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Community and Q&A

Advice on knee wall attic insulation

bHqUcamKme | Posted in GBA Pro Help on

I am looking for help giving advice to a homeowner where I performed an energy audit yesterday. The home is a 1 ½ story home with knee walls on the south side of the second story. When I accessed the knee wall attic, I discovered that both the roof deck AND the knee walls were insulated with fiberglass batts. The knee wall cavities were filled with unfaced R13 batts and the roof rafters were filled with R19 faced batts, although several of the batts had come unattached and are lying on the floor of the knee wall. The floor joists below the knee wall are filled with unfaced batts (6”).

At first I was puzzled by why this system was installed. I think it was done because there are several heating/cooling ducts running in the area on the floor of the knee wall attic and although the ducts are insulated, perhaps they wanted to make it warmer in the attic to avoid loss of energy through the duct.
What advice should I give the homeowner in my audit report? My first inclination is to remove the batts from the roof deck, increase the knee wall insulation, and blow cellulose on the floor of the knee wall attic to R49. Is there another approach I should advise?

We are in Zone 6 (northern Iowa). I have attached a photo. Thanks in advance for your help!

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  1. Riversong | | #1

    The critical issues are the location and integrity of the air barrier and whether the roof is vented or should be.

    If there is a well-defined air barrier following the upper roof slope to the knee walls and down into the floor joists (sealed blocking), and the lower slope insulation is not blocking roof ventilation, then there's no harm in leaving the lower slope insulation (or repairing it as needed). If it keeps the kneewall crawl spaces warmer to lower duct losses, that's a good thing - as long as the ducts are well sealed, since they are outside of the air barrier (perhaps?).

    Insulating just the kneewall spaces to R-49 would not be cost-effective unless you could bring the entire "hat" of the house up to something approaching that level.

  2. GBA Editor
    Martin Holladay | | #2

    The double insulation does no harm, and is possibly doing some good.

    But as I'm sure you are aware, this kneewall area is a mess. If the homeowners can afford the work, this is what should be done:

    - Do some exploration to investigate the integrity of the air barrier. To do this work, you'll need to choose where the air barrier is, and that isn't clear. If the investigation shows defects -- and I predict it will -- then air sealing work will be necessary. That may include air sealing in the joist bays beneath the kneewall bottom plate, if that is where the air barrier will be located.

    - Either the sagging fiberglass batts need to be put back in place and some mechanism devised to keep them there, or all of the fiberglass batts need to go to the dumpster so that new insulation can be installed.

    - The question of roof ventilation needs to be investigated. Are ventilation chutes present? Is the ventilation continuous? Do you want ventilation? Once these questions are answered, remedial work may be required.

  3. Riversong | | #3

    Not to mention that the batts are two different colors. Color matching is the key to good thermal performance ;-)

  4. Danny Kelly | | #4

    In addition to the color matching, if you decide to leave the yellow stuff in the knee wall, after blocking between the joists, "back up" the knee walls and seal all the joints. Check to see if there is a top plate for the knee wall - need to air seal the top as well. To stay on a budget, back up with thermoply - to increase the R-Value of the knee wall use Thermax.

    Trying to think if changing the conditions will have any unintended consequences with the unusal "double thermal boundary" you have there. I think as long as there is proper ventilation - everything should be ok.

  5. bHqUcamKme | | #5

    It sounds like there's no harm with leaving the insulation in the sloped roof (after repair and securing it better).

    I like the idea of backing up the knee walls with XPS or Thermax, but the access to that area is very limited...they'd either have to cut the foam into small strips to fit through the 30" high access panel to the knee wall attic or break out some of the wall to get back there.

    How about using Tyvek over the knee wall and sealing all the joints? Would that give him the air barrier he needs?

  6. GBA Editor
    Martin Holladay | | #6

    It's unlikely that you will be able to establish a decent air barrier with Tyvek.

    If you can't install solid panels at the back of the kneewall, then it will probably be necessary to perform your air sealing at the drywall layer on the other side of the wall.

  7. bHqUcamKme | | #7

    The blower door test showed 1,550 CFM. House is 2462 sq ft conditioned, volume is 18,146 cu ft, so it appears that they are close to their building tightness limit. I'm not sure they need to do a lot of air sealing unless they install a mechanical ventilation system.

  8. Riversong | | #8

    That's 5 ACH50 - not very tight at all.

    With fiberglass insulation in the attic, it's imperative to have a sound air barrier to prevent mold and rot, as well as to save energy.

  9. Danny Kelly | | #9

    Curtis - Backing up of the knee walls will not really affect the blower door numbers/ACH if there is a good air barrier on the conditioned side of the fiberglass. The purpose of backing up the insulation is to prevent the cold air from the attic infiltrating the fiberglass reducing its R-Value. Blocking between the joist under the knee wall should tighten up the house some though - this will prevent cold air running through the floor system. If you can't get rigid foam in there, the thermoply can bend and fold a little bit so you can get larger sheets in.
    Tyvek may do an ok job at first but long term will pull away from its fasteners due to positive/negative pressures - that is why all air barriers must be rigid.

  10. bHqUcamKme | | #10

    I know this thread is a few weeks old, but I'm hoping some of you will see this follow-up question. Could the homeowner fill the entire knee wall attic area with cellulose (after re-attaching and securing the batts between the rafters)? Since their is no ventilation in that area anyway because the rafters are insulated, I'm thinking this would be a way to increase his total R value above R49, and would slow air movement. Thoughts?

  11. GBA Editor
    Martin Holladay | | #11

    I know that some weatherization agencies use your suggested technique -- just fill up the whole triangle with cellulose. Cellulose is relatively cheap.

    I don't have enough experience with the technique to know whether there have been any problems in homes insulated this way.

  12. bHqUcamKme | | #12

    What about removing the fiberglass from the roof deck and spray foaming it?

  13. 2tePuaao2B | | #13

    This is the best way to deal with your situation without worries provided you will have no reason to access the area in the future. We have a cape cod that was full-filled in the kneewall area with cellulose 9 years ago. Huge difference with no problems.
    Foam would be more expensive and there could be problems, health related and other depending on the type of roof system.

  14. bHqUcamKme | | #14

    Another option I"m thinking about...
    1. Remove fiberglass from between rafters
    2. Secure 2" XPS board to the rafters and dense pack cellulose between XPS and the roof deck
    The total R value will only get to R30 and he would need R49. So where do we get the rest?

  15. hkUpvzbk4A | | #15

    Here's what I'd do. I would use thermoply or "bubble wrap" (secured properly) and enclose the attic knee wall insulation, making sure there is a top plate. I would leave the insulation in the rafters (if doing no harm) and create an air barrier there over the batts, in hopes of capturing some heat. This is all assuming you have good ventilation between the knee wall and rafters. I would leave the batts over top of the duct work, so long as they weren't compressing any insulation around the ducts. Why not blow fiberglass or cellulose over top and around all of that to attain an average of R-49. Should you seal the ducts before blowing insulation over top? Checking to see that the floor system is not open under the knee walls will also be a benefit for comfort and savings. Like someone else said using a rigid foam board or t-ply between the joists picture framed with a 2 part foam can is ideal. Yes I summarized what others suggested, but this is what I would do.

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