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Community and Q&A

Insulating and air sealing kneewalls

roscoe42 | Posted in Energy Efficiency and Durability on


I recently bought a house, and over the last few months have figured a number of issues with the insulation in the knee walls. A few notes about the house:
*In MN (Climate Zone 6, almost 7).
* ~20 years old
*Built with a poly vapor barrier on the inside behind the drywall
*2×6 framing, currently no sheathing on the attic side, R-19 fiberglass is currently in the kneewalls
*Only way into attic is through a 24×30 opening
*Due to the architecture of the house, I have many knee walls, ranging from 7.5′, 4′, and two that progressively get shorter as you go down a scissor truss.
*Most of the walls have plenty of space behind them on the attic side, except for one only has about 6″ of clearance from some trusses near it.

I had a contractor bid it, came in way more expensive than I can afford or want to pay for doing the entire attic, and I’m looking for a challenge… Part of their quote was to take put a horizontal layer of R-19 down, and then cover it with housewrap.

I know at a minimum, the kneewalls at least need an air barrier to stop windwashing, and having the interior vapor barrier, I know that rules out the use of rigid foam. So here are my questions:
1. Is adding an extra layer of R-19 to these walls necessary, or is it overkill?
2. If I did go horizontal, I was thinking I’d get 2×6’s, nail the bottom one to the joists, and then use long lag screws to attach the ones above it to the existing vertical 2×6’s. Is that the best way, or is there a better way to do this?
3. I understand housewrap can be used as an air barrier. Just from the sheer effort of having to cut OSB down to fit through the opening, carrying it through the house, and then up and across the attic and installing it, if housewrap can do the job, it would probably save a lot of damage to the inside of the house, and my body. What are the guidelines for installing it “correctly”. All I’ve been able to find so far is it can be done well and can be done poorly. If I went this route I was thinking it might be good to put the housewrap down, and then attach furring strips every 1-2 feet to keep the fiberglass from falling down. Is this viable?
4. This goes with questions 2 and 3, but If I did go horizontal and used housewrap, a local home center can get fiberglass rolls that are 48″ wide. That seems attractive as its a continuous piece, and would cut down the number of 2×6’s that need to go in. Would 48″ be too much for the housewrap to hold? With OSB, this seems like it might be tough not to have it nailed down in the center…

Those are the questions that keep circulating in my mind right now as I start preparing for this job. Any other tips or advice related to this would be greatly appreciated.

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  1. Dana1 | | #1

    R19 walls meet code in MN (where codes are still based on IRC 2003 & 2006). Batts that have the fluff showing won't be performing anywhere near R19, but they might be hitting R10 even with the convective losses. Fully compressed to 5.5" in a 2x6 cavity R19 batts perform at R18, according to the manufacturers' compression charts.

    Simply installing housewrap stapled to the studs on the far side of the kneewalls (and detailed at the edges & seams as an air barrier) will roughly double the mid-winter performance, which is a pretty cheap fix compared to using rigid air barriers or adding thickness to the assembly. Unless you are planning to retrofit the entire house to much higher performance adding another layer of 5.5" thick batts with horizontal 2x6s wouldn't be "worth it". For the same time & money you could probably reap a much better ROI elsewhere in the house.

    If you DO go ahead with thickening the kneewalls, put the 2x6s 24" o.c. and use R23 rock wool, which is far more air-retardent than low density R19s. If fiberglass, R21 "cathedral ceiling" batts are similarly air retardent. Unlike R19s, these products perform at the labeled R-value when compressed to 5.5" thickness. (All batts are manufactured with slightly higher loft than framing lumber nominal dimensions to insure a complete cavity fill, but R19s are manufactured at about 6", and you're losing a full 1/2" of thickness when installed in 2x6 framing.)

    With the housewrap air-barrier in place you may be able to install R15 rock wool or fiberglass batts (also high enough density to resist wind washing & convection losses) directly to the studs using cap nails, with the batts butted up to one another, no supporting framing. Continous R15 insulation would slightly outperform R20s thermally bridged by framing anyway.

  2. GBA Editor
    Martin Holladay | | #2

    Although the existence of polyethylene on the interior side of your kneewalls isn't ideal, I don't think that there is any reason to worry if you install rigid foam on the attic side of your kneewalls. I wouldn't anticipate any problems from this approach, even if the wall assembly can't dry to the interior.

    If you want a continuous layer of insulation (and an air barrier) on the attic side of your kneewalls, install EPS with taped seams. (I suggest that you use Siga Wigluv tape.)

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

  3. Expert Member
    Dana Dorsett | | #3

    "*Only way into attic is through a 24x30 opening"

    That is a nearly impossible access port for getting even 2' x 4' sheets of rigid foam in there. I'm not sure even how they would even get 2x6 framing timbers in there. Air sealing it with house wrap seems a lot easier than with rigid board & tape, since it would have but a tiny fraction of running seam to seal, and you can cut large piece of housewrap and fold or roll them up in a flexible fashion (without the center cardboard tube roll) and still get it in there without much trouble.

    Unless that's 24 x 30 FEET of access port (rather than inches), in which case you can just back the truck right in! :-)

  4. Syau | | #4

    Dana, just to be clear, you are suggesting possibly putting all the R value on top of roof deck because my attic would be so awkward to work in? I agree. I have been considering that as an option, having climbed up there myself.

  5. roscoe42 | | #5

    Thanks for the responses!

    It took me a while to figure out why the contractor including adding R-19 to these walls, but my assumption was to resist a higher difference in temperature in the summer. In my research though, I believe Georgia only requires R-19, so maybe the benefits aren't as great in higher temperatures.

    I'll have to look into rock wool and EPS. I'm not too familiar with either of those products. Although the attic port is 24x30, it has a good vertical clearance over it. I've moved 2'x4' pieces of plywood up there already so I have a nice surface to kneel on. I'm pretty sure i could get something 2'x8' in there without too much trouble (aside from dinging the drywall around the hole).

  6. Expert Member
    Dana Dorsett | | #6

    Sonia: In this thread I was recommending putting insulation on the exterior side of the knee wall after putting up housewrap as an air barrier on the knee wall studs.

    Ross: Yes, you can get 2' x 4' pieces in there, but the amount of seam you would have to detail is immense compared to making a reliable air barrier with wide-roll sheet goods like housewrap.

    EPS is usually sold in 4' x 8' sheets, as is rigid rock wool, and you'd have to cut them up to get it in there.

    But rock wool batts are flexible, come as wide as 23" wide (designed for 24" o.c. framing) or 15" wide (for 16" o.c. framing) x 4' long pieces that would be very easy to get in there. Since they are dense enough and air retardent enough to not convect a lot of air through them, they don't absolutely need air barriers on both sides to hit their R-rated performance the way low-density R19s do. So in this application you can mount them horizontally, gently nailed to the studs with long cap-nails or cap-screws, and snug 'em up against one another for a continuous layer of insulation, unbroken by thermally bridging framing. R15s are designed for 2x4 framing, but you don't need additional framing to support the batts. Just try to avoid compressions & warping when you tack it in place. This would be SO much quicker and better than trying to install additional framing or cut-up rigid sheet foam with taped seams.

    The big box stores in my area both carry Roxul brand rock wool. If yours doesn't, R15 fiberglass has similar characteristics, though the fiberglass has more issues with airborne fiber dust than rock wool if you don't add another air barrier. (If you've lived with the shaggy R19s facing the attic crawl, R15 fiberglass won't be any worse than that.) eg:

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