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Community and Q&A

Kneewall insulation in Climate Zone 3A

NormanWB | Posted in Energy Efficiency and Durability on

New construction Zone 3A – mixed humid

I am following the recommendations in “Building America Special Research Project: High R-Value Enclosures for High Performance Residential Buildings in All Climate Zones” (an excellent resource!), but don’t see any mention of kneewalls. Should I treat them as a wall (R-20) or attic (R-50)? I am thinking the former, but want to be sure.

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  1. GBA Editor
    Martin Holladay | | #1

    For a variety of reasons, it's better to install insulation along the sloping roofline behind the kneewall rather than it is to insulate the kneewall. To understand why, and to read about ways to insulate the roof, see this article: “Two Ways to Insulate Attic Kneewalls.”

  2. NormanWB | | #2

    Thanks, Martin. That helps with the kneewall that is attached to the sloped portion of the roof. However, my loft area with the kneewall is only part of the attic area which will be vented with blown insulation over the ceiling joists (HVAC in the conditioned space), so the transitions from the sloped area to the rest of the attic will still need wall insulation. So, what value? The delta-T from 140 degree attic to a 78 degree living space would seem to indicate a lot more than R-20, so I am thinking of some combination of cavity insulation (R-13 to R-19) plus 4-6" of foil faced poly iso sheathing.

  3. GBA Editor
    Martin Holladay | | #3

    When it comes to choosing an R-value, the answer is usually something like, "The R-value should at least meet minimum code requirements, and can be up to twice code-minimum values if your budget allows higher levels and if cost-effectiveness calculations justify the investment."

    In your case, remember to install blocking under the kneewall bottom plate, and blocking above the kneewall top place, with air sealing measures around the perimeter of each piece of blocking.

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