble method would be risky in a continuous sheathing scenario I agree, however since it is plank board it will promote drying from the miles of cracks in the sheathing.

        Have you ever air sealed and tested the integrity of fiberboard or is that a theoretical solution? I dont see how anything would reliably stick to fiberboard.

        1. Expert Member
          Dana Dorsett | | #11

          Counting on seams & cracks in plank sheathing for drying seems dubious. The whole point of the (quasi) air barrier is to reduce the amount of air moving through the exterior sheathing layers.

          Like the sprayed on asphalt, roofing patch caulk bonds very well to fiberboard. So does polyurethane caulk. Under mechanical stress the fiberboard itself will give up before the polyurethane does- it's a lot like construction adhesive, but doesn't shrink as much and doesn't get brittle (at least not as quickly as construction adhesive). Almost anything sticks to fiberboard, but whatever is used in this application has to be able to handle a wider temperature & humidity range. Polyurethane caulk is nicer stuff to work with than roofing patch with fewer longer term volatile organic compound emissions, ergo polyurethane as the preferred material.

          Minor air leakage around the fiberboard doesn't have the same moisture consequences of less vapor-permeable goods, since the moisture can still move in either direction via diffusion.

          But I have not personally tested the air tightness or longevity of this approach.

          I have used can-foam to seal cut'n'cobbled foam, and it's not as air tight as it looks to the naked eye. I have quite a bit of cut'n'cobbled 3" foam board on the band joists in my own house, and in the few places where I've had to remove some for plumbing/electrical it hasn't been too impressive. Adhesion to the wood was less than anticipated, and the completeness of the fill wasn't as good as expected. Of the hundreds of linear feet of foam-filled cut'n'cobble it would be surprising if there weren't at least a few leak points. Whether those leaks are in consequential locations will have to wait for time to tell, but I'm not crazy about recommending the method even though I've used it. I'd be even less inclined to use it inside a finished wall than on a band joist/foundation sill, where it's more inspectable/accessible.

  6. Jon_R | | #6

    Reviewing below, I wouldn't be concerned about wind washing of anything other than possibly fiberglass.

    On the other hand, interior and exterior side air barriers are of benefit.

    For better or worse, the Canadian NBC allows the air barrier to be anywhere in any wall.

  7. GBA Editor
    Martin Holladay | | #7

    T. Carson,
    Q. "Dana, flash and fill is risky?"

    A. You are confusing flash-and-fill (also known a flash-and-batt) with cut-and-cobble. Dana wrote, "Cut'n'cobbled rigid foam is risky. If going with foam, an inch or more of closed-cell polyurethane would be needed for dew point control on a 2x4 wall in a climate zone 7 location."

    For more information on these two different approaches, see these two articles:

    "Cut-and-Cobble Insulation"

    "Flash-and-Batt Insulation"

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