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Community and Q&A

Insulating knee wall space

Corkfish | Posted in Energy Efficiency and Durability on

I have a standard knee wall that I would like to insulate. It is a non ventilated space so I am concerned about mold. The advice seems to be all over the map. Should I use a triangular approach putting batts on the attic floor, wall and rafters? That would seem to cover everything, but I have also read that you shouldn’t insulate the rafters.Should I use faced or unfaced batts ( does it really matter?) And, I assume that if I used faced batts the vapor barrier should face the attic space. Can I put foam board over the batts against the interior wall to get added r value or would that create a space for mold?

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  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to an article that tells you everything you need to know: “Two Ways to Insulate Attic Kneewalls.”

    1. Don't worry about ventilation in the triangular attic behind the kneewall. If you don't currently see any signs of moisture accumulation, mold, or rot, there is no need to worry. That said, you can't install a fluffy insulation like fiberglass batts between the rafters unless you are able to provide a ventilated gap between the top of the insulation and the underside of the roof sheathing.

    2. Pay attention to airtightness in all your work.

    3. It's better to insulate along the sloping roofline than it is to insulate the kneewall.

    4. Here is a link to an article that describes safe ways to insulate a sloping roofline: How to Build an Insulated Cathedral Ceiling.

  2. cowsnearby | | #2

    Martin - I read the article "Two Ways to Insulate.." that you linked to, and I have a follow-up question.

    I have a room on the second floor of my Maine 1970s home where the ceiling slopes all the way down to the floor at ~45 degrees. It's not very useful space, but I have built some cabinets with doors that are effectively a bit like a short kneewall (about 3' high). The ceiling is drywall and as best I can tell has some blown insulation above it in the attic. So, long story short - this is all currently conditioned space using method two from your article, with the sloped ceiling being insulated.

    I'm in the process of putting hardwood flooring into the room, and it's going to be a challenge to nail the last few rows all the way to where the ceiling slopes down to meet the floor. As such, someone suggested putting in a mini knee wall at 10-12" (wherever I can't practically get nails in beyond) and just extend floor to there. Seems reasonable.

    My question for you: is there any reason NOT to insulate that little triangle of space behind my 10" kneewall? Putting my hand into that space and onto the ceiling above it, I can feel that it is pretty cool. I am concerned about causing moisture issues (there are presently none). I have some rigid foamboard lying around, and I was thinking of cutting it and basically filling that space.

    The front face of the 10" kneewall will probably be just 3/4" pine. It's going to be invisible to the room (in the back of the cabinets), so I don't want to hassle with drywall. I also am not planning to cut into the existing ceiling drywall to attach my 10" kneewall - I'll just fasten the wall's top "plate" on the room side of the drywall, screwing through to the rafters. Should I try to airseal the 10" wall or is that unnecessary?

    I'm just flying by the seat of my pants, so if you or anyone else has advice, I'd be most appreciative. Thanks!

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Is there any reason NOT to insulate that little triangle of space behind my 10 inch kneewall?"

    A. No.

  4. cowsnearby | | #4

    Thank you, Martin. Is air sealing around the boards that will make up the face of the 10" wall worthwhile or just a waste of time in this case?

  5. GBA Editor
    Martin Holladay | | #5

    Air sealing is never a waste of time. Do it.

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