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I need help insulating a kneewall

trguitar | Posted in Energy Efficiency and Durability on

Hello All.

I recently had an energy audit done, and, not surprisingly, big issues were found with the knee walls on the second floor. This is a “contemporary” Cape in Massachusetts.

I’m in the middle of rectifying the problems. Here is a description of the issues.

The knee walls themselves have zero insulation, except for one room where they have R19 in the wall. The floor of the knee wall space has what looks like R13, except for this one room where it looks like R30. There are no ventilation baffles on the sloped ceilings at all. There are soffit vents, and a ridge vent in the top attic, as well as gable vents in the top attic with thermostatically controlled fans. No gable vents in the knee wall space. The rafters are 2 x 8, 16″ on center.

The sloped ceiling on the north side has R19 fiberglass. Half of this knee wall is 7 feet tall, and the rest is about 3 feet tall. There is a skylight in the sloped ceiling. For the 7′ wall part I basically have full access to the sloped ceiling, but for the rest only a few feet. This rest makes up cathedral ceiling in the house.

The sloped ceiling on the south side has what looks like R11 on top of R13. This wall is only 21″ tall, and there is a dormer, as well as a skylight in the sloped ceiling. I have access to only a few feet of the ceiling, and the rest is cathedral in the house.

The thermal boundary is clearly ambiguous.

What’s the best way to go about solving this?

How is this for a plan? (I realize this continues the thermal ambiguity.)

1) insulate the knee walls themselves with R13 covered with radiant barrier, and air seal under the knee wall
2) put R30 in the floor of the knee wall attic space
3) leave the sloped ceiling insulation and install ventilation baffles behind it as best I can. I’m not sure I can get the baffles all the way up to upper attic because of the cathedral part of the house. On the 7′ part of the knee wall I definitely can, though.

The thermal boundary is already ambiguous because there is insulation on the sloped ceiling, floor, and knee wall.

Or is it better to try to get closed cell foam on the sloped ceiling? Can I really get the desired R-value with 2 x 8’s all the way up through the cathedral (and skylights)?

Thoughts, comments?

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  1. GBA Editor
    Martin Holladay | | #1

    This is a big job, and is tough for many reasons. To do a good job may require opening up some existing ceilings (removing drywall).

    First, unplug the thermostatically controlled attic fans, or find a way to disable them permanently.

    Second, you want the thermal boundary to follow the sloped roof -- from the top plate near the soffit all the way to the upper attic. You don't need any insulation in the kneewalls. This insulated sloped roof can be either vented or unvented.

    Here's a link to an article about insulating sloped roofs in homes homes with kneewalls: Two ways to insulate attic kneewalls.

    Here's a link to an article about insulated sloped roofs, whether vented or unvented: How to Build an Insulated Cathedral Ceiling.

  2. trguitar | | #2

    Martin, thanks for the links.

    One other issue, I'd love some feedback on.

    There is a raised top plate at the eaves. Does the whole blocking lumber need to be air sealed at the perimeter, or just the bottom of it, or the bottom and top?

    I don't think there is any rigid insulation on the exterior side of the blocking lumber. Should I seal it like a basement rim joist with 2" rigid foam, and canned foam around the entire perimeter?


  3. GBA Editor
    Martin Holladay | | #3

    Q. "Does the whole blocking lumber need to be air sealed at the perimeter, or just the bottom of it, or the bottom and top?"

    A. If the blocking consists of a stack of 2x4 or 2x6 plates, then you need to air seal the blocking and insulate the area, just like a rim joist. All cracks need to be air sealed.

    Q. "I don't think there is any rigid insulation on the exterior side of the blocking lumber. Should I seal it like a basement rim joist with 2 in. rigid foam, and canned foam around the entire perimeter?"

    A. That would work.

  4. trguitar | | #4

    Thanks for the response, Martin.

    The blocking is a 2 x 10 band joist. No stacked lumber.

    If I decide to insulate the attic floor right up to the band joist, leaving the attic knee wall space outside the thermal boundary as in the link you gave, is it a requirement to have rigid insulation (XPS) attached to the band joist and the perimeter sealed with canned foam (like a basement rim joist)? Or would it be sufficient to just seal the band joist with no rigid foam?


  5. GBA Editor
    Martin Holladay | | #5

    If you want to leave the triangular attic behind the kneewall outside of the home's thermal envelope:

    1. You need to seal the rim joist, but you don't necessarily need rigid foam. The insulation installed between the floor joists is sufficient (as long as it is deep enough to meet minimum code R-value requirements).

    2. The trickiest step (if you follow this method) is installing blocking between the floor joists under the kneewall bottom plate, as well as blocking between the rafters above the kneewall top plate. Each piece of blocking needs to be air-sealed at the perimeter.

  6. trguitar | | #6

    Thanks for the responses, Martin.

    Now I need to digest this.


  7. Expert Member
    Dana Dorsett | | #7

    Mind you, retrofit air sealing the attic spaces behind kneewalls is a fool's errand (speaking as one who has played the starring role of "fool" in that movie multiple times. :-) ). With enough time and effort you can eventually get there, it's not going to be cheaper or better than other methods, and if you have access ports to the space you have ready-made leak points to fail in the future.

    From a "time is money" point of view it's easier to get it right insulating at the roof deck and bringing the mini-attics inside of conditioned space, and it makes the pressure & thermal boundary of the envelope less ambiguous.

  8. trguitar | | #8

    Well, my eyes are wide open now. :)

    I've come across another issue that I'd like to run by you.

    While inspecting the knee wall area on the south side of the house I notice that a portion of the attic floor is actually outside. This is above the front door area, where the front door is recessed in about 9". So, the area of the knee wall attic floor that is outside is about 4' by 9".

    Does anything special need to be done here, even if I end up making the sloped roof the thermal boundary?

    There's a top plate here, that should probably be air sealed. But, do I need to treat this like a cantilevered floor?

  9. trguitar | | #9

    So, in thinking about this a bit more, I guess the question I'm asking is does rigid foam need to be used on the knee wall attic floor space in addition to the insulation that will go down (either for the band joist, or for the whole floor, depending on how I decide to do it)?

    I see in the article on insulating cold floors that rigid foam is used on the part of the floor over the cold space, so my question is does it need to be used for this 4' x 9" area? Is it important?

    From the article:

    "It’s always a good idea to install a continuous layer of rigid foam on the underside of the floor joists, especially if the joist bays are insulated with fiberglass batts or cellulose. Rigid foam stops thermal bridging through the floor joists and helps with air-sealing, especially if the perimeter of each piece of foam is sealed with caulk or high-quality tape."

  10. GBA Editor
    Martin Holladay | | #10

    1. Ideally, you will install your insulation in the sloped roof assembly above the triangular attic -- not in the floor below the triangular attic.

    2. If you insist on keeping the triangular attic outside of your thermal envelope -- a difficult approach -- then the insulation on the floor of the triangular attic is detailed the same way as the insulation on the floor of any attic. This is not really "floor insulation." It's "ceiling insulation." You are preventing heat loss from the heated room on the first floor -- so you are insulating the ceiling of the first floor.

  11. trguitar | | #11


    I understand. But, either way there will be insulation at the band joist area of the floor, as shown in your article, right? In my situation this is the area that is "outside". For a 4' x 9" area the band joist and the knee wall attic floor are outside.

    My question is, since there is an area of this "ceiling" that is outside, or "cantilevered" if you will, where insulation would go in either knee wall method, do the concepts of insulating a cold floor need to be applied here?

    I'm hearing you say that trying to prevent any thermal bridging doesn't matter in this case because it's not a floor. I should be thinking about this as a ceiling, so I don't need to worry about anything here.

    I just want to make sure I'm doing the right thing here.

  12. GBA Editor
    Martin Holladay | | #12

    I wasn't aware that the floor of the triangular attic is cantilevered over cold space. If it is truly cantilevered over cold space, and if you insist on keeping the triangular attic outside of the thermal envelope, then there is no need to insulate the floor of the attic in any way -- right? During the winter, it's cold underneath the floor, and it's cold above the floor -- so why install any insulation?

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