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How to insulate behind a kneewall in a finished attic?

greenskies | Posted in Green Products and Materials on

I have a finished attic in my vented roof home. The vent’s size is about 50-60% of the length of the roof.

The attic level is 27′ wide 40′ long, with the soffits running along the 40′ length. About 6 feet in from the soffits on either side is a vertical finished wall that meets the ceiling. R-11 batts run the width of the home in the 12″ floor joists, and the insulation is also placed in between the rafters from just above the soffit to the vented and non-vented portions of the roof, and also behind the kneewall.

The space desperately needs air sealing and additional insulation as it is responsible for at least 1.5ACH on that level alone.

I got a few quotes with multiple different approaches, (closed cell, foam boards on rafters, blown-in, duct-sealing) but all quotes were too expensive (10k+) for my taste so I am going to do it myself.

I am primarily interested in placing 4″ of foam board behind the kneewall and between the joists below and air sealing with can foam, adding baffles to the rafters to prevent wind washing and also blowing in ~12 inches of cellulose

I am curious though about the viability of placing foam boards on the rafters themselves and air sealing there. I am not sure what the implications of this would be in my open vent design roof… but I also already have batts in the rafters… would I be creating a problem by foamboarding over the rafters from soffit to the top of the knee wall?

Would the baffles not effectively create an air seal between the roof and the attic by themselves anyway? In which case, if I use the foam board on the rafters, do I even need baffles?

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  1. Expert Member
    BILL WICHERS | | #1

    It sounds like both the floor of the attic space AND the roof are insulated, along with the knee wall? You really need to insulated only the roof OR the floor. There is no need to insulate both. Insulating the roof brings the attic into the conditioned space, and can make air sealing easier. Insulating the floor and kneewall requires more careful air sealing but is sometimes easier to insulate since you can use blown insulation.

    If you want the attic to be “outside”, then Using foamboard on the attic side of the knee wall is a good idea. You can use foam board to make baffles near your soffit vents and then blow in enough insulation to reach the R value you want. You’ll need to air seal all the penetrations and leaky spots like joists under the kneewall. There are many good articles about doing this.

    If you want the attic to be “inside”, then you don’t need to insulate the floor or the kneewall, but you do need to insulate the roof area to code. You can use spray foam and not vent it, or you can space the insulation down from the sheathing to provide a ventilation channel the full length of the rafters from soffit to ridge. You can use batts between the rafters and foam board inside to support the batts. There are many ways to hold the insulation in place.

    First thing to do is to decide if you want the attic to be inside or outside you’re houses thermal envelope. Here is an article to help you with that:


  2. greenskies | | #2

    I didn't build the house so I'm not privy to why they insulated every surface, but you are correct, every space between joists, studs and rafters has batt insulation.

    I have duct work in the attic space so id like to have that conditioned if possible.

    I have batts between the rafters currently. Can I just put foam board on top of the rafters, air seal, and call it a day?

    I would probably still take steps to air seal and insulate in the attic anyway though.

    1. Expert Member
      BILL WICHERS | | #3

      If you have duct work in the attic that you can’t easily move, then it makes sense to condition the attic and insulate the roof. You need to make sure you have proper venting, which includes a vent channel above the batts. If your attic is accessible, you’ll need to cover the exposed foam board for fire protection unless you use a type that is approved to be left exposed (like Dow Thermax).


      1. greenskies | | #4

        Maybe half of the rafter space w/insulation is covered with drywall, it was my plan to tack some baffles together and slide them between the sheathing and batts and slide it up towards the vent...

        Do I need baffles where there is no vent connection?

        Thanks for the tip about Dow Thermax

    1. greenskies | | #6

      This is pretty much exactly what it looks like yeah. The interior side is all Sheetrock for me.

      As I mentioned to Zephr7, all the spaces (rafters, joists, and stud-space) are filled with batts, there is no blocking, baffles, foam, blown-in or anything else in that space aside from batts.

      The batts appear to run from ridge to soffit in every rafter.

      I'm was planning to do foam blocking, and foam boards behind the studs/batts on the kneewall, and foam all intrusions in the ceiling below, and do blown-in. Obviously doing foam board on the rafters is more economical, I just don't want to impact the way my house is breathing.

      To help vent I was planning on also sliding baffles behind the batts that are behind the drywall and up through to the ridge vent, and down to the soffit, but it occurred to me that just air sealing the foam board would have the same effect but with less air restriction and more economical.

      I more or less want to know the viability of putting foam board sheet across the rafters, and if that will affect my venting situation in a negative way.

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