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Another homeowner lamenting over insulating a kneewall — I need a little advice

DavidfromPNW | Posted in Energy Efficiency and Durability on

Hi. My name is David and this is my first post.

We’ve just added a second story to our 1960 split level using monster attic trusses that span 30 feet and are 16 feet high. The main purpose was aesthetics at first as our house looked odd with the 3/12 pitch roof it had, plus we did an addition in 2015 with a 12/12 pitch knowing we’d eventually remove the 3/12 on the main house and replace it with a 12/12.

The new attic trusses have yielded an upstair space of 40′ x 16′ with 6′ high knee walls and 8’2″ ceilings. So, it’s good sized. My problem is that I don’t really have knee walls. The vertical walls are simply the truss webs so insulating those “knee walls” and they sheathing to the unconditioned space is not a great option for me. I did forget to mention, the trusses have an 18″ energy heel which it how I got the height.

I’ve read about every article I can find and it seems, although it’s going to suck big time, my best bet is to insulate the slope behind the knee wall thus bringing the knee wall into the conditioned space. The house also has 4 gable dormers so, with the stairwell which is a defacto 5th dormer, I’m required by code to have access behind the knee wall in six different areas.

Here is what I’ve done so far. The floor of the new level is fully insulated with r30 fiberglass noise reduction insulation. The floor joist (bottom chord of the trusses) are 2×10. I’ve also built my own baffles per this article ( that extend several feet below the were the knee wall and sloped ceiling meet and several feet above where the slope ceiling meets the flat ceiling towards the peak. I’ve done r49 batt in the flat part of the ceiling and all the dormers have been done with r21 in the walls and r49 in the dormer ceiling.

Apologies for the long set up. My question is:

Q. If I decided to insulate the slope down to the soffit with r49 fiberglass, can I use tyvek on the interior side of the batts behind the knee wall for an ai barrier. I do know that I will have to continue the site built baffles down to the soffit and that’s not a problem. It would just be very difficult to use a rigid sheathing on the slope behind the knee wall due to the engineering on the trusses.

Q. Since I insulated the entire new story floor with r30 and have a 18′ energy heal allowing me to insulate right over the lower wall top plate, do I even need to do blocking where the knee wall meets the floor joist?  (note: soffits are at the top of the energy heal, so I have about 15″ in the joist bay at the outside top plate before I even get to the soffits on the outside wall.

Q. If I end up insulating the slope behind the knee wall, is it okay to insulate the knee wall as an added measure?

Sorry for the long winded questions. I am the homeowner. I’ve GC’d both projects and done a lot of the work, but here in Seattle, it is very hard to find a competent company who understands knee walls. I get a lot of blank stairs so it one of those, “if you want it done right…” scenarios.



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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Can I use Tyvek on the interior side of the batts behind the knee wall for an air barrier?"

    A. Probably, but the final determination belongs with your building inspector, and that determination may depend on the flammability of Tyvek, and whether Tyvek is approved to be left exposed in this type of location.

    Q. "Do I even need to do blocking where the knee wall meets the floor joist?"

    A. No. If you are insulating the sloped roofline down to the soffit, the kneewall is entirely within the home's conditioned space, and there is no need for the blocking you describe. More information here: “Two Ways to Insulate Attic Kneewalls.”

    Q. "If I end up insulating the slope behind the knee wall, is it okay to insulate the knee wall as an added measure?"

    A. It's OK but unnecessary.

  2. DavidfromPNW | | #2


    thanks for the reply. Not sure if I mentioned that the knee wall will have 5/8 firecode dywall to the interior space side. I am having inspector out tomorrow for partial cover approval so I will ask. Any other thoughts are very greatly appreciated.

    Also, I am using 1/2' Rmax (off the shelf from Home Depot) for baffle material. Based on my readings, I am assuming that is good.



  3. Peter Yost | | #3

    Hi David -

    I strongly advise you to have your work either guided by or confirmed with air leakage testing. If you have not done this sort of work before, a blower door test (or two) is your way of making sure you actually connected all the parts and pieces of your continuous air control layer in your roof assembly.

    And I speak from personal experience: I was a builder/remodeler, convinced I understood what air tight meant, and got my head handed to me the first time I put my air sealing efforts to the test, literally the blower door test.


  4. DavidfromPNW | | #4


    Thanks for the advice. I will. That said, although I am the homeowner, I've got an advanced knowledge of residential building. The air sealing is a lot easier than it use to be for sure. My big issue is the information and misinformation on how to manage the knee walls. I've talked to multiple insulation contractors and not one had even mentioned insulating the slope. They all basically say "put faced r15 in the knee wall and drywall it and you're good." So, finding someone here in Seattle that knows any more than me is virtually impossible. It seems the answer to everything is just spray foam it, which I do not want to do.

    What do you usually air seal the inside of the slope behind the knee wall with?

    1. Matthew_M | | #5

      Coincidentally, I'm in a very similar situation as you, giant trusses making up a 2nd floor and dormers, with knee wall areas to deal with, but new const. vs existing.
      I'm using zip sheathing as the primary air barrier on the exterior, wall & roof.
      I don't really want to use spray foam either, but I think it might be the best/simplest solution for these areas.

      Subscribing to this post to see if anyone has more ideas.

  5. DavidfromPNW | | #6

    bumping this up to see if anyone has any more ideas on air sealing the insulation on the slope behind the knee wall. talked with my inspector and he said he was okay with it but felt there had to be a better solution on the market. I see that dupont has a fire retardant tyvek called firecurb but it looks like you can only get it in the UK? I've sent a Whiskey Tango Foxtrot emaIl to dupont to see if I can get some in the US?

    Any additional thoughts for that air barrier on the slope behind the knee wall that still allow vapor to escape would be great. Anyone used this product?

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