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Another slightly different knee wall insulation question

Ben_Brewer | Posted in General Questions on

What about putting rigid foam underneath the fluffy stuff on top of joists behind the knee wall?

My third floor is gutted. I am working to put it back together and then will move to gutting the second floor.

I reviewed other knee wall threads and watched Mike Guertin’s video. Two potential downsides to that approach for me:

1. If I install rigid foam blocking in joist bays below/in line with the knee wall, I will be trying to seal it at the bottom to old keyed plaster and lathe, which wont be an easy seal and which is getting torn out. Also I’m going to install HVAC duct in the joist bays and a couple of the second floor supplies will be under the third floor knee wall space. 

2. I can’t put fluffy insulation in the joist bays, because it’ll all fall out when I gut the second floor, and I need open bays for ductwork. 

What if instead, I put the vertical rigid foam blocking at the end of the joist bays between the top plate of the second floor and the bottom plate of the third floor, and then also put rigid foam (or drywall or plywood) horizontally on the uncovered joists, from where the tongue and groove flooring ends to the newly installed foam blocking at the end of the joist bays. Then I would put the fluffy insulation on top of that. The end goal being that my joist bays are air sealed but empty, and the fluffy insulation is above rather than inside the bays. That way, when I gut the second floor, the knee wall insulation stays put, the dust from the second floor demo can’t go up behind the knee walls, and I have uninterrupted joist bays to install ductwork.

Another question: if I do the above and then also create baffles that run continuously from the bottom of the roof all the way past the knee wall up into the attic above the third floor, do I still need to install vertical rigid foam on the knee walls to encapsulate the fiberglass insulation? I imagine not. I figure if I have to baffle at bottom of the roof and again where it intersects the knee wall, I might as well just connect them and have one continuous baffle…


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Ben,

    If your plan is to insulate in such a way that the attic space behind the kneewall is unconditioned (in other words, you're insulating the kneewalls and floor behind the kneewalls), then you need a continuous air barrier from the walls below, along the ceiling and up the kneewall.

    If you are using fibrous insulation, is best if the air barrier is on the interior side--commonly using the drywall or a vapor retarder along the ceiling and rigid foam insulation, vapor retarder, or drywall on the inside of the kneewall studs. The blocking allows you to connect the air barrier from the ceiling to the kneewall.

    I'm not sure why you can't add the blocking when you have the ceiling below gutted, but maybe I am misunderstanding your process.

    Architects and builders often use the "red pen test" for their air barriers. Draw out your details and then trace the air barrier with a red pen to make sure that it is continuous and that you can identify what is doing the air sealing everywhere along the line. See if you can do that with your assembly. And if you can, then ask, "Is the air barrier really in the ideal location?" You want to do you best to keep conditioned indoor air out of these assemblies.

    And, if you haven't already, check out this article: Insulating Behind Kneewalls

    By the way, where are you located?

    1. Ben_Brewer | | #2

      Hi, Brian.

      Thanks for your response, and for all your insight on the FHB podcasts!

      I think I understand your advice, and it makes sense.

      I live in Pittsburgh.

      Here's my attempt at writing out the red line test. Also attaching am attempt at a drawing. Let me know if this makes sense. Air barriers from the second floor going up:

      2nd floor assembly
      --Dry wall on the exterior wall of the second floor
      --Dry wall on the ceiling of the second floor

      3rd floor assembly
      --Dry wall on the second floor ceiling
      --Vertical rigid foam blocking above the exterior wall of the second floor, between the top plate of the second floor to the bottom plate of the third floor
      --Horizontal rigid foam sheets on top of the floor joists from the rigid foam blocking on the exterior wall into where the third floor tongue and groove flooring starts
      --Dry wall on the knee wall in the third floor
      --Dry wall on the ceiling of the third floor

      Thanks for the help!


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