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Knee wall insulation

neednewrouter | Posted in Green Building Techniques on

Hi I have a cape in NY with knee walls on second story that were batted 50 years ago. The knee wall studs are only 2.5″ wide (not usual 3.5″). The insulation is pretty beat in places. The floor of the knee wall is insulated over the ceilings below. There is a new roof cap in place that allows air to move up the rafter area. My question is what is the best way to insulate or re insulate the knee walls?

I was thinking of just adding a 2″ rigid foam over the existing knee wall studs. Probably should replace the batting also though? I really dont want to replace the entire knee wall studding.

Thanks for any advice


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  1. GBA Editor
    Martin Holladay | | #1

    You have two choices. You can either insulate the small triangular attic behind the kneewall by insulating the sloped roof assembly above the attic, or you can insulate the kneewall. The first approach is preferred.

    If you decide to follow the second approach, you need to install airtight blocking between all of the floor joists under the kneewall bottom plate, and you need to install airtight blocking between all of the rafters above the kneewall top plate. You also need to beef up the insulation on the kneewall itself with rigid foam, as you propose.

    For more information on this topic, see Two ways to insulate attic kneewalls.

  2. neednewrouter | | #2

    Thanks Martin

    Why do you block between the joists and the rafters?
    And if you block between the rafters, wont you impede the good airflow up from the soffit through the rafter cavities to/out the ridge cap?

  3. GBA Editor
    Martin Holladay | | #3

    The answers to your questions are provided in the article I linked to.

    The reason that you need blocking -- with each rectangle of blocking carefully air-sealed -- is to prevent the flow of cold exterior air into uninsulated joist bays or through air-permeable insulation.

    In the case of the blocking between the rafters, the blocking needs to extend from the top plate of your kneewall to the underside of your ventilation baffle. The ventilation baffle needs to be continuous, so that airflow is unimpeded from the soffit vents to the ridge vent.

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