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Community and Q&A

Sealing/Insulating Attic with Knee Wall

bbSGYnAJUU | Posted in Energy Efficiency and Durability on

I know there’s a recent Q&A thread about an attic in a relatively new construction. My question is more specific. Picture a home with multiple attic spaces. The space with the air handler is a side attic on the 2nd floor. The cross section on the attic is shaped like a right triangle, with the base being over the 1st story conditioned space and outside porch, the left vertical side being a wall to conditioned 2nd floor space, and the hypotenuse being the roof. There’s a knee wall of sorts that sits on the exterior first floor wall dividing conditioned space from porch. If I were to turn my attic into conditioned space to reduce the HVAC load, I could reduce the materials cost of the project by starting the attic sealing/insulating at the knee wall. My question is the order and orientation of material application. Do I install an R Matte Plus 3 with foil side facing the porch side or the HVAC (to be conditioned) side of the knee wall studs, and which side of the studs should I attach it, assuming I’ll be putting fiberglass batts between the knee wall studs?

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  1. GBA Editor
    Martin Holladay | | #1

    What's your climate?

    If you are insulating kneewall studs with fiberglass batts, both sides of the kneewall need to be enclosed with an air barrier. One of these air barriers (on either side of the wall) can be foil-faced polyisocyanurate foam, if you want, as long as your local building inspector doesn't object to the exposed foam. In some cases you may be required to cover the foam with drywall as a thermal barrier to the foam.

    The foil facing will only be effective as a radiant barrier if it faces an air space. Don't worry about this too much, though -- the additional R-value of an air space plus a radiant barrier is only about R-1.

  2. bbSGYnAJUU | | #2

    Sorry. Zone 2A. Houston.

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