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Community and Q&A

Insulating attic/kneewalls

MarkRV | Posted in General Questions on

I have 3+ car garage with a second floor that I am finishing off as living space.  The space has kneewalls ~5 feet high, and I am trying to determine the best way of insulating the attic space/kneewalls.  From research I’ve done, most seem to agree that the best solution is to insulate the roof rafters (bringing the kneewalls into the thermal boundary), as opposed to insulating the kneewalls themselves.  In my scenario, since the living space is above an unheated garage (which has R-19 in the walls), I have already insulated the garage ceiling/2nd-level floor with R-30.  One other factor that I’m not sure of it’s impact –  there will be a small hvac system (gas) in the attic space to heat/cool the living space.

While I would be able to insulate the attic rafters in my scenario, wouldn’t I lose a lot of heat through the uninsulated kneewalls into attic space? So in addition to insulating the roof rafters in the attic, could I also insulate the kneewalls? All of the articles I have read on this give an either/or scenario, but none has mentioned doing both as a possibility.  Where I live (upstate NY), I am most concerned with impact on heating, and not cooling.

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  1. Expert Member
    BILL WICHERS | | #1

    The usual idea is to insulate the rafters all the way to the exterior wall, which then ties the entire attic -- and your living space -- into the thermal envelope of the rest of the home. This assumes that the "floor" of the attic/living space is also the "ceiling" of the upper floor of the home, which isn't the case if you're over a garage.

    In your case, since the garage is unconditioned space, you'll want to insulate all the way around your living space which includes those kneewalls and the floor under the living space too. What I would do is put a layer of polyiso over the back (attic) side of the kneewalls, then fill the stud bays with mineral wool. If you have the ability to put polyiso under the joists, you can use loose fill in the cavities between joists. Polyiso is easy to air seal, which is extra important with living spaces connected to a garage.

    Try to allow for vent channels under the roof sheathing for the portion over your living space if you plan to build a cathedral ceiling in there. Vented assemblies are much less prone to moisture issues.


    1. MarkRV | | #2

      Bill - thanks for helping to clarify this for me!

  2. MarkRV | | #3

    Follow-up question - is using Polyiso on the backside of the kneewall a better solution than XPS or would either work? I've read that the R-value of polyiso drops significantly in cold weather.

    1. Expert Member
      BILL WICHERS | | #4

      Polyiso is usually the best choice. The R value issue has been overblown, and even when derated, polyiso is usually at R5 per inch -- the same as XPS. You loose nothing relative to XPS in cold weather, but in shoulder seasons (spring/fall), and in summer, you win with higher R value per unit thickness compared to the other rigid foams.


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