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insulating the interior wall of an attached garage

JayDS | Posted in Energy Efficiency and Durability on

Hell0, my question pertains to how one should insulate a wall that separates an attached and uninsulated garage from the heated home and the potential issues that may arise using different methods. I have searched this site as well as others but have not come across answers specific to what I am asking here. Apologies if those answers already exist and I missed them.

I live in climate zone 5 (western NY near Lake Ontario). The wall that separates the cold garage from the heated house is made of 2x4s, is currently uninsulated and has drywall on the warm side (kitchen side) only.
Is flash and batt (1 inch CC spray foam applied to the existing drywall + R11 fiberglass from the garage side) an acceptable approach here? After this installation the garage side of the 2x4s will also be finished with drywall. Would this situation cause moisture issues in between the fiberglass and 1 inch of CC spray foam?
Alternatively, would using 3 inches of CC spray foam be superior? Though this seems superior, my concern with 3 inches of CC spray foam is having electrical cables encased within the CC spray foam, (including a 6-3 cable for an electric oven). Does encasing electrical cables (Romex) in CC spray foam raise any red flags?
If that is not acceptable either, would R13 fiberglass (with kraft paper toward the kitchen drywall) be acceptable?
Thank you for any responses.

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  1. walta100 | | #1

    If this home is still a set of drawing, I say any large areas of spray foam are huge red flags for lazy solutions put in place by someone who has little or no regard for your budget.

    Spray foam is the most expensive least green and riskiest way to get an R of insulation give the smallest amount of planning is totally avoidable in new construction.

    I would insulate this wall in the same way as the other exterior walls.


  2. JayDS | | #2

    Thank you Walta. I should clarify the house is old (1870) and there are several additions from the 40's and 60s. The kitchen and garage are from 1960. I want to insulate the garage/kitchen wall to reduce heat loss from kitchen during winter. I would do R13 fiberglass myself but I've asked around and some say that that is not enough and I should bump to ~R20 by doing flash and batt...and there are NYSERDA rebates for that as well to save money. My concern is whether that method presents a water condensation issue. Alternatively, 3 inches of spray would give ~R20 but I worry about encasing electrical cables. Is that potentially hazardous?
    I guess my primary questions are 1) whether the flash and batt done in this particular fashion (CC spray foam on the warm-side drywall + fiberglass on cold side) would create a condensation issue? And 2) whether CC spray foam encased electrical cables is hazardous?

  3. walta100 | | #3

    I still think you should treat this wall in the same way as your other exterior walls.

    R20 would be nice and R13 is OK but I think you are unlikely to recover the cost of spray foam from the change in R value.

    Given the age of your building it is going have huge air leaks and spray foam may well be worth wile in stopping them if the walls are open. One wall is not going to make a noticeable change.

    If the wires are modern with a plastic outer jacket covering them in foam is not a problem if the wires have a cloth jacket upgrading while the wall is open is the way to go.

    Given how leaky this house is I do not think excessive moisture will be a problem no matter what you do.


  4. Expert Member


    For me the simplest solutions, which would allow you to do all the work yourself, would be to either:
    - Use batts in the existing wall, and cover the exterior with 1" foam before drywalling.
    - Fur out the studs and plates with 2"x2"s, which would make the cavities deep enough to use R-20 batts.

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