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Community and Q&A

Insulating a wall

Acfrqflyer | Posted in General Questions on

Hi, I live in Toronto.Canada
Situation: I have a garage attached to my house. The wall that separates the garage to the living room, I removed the drywall and all the kraft faced insulation between the studs. The drywall on the living room side I don’t want to remove because I installed laminate on the wall with a fireplace. How would I insulate this wall from the garage side? I was thinking of installing 3.5″ of rigid xps inbetween the studs then re drywalling. Thank you

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  1. Expert Member
    Dana Dorsett | | #1

    Using expensive high R/inch foam between studs is a waste of good foam. see:

    What's worse, XPS loses performance over a few decades as it's climate damaging HFC blowing agents leak out, eventually falling to under R4.3/inch ( R15 @ 3.5").

    From a price/performance point of view you're better off caulking the wallboard to the framing with polyurethane caulk (not the cheap stuff) and installing well fitted 3.5" ( R15) rock wool in that partition wall, and NO vapor barriers, and either standard or exterior grade drywall on the garage side, painted with a standard interior latex paint.

    The high vapor permeance of the drywall prevents moisture accumulation inside the studwall, since can dry easily into the wall. If that's not good enough for the local code inspectors, paint the interior side drywall with "vapor barrier latex" before insulating the cavity to bring it within the NBC code definition for "vapour barrier".

    Rock wool has the additional benefit of being completely fireproof, whereas polystyrene (XPS) would melt and burn in the event of a garage fire.

  2. GBA Editor
    Martin Holladay | | #2

    Dana Dorsett referred to your "interior side drywall," but the way I read your question, there is no interior side drywall. It sounds like you covered the wall with laminate.

    One other comment: Dana recommended "standard or exterior grade drywall on the garage side." Note that most building codes require this drywall to be 5/8 inch thick (because this is a garage).

  3. Expert Member
    Dana Dorsett | | #3

    "The drywall on the living room side I don't want to remove because I installed laminate on the wall with a fireplace."

    Reads to me like the laminate was installed over the drywall, not replaced by laminate.

  4. GBA Editor
    Martin Holladay | | #4

    I read your advice to Vernon -- "paint the interior side drywall" -- to mean "paint the drywall facing the living space." That seems impossible to me.

    But perhaps you were advising Vernon to paint the back side of the drywall -- the side of the drywall facing the stud bays.

  5. Acfrqflyer | | #5

    Dana and MARTIN, update: the living room wall is fully drywalled with laminate installed on top of the drywall in the middle of the wall . The wall is approx 16 ‘ long. The laminate takes up approximately 8’ . So to understand what your telling me, I paint the garage side drywall (back side of living room drywall) with a vapour barrier paint. Then caulk were the studs meet the drywall in each cavity with polyurethane caulk. Then install rock wool batts in each cavity, then install drywall over that. Then paint wall with latex paint. Also, what brand is considered a good polyurethane caulk? Dap?

  6. Expert Member
    Dana Dorsett | | #6

    I was indeed suggesting that the cavity facing side of the interior side drywall be painted prior to insulating, but only if the inspectors insist.

    Caulking the drywall to the framing it probably best done prior to any painting- the paint would probably not create as strong a bond as the caulk, and it's hopefully going to remain sealed for a century or more. It's also worth caulking the seams of any doubled-up framing such as top plates, jack studs, etc. Seal up all electrical & plumbing penetrations too. Toronto is at the climate zone 5/6 boundary, and with the exterior sheathing being highly permeable wallboard, vented generously with the air in the garage with the true exterior siding exposed to the weather being a LONG way away, it would qualify as "Vented cladding over gypsum" in the IRC prescriptives that would allow a Class-III vapor retarder for the interior side of the assembly:

    Siding with a 6mm rainscreen would also qualify, if using an exterior grade gypsum board, but you have a whole garage between the gypsum board and the true exterior. (That's one HUGE rainscreen! :-) )

    Any general purpose polyurethane caulk should be good enough, just avoid specialty types that are thinner, such as "self leveling" caulk designed for sealing cracks in concrete. Just avoid cheaper stuff such as painter's caulk, which is less likely to survive the test of time.

  7. Reid Baldwin | | #7


    I am surprised nobody asked what you were attempting to accomplish by removing the garage-side drywall and the existing insulation. Was there a problem with them? If your objective was to significantly improve the R-value of this wall, then the advice might be different.

    BTW, While your wall is open, you should put in the wiring for an electric car charger.

  8. Acfrqflyer | | #8

    Hi Reid, thanks for answering. I installed an electric fireplace on the opposite side of the wall. There was no electrical for it. So I needed to run a wire. Also, the wall was very cold to begin with so I wanted to air seal and insulate always. The only thing now, I can’t remove the drywall in the living room because I don’t want to remove the fireplace and laminate feature wall I installed on top of the drywall. What’ is your thoughts?

  9. Reid Baldwin | | #9

    I would trust Dana's advice regarding how to improve the air sealing. Air sealing a garage/house wall is especially important because of indoor air quality concerns. If the air sealing was relatively bad before, that would explain the wall being cold. Air flowing around the old insulation may have caused the wall not to perform up to its nominal R-value. Unless the other walls in the house have higher R values, then there is no reason to think the R-value of this wall needs to be improved beyond what Dana suggested.

  10. GBA Editor
    Martin Holladay | | #10

    I agree with the advice you've heard so far. Air sealing work is essential. If you want to improve the R-value of the wall, you've got a good opportunity to do so -- but you don't have to.

    One option is to install a continuous layer of rigid foam on the garage side of the studs before installing a layer of 5/8-inch drywall. Toronto is in Climate Zone 5 or 6 -- so the minimum R-value for exterior rigid foam might be as low as R-5 for a 2x4 wall in Zone 5 or as high as R-11.25 for a 2x6 wall in Zone 6. I don't think you need to worry too much about these minimums, though, because your garage won't be as cold as the exterior air. For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

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