GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Bungalow attic knee wall insulation

MikeSG | Posted in Energy Efficiency and Durability on

Hi Everyone,
I have a few questions regarding attic knee wall insulation. I have ready many posts on this website but haven’t been able to answer my particular question. My current attic knee wall has a bit of fiber glass insulation and a shotty job of a previous home owner/contractor. Also the attic floor has just a bunch of fiberglass tossed on it. In the winter our master (conditioned attic) becomes very cold. I am in a Zone 5 climate (Southeast Michigan). I plan on putting insulation in the attic space and would like to know the best strategy. Insulating the attic roof side is not an option as it is too expensive and much of the roof is inaccessible since drywall is already up. My current strategy is to insulate the knee wall as follows: from room (conditioned space) to attic (unconditioned space) I plan to put 2″ XPS foam (for air sealing) between the 2×4 studs in direct contact to drywall surrounded by great stuff foam, then R15 rockwool (or fiber glass) batt insulation within a double stud wall construction in direct contact to XPS foam for a total of R25 and cover studs to prevent thermal bridging. Will this work for a knee wall? Do I need a vapor barrier or will the 2″ foam suffice? Also, will there be any moisture problems in the insulation, or is the ability to dry to the attic space enough? I have read there is a foam to fiberglass/rockwool ratio if I want to pursue a flash and batt type insulation. Do this apply if I do not seal in the insulation with drywall/OSB board from the attic (unconditioned space) side? Also, do I need to use some kind of fire retarding paint on the XPS foam to bring it to code, since I read the foam is flammable, or will the drywall/batt insulation suffice?

With regards to the attic floor, I plan on putting loose fill cellulose about 18″ high after sealing air leaksleaks great stuff foam cans. Will this suffice or do I also need a vapor barrier between the first level plaster and cellulose? Will this result in moisture issues? Do I need to build boxes out of drywall around junction boxes and light fixtures to provide 3″ or clear space for code to prevent fire? 

Also, I plan on using XPS foam to seal the gaps under the conditioned space as it is open to the attic. Is this ok?

I have a vented attic. A local contractor quotes me at 15k for the job which sounds insane to me so I plan to do it myself.  Any suggestions/guidance would be appreciated as this is my first time working with attic insulation.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    First, read this article: “Two Ways to Insulate Attic Kneewalls.”

    I assume you will be insulating the kneewalls from the exterior side. If there is enough room for you to work on that side of the kneewalls, consider installing a continuous layer of rigid foam on the attic side of the kneewalls rather than cutting the rigid foam into narrow strips and inserting the foam between the studs. A continuous layer of rigid foam will perform better.

    Green builders generally prefer polyiso or EPS to XPS. To learn why, read this article: "Choosing Rigid Foam."

    Your biggest challenge isn't insulating the kneewall. (Insulating the kneewall is trivial.) The biggest challenge is making sure that your air barrier is continuous -- and that effort requires air-sealed blocking in each joist bay under the kneewall bottom plate, as well as air-sealed blocking in each rafter bay above the kneewall top plate. Sealing these little rectangles of blocking is the hard part.

    1. MikeSG | | #2

      Hi Martin,
      Yes I have read the mentioned posts. However, with your suggestion of placing the foam on the exterior of the studs with fiberglass insulation between the foam and drywall, won't that cause moisture/condensation issues inside the wall in the fiberglass? With the fiber glass exposed I figure any moisture would dry.


      1. GBA Editor
        Martin Holladay | | #3

        Exterior rigid foam is commonly used on walls. If you want to be super-conscientious, you can follow the rules outlined in this article: "Calculating the Minimum Thickness of Rigid Foam Sheathing."

        But really, when it comes to kneewalls, you can cheat a little on these rules, because the air temperature on the "exterior" side of the rigid foam is attic air, not exterior air -- and attic air isn't as cold as exterior air. Thinner foam is very unlikely to lead to problems -- and the advantages to having continuous rigid foam (instead of foam cut into narrow rectangles and installed between the studs) is huge.

  2. MikeSG | | #4

    Hi Martin,
    I see the reasoning on that suggestion now. So in general, the 2 in foam would act as a vapor retarder or would I need a vapor barrier at the drywall? I have read that 2 in XPS/eps act as vapor retarder when used at 2 in thick.

    Also, in the case of installing foam on the exterior of the studs, wouldn't code require that I use some kind fire block paint or similar since the foam is flammable or would the fiber glass/roxul in between the foam and drywall suffice?


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |