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Air sealing an attic kneewall

Mill_house | Posted in General Questions on

I watched the GBA video: Air sealing an attic kneewall
The approach used in the video is opposite to what I am thinking of doing.  My approach: installing batt insulation that extends into the attic past the 2×4 studs – making sure it doesn’t flop over into the attic by adding some kind of bracing – and then adding a 2″ piece of rigid board insulation on the non-attic side of kneewall, followed by drywall.
In the video, it shows batt insulation in the kneewall studs, then a piece of polyisocyanurate foam on the attic side of the kneewall.  Explanation offered in the video is that: “the insulation in the knee walls of an attic is usually uncovered on the  attic side.  That has the potential to degrade the insulation as convective air loops through the insulation.”
Now i’m wondering if the approach I was thinking of using is flawed and needs to be adjusted.  Please advise.  Thanks.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mill House,
    The video is correct. Fiberglass batts don't do much unless they are encapsulated on all sides by an air barrier (drywall, ThermoPly, plywood, OSB, rigid foam, or carefully installed housewrap).

    For more information, see this article: “Two Ways to Insulate Attic Kneewalls.”

  2. Expert Member
    ARMANDO COBO | | #2

    Be aware, in many situations, kneewalls are structural walls and need to have structural sheathing, just like a typical exterior wall. I happen to add tapped exterior rigid foam on all exterior wall assemblies, thus adding tapped rigid foam to kneewalls is a given.

  3. Expert Member
    Dana Dorsett | | #3

    Be sure to install adequate air-barriers for any joist bays that extend from the attic space under the kneewalls and below the conditioned space floors. It can be a real PITA as a retrofit, but necessary- joist bays under the floor can be a HUGE thermal bypass, even if semi-blocked aka "filtered" by fiber insulation that extends from the attic floor to under the kneewall.

    1. Mill_house | | #4

      I must have gotten lucky as the majority of the joist bays have wood blocking in them already. Some bays have about a 1/2 gap between the bottom of the blocking and the plaster board ceiling below, however. For these larger gaps I will use spray foam; for the very small gaps - 1/8" to 1/4" - I will use acoustic sealant.
      If it's necessary to cover the batt insulation in the kneewall by adding - say rigid board on the attic side - do I also need to cover the insulation that will be added in the joist bays of the attic too? If not, why the difference in treatment? Climate Zone 7. Canada.
      Because the batts of insulation on the room side of the kneewall will not have poly over top - as the rigid board on the attic side serves as the air/ vapour barrier - how will the electrical outlets reside in the batts? I'm used to a layer of poly over top and the electrical boxes having the plastic insulation shield with a bead of acoustic sealant connecting the shield to the poly.
      And how close can I bring the batt insulation to the roof rafters that tie into the wall top plate/outside wall of house? All i have in terms of ventilation is mushroom vents and two gable vents on the flat portion of the attic (peak). No soffit ventilation.
      Which brings up my last question: The ceiling on the main floor beneath the kneewall may eventually be removed one day. At that point the seal at the bottom of the blocking will be broken. I'm assuming I will be installing some kind of vapour barrier on the portion of the ceiling that is directly under the attic portion of the kneewall. Once this happens, am I up in the attic again resealing the bottom portion of the blocking to the poly?

  4. GBA Editor
    Martin Holladay | | #5

    Mill,
    Q. "And if it's necessary to cover the batt insulation in the kneewall by adding say rigid board on the attic side, do I also need to cover the insulation that will be added in the joist bays of the attic too? If not, why the difference in treatment?"

    A. Batt or cellulose insulation installed on an attic floor doesn't have a top-side air barrier, and it's true that the lack of a top-side air barrier means that the insulation doesn't perform as well as it might. The reason that we don't solve this problem by installing housewrap above the insulation layer is because there is a less-expensive alternative solution: Just install deeper insulation to improve the thermal performance of the insulation layer.

    Blowing more cellulose is cheaper than adding housewrap, and it raises the R-value enough to improve the performance of the insulation layer.

    1. Expert Member
      Dana Dorsett | | #6

      Rigid foam board would put a vapor retarder on the cold side of the insulation, creating a potential moisture trap. If putting a top-side air barrier on the floor insulation it's best to use something vapor permeable, and NOT foam board.

      Low density R19s & R38s are more like air filters than air-retarders and are pretty lousy without top side air barriers in a cold-side-up orientation. For filling in the gaps & depressions in batts and boosting performance of low density batts, an overblow of 3" or more of cellulose is sufficiently air-retardent to limit convective air transfer between low density fiber insulation and the cold attic, restoring full performance to the batts below.

      1. Mill_house | | #7

        Dana, your answer is confusing me: "Rigid foam board would put a vapor retarder on the cold side of the insulation creating a potential moisture trap." We've moved from air barrier to a vapor barrier discussion. So to clarify for me would the following design be ok:
        Kneewall: from the room side moving towards the attic side, the wall would be assembled like so: drywall, membrain, roxul batt insulation, EPS rigid board.

        Can the rigid board be installed with normal screws or does it require special screws? And do the joints need to be taped? Any details you could provide would be most appreciated.

        1. NormanWB | | #8

          Regular screws will work, as long as they have 1/2" or more of bite, but you need some sort of washer. I used a roll of plastic "tabs" for a pneumatic stapler that were left over from construction.

  5. GBA Editor
    Martin Holladay | | #9

    Mill House,
    For a wall, it's OK to have rigid foam on the exterior of the wall, as long as the rigid foam is thick enough. (The thickness rules are often ignored for kneewalls, however, because the triangular attic behind a kneewall isn't as cold as the outdoors.) For more information, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

    Adding horizontal rigid foam above horizontal fluffy insulation on an attic floor would be a mistake, for the reason Dana explained. It would be hard to get the rigid foam thick enough to follow the rules for the foam-to-fluffy ratio, and it's a risky detail. For insulation on an attic floor, stick to fluffy insulation like cellulose or fiberglass.

    1. Deleted | | #10

      Deleted

    2. Mill_house | | #11

      Martin,

      Thanks. Here’s the plan I've come up with based on what I’ve read/videos I’ve watched. I’m just looking for some reassurance that I’ve got it right. Hoping you can review.

      Climate zone 7/8; unvented roof. Chart you referred me to says R10 foam for 2x4 walls.

      Kneewall (from the inside of the room moving out towards the attic): drywall, roxul batt r14 in the stud bays, 2inches XPS screwed, seams taped or sealed (?) to backside of kneewall. Seal above top plate and below bottom plate. No poly. Can I use membrain?

      Outer attic floor (behind kneewall): roxul batt insulation in between and on top of joists - not allowing insulation to come into contact with roof sheathing. Insulate vertical wall on shed dormer with roxul batt and rigid board. (I can't use fluffy sprayed insulation on the floor because the ceiling below may be removed/replaced one day. At the time I’ll need to keep poly or membrain continuous along the main floor ceiling behind the knee wall area as it could introduce cold air in the floor joist area of the second floor area, right?)

      Sloped ceiling: Add 2x2s to existing rafters for more depth. Spray foam the sloped ceiling from top plate of kneewall to just above collar ties, drywall over. No poly, no membrain.

      Attic ceiling (from the inside of the room moving outwards): drywall, poly or membrain, roxul batts in ceiling joists, additional batts over top of joists.

      I hope this is right...I'll await your blessing.

    3. Ben_Brewer | | #14

      Hey, Martin.

      What about putting rigid foam underneath the fluffy stuff in the floor 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 your comments 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.

      2. I can't put fluffy insulation in the joist bays yet, because it'll all fall out when I gut the second floor.

      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 and the dust from the second floor demo can't go up behind the knee walls.

      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...

      Thanks.

  6. GBA Editor
    Martin Holladay | | #12

    Mill House,
    The only issue I see is that you are choosing to use XPS, a type of rigid foam that most green builders avoid for environmental reasons. Either EPS or polyiso would be a "greener" choice. (For more information on this issue, see "Choosing Rigid Foam."

    1. Mill_house | | #13

      Thank you. I appreciate you bringing the greener choice to my attention.
      Two quick questions I had:
      Kneewall (from the inside of the room moving out towards the attic): No poly. Can I use membrain?
      Join pieces of rigid board with sealant and tape?

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