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Community and Q&A

Can you solve these two kneewall problems?

FrankFulton | Posted in General Questions on

As part of a moderate (and in places complex) retrofit, we are nearing completion of moving the thermal boundary on a 50′ kneewall. We inserted 6″ of EPS between the rafters (leaving a 1″ air vent) and then sheathed 2″ of RMax to address thermal bridging. In the slopes leading to the vented top attic, we continued the 1″ vent behind 4″ RMax. Photos attached.

While testing results of the project, we found two problems. The installers are standing by their work and returning to fix the issues. I’d value the input of this thoughtful community. How would you solve these problems?

1. How would you seal and insulate floor above an unconditioned front porch and associated open joist bays?

The front portion of the kneewall is above an unconditioned front porch. Because of installer error, it was misjudged where the porch ends and conditioned space begins. Thus, the vertical foam board in the attached photo wall actually encapsulates an unconditioned “floor” (ie, porch ceiling), and these bays remain wide open. As a solution, we have discussed using an “onion bag technique” to dense pack this area, which would hopefully air seal the joist bays and also insulate the “floor.” One of the crew also asked about using rigid foam board to insulate the floor and air seal the bays.

2. How would you eliminate air infiltration behind the foam board in the 4′ slopes leading to the top attic?

These sloped ceilings feel colder to the touch, and infrared camera shows a >7 degree difference, relative to adjacent interior walls, in some areas. Thus, the slopes might be R0 in places, rather than R26. I believe the leak originates in the area right above the vertical kneewall, where the top of the “kneewall” foam board meets the bottom of the “slope” foam board. (Unfortunately, the installers completed the kneewall portion of the project first, and then sought to insulate the slopes from the top attic, using a 4’ foam gun and placing the foam boards from above.) To solve this problem, my two thoughts are: 1) cut into the foam board at the top of the kneewall and do a better job spray foaming the space where the foam boards meet, then try to “clean this up” cosmetically. Or, 2) work from the top attic, either removing the foam boards, reinserting them as baffles and dense packing beneath them, or some other approach. This would be much more labor intensive.

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  1. FrankFulton | | #1

    Photos. I'm not sure how to rearrange/flip these to make them easier to view.
    1. Overall kneewall space
    2. Floorboard removed - note where porch ends
    3. Joist bay leading to porch (there is eventually a soffit/board)
    4. Top of kneewall intersection w/slope leading to top attic

  2. CMObuilds | | #2

    Ive usually done an onion bag plug as in any house like this their is usually nothing on the exterior edge of the floor joists. In your case a plug back to the lower level exterior wall and then a closed floor dense pack to the foam board. If the porch continues beyond the bottom of your foam board you would use 2 plugs.

    Or tear the floor up and use spray foam or foam board.

    The exterior wall will always be colder than an interior wall, depends on outside temp at the time of the scan. are you sure part of the additional temp difference isnt due to the short circuit and the knee wall attic space being colder than the interior rooms? Also your scanning a very reflective surface, did you put tape on the surface to get an accurate temp, you wont get a reliable temp scanning foil faced board.

  3. FrankFulton | | #3

    T, Thank you, appreciate your input.

    1. "In your case a plug back to the lower level exterior wall and then a closed floor dense pack to the foam board. If the porch continues beyond the bottom of your foam board you would use 2 plugs."

    Can you please explain both options in a bit more detail? And, which do you recommend?

    a. What do you mean use two plugs for onion bag? My thought was, insert onion bag into the floor joist area/under lifted floor board (as in photo) and fill it OUTWARD - extending just beyond the bottom of the foam board makeshift wall. (The porch does continue 3-4 more feet beyond makeshift wall). Then, maybe insert a foam board as backing on the inside end of the onion bag. What are you suggesting?

    b. We could lift the floor boards. Based on what you see here, what would need to be completed? Block the joist bays (like them were rim joists) and seal foam board to the floor/porch ceiling? Or?

    2. Good suggestion re: using tape on reflective surface. In this instance we took the temp on the inside of the drywall, from the inside of a closet with a sloped ceiling. Hopefully, you are correct that all s "OK." How else would you test for air infiltration behind the foam board in the sloped ceilings?

  4. CMObuilds | | #4

    You mentioned the porch ceiling is under the wood floor boards, to the inside and beyond your sloped ceiling foamboard. So where is the lower exterior wall?

    The cavity from the inside face of your lower exterior wall, to the exterior face of your knee wall space foamboard insulation is your short circuit and needs to be insulated.

    You dont want to fill the entire floor cavity where it becomes the ceiling of your main floor so you need a plug or backstop if you are going to dense pack. You also dont want to insulate your porch ceiling where it goes beyond the point of your sloped ceiling foam so you install plug number two and dense pack in between the two plugs.

    This is my guess based on what Im reading to provide a continous barrier, maybe I am misunderstanding your layout below the floor boards.

    Test for air infiltration or quality of seal of the newly installed foam board with a blower door and one or combination of 1. Your hand, 2 smoke stick 3 thermal imager 4 some other indicator of air movement.

  5. FrankFulton | | #5

    Very helpful. Thanks. You are understanding correctly.

    1. Would an "onion bag" serve the purpose of the backstops you describe, to dense pack over the floor space and air seal the joist bay? Or do we need both an onion bag AND a foam board?

    2. With the blower door running, where would you look for air? From the top attic, near the top of the kneewall? Someplace else?

  6. GBA Editor
    Martin Holladay | | #6

    T. Carlson is providing good advice. The risk here is, indeed, "short circuits" -- hidden airway paths that defeat the insulation. It seems to me that lifting the floor boards and installing rigid foam blocking under the kneewall would be the best approach -- but cellulose "plugs" using the bag technique (we've always called them grain bags, not onion bags) can provide a fairly good air seal.

  7. FrankFulton | | #7

    Can you please clarify where/how you would place the foam board? If we block vertically under the makeshift vertical wall, the floor/porch ceiling will remain unsealed and uninsulated. Thus, are you suggesting that we block vertically at the far end of the bay (under the makeshift vertical
    wall) AND also lay 3’ foam boards flat along the unconditioned floor/ceiling? I assume we’d want to use canned spray foam to air seal the heck out of all edges, and then cover the foam board witb cellulose prior to replacing the floor boards?

    Or, are you suggestinganother approach?

  8. CMObuilds | | #8

    I haven't had much luck dense packing against a piece of vertical foam board in a floor cavity, Ive snapped the foam board in half from pressure once dense pack starts.

    Dense pack does a pretty good job slowing airflow. Sealed foam board performs better if you have access, deciding factor for me is ease of access taking into account labor hours as well.

    Emerson, I think you got it in #7. Think of it this way, your inside space needs to wear a coat, define inside and outside and make sure there is continuous insulation between the two.

  9. FrankFulton | | #9

    T, Thanks. To be clear, this would be the suggested approach:

    1. Remove floor boards
    2. Block vertically at the far end of the bay (under the makeshift vertical
    wall), to create a continuous barrier between inside and outside
    3. Lay 3’ foam boards flat along the unconditioned floor/ceiling
    4. Use canned spray foam to air seal the heck out of all edges along all edges of the horizontal foam boards.
    5. Cover the foam board w/cellulose - this would increase R value to R25 (2" foam board=R10, 5+" cellulose=R18) in this 50sqft floor space
    6. Replace floor boards

    Any other suggestions to improve results?

  10. FrankFulton | | #10

    re: possible air infiltration behind foam boards in sloped ceilings
    2. With the blower door running, where would you look for air? Near the seam at the top of the kneewall? Someplace else?

  11. CMObuilds | | #11

    #9 sounds good.

    You look for air leakage all over the exterior surface, particularly at edges, penetrations, changes in direction, corners, tricky areas such as your porch issue. It will not be perfect, you should quantify the total house leakage, and if unacceptable look for major leakage paths as well as checking new work which should be nice and tight compared to old existing areas. Thermal camera is the fastest way to scan for temp anomolies that can indicate leakage or missing insulation. Can also scan exterior.

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