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Community and Q&A

Kneewall and radiant barrier?

cmcgillawee | Posted in Energy Efficiency and Durability on

I recently bought a house in Jacksonville, FL. Part of the attic is finished into a loft with a knee-wall construction. It got quite in the loft last summer relative to the rest of the house, and I’m looking at ways to cool it down a bit.

The existing knee wall is drywalled and filled with fiberglass batts, which are in good shape, but they do not have any additional barrier between the attic space. My research suggests one option I have is to install a radiant barrier overtop of the batts on the backside of the knee wall. I’m questioning the logic because ceilings typically only have insulation and no air or reflective barrier overtop.

Can someone please explain if the reflective barrier will work, and if so, why this isn’t done over ceilings as well?


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  1. Expert Member
    Dana Dorsett | | #1

    Without some sort of air barrier on the back sides of vertically installed low to mid-density batts their effectiveness is cut by about 1/3.

    Rather than radiant barrier as that air-barrier for the kneewalls you'll get a bit more performance out of 1" rigid polyiscynaurate foam board, which would roughly double the performance of that kneewall without the back-side air-barrier, and would reduce the peak gains a bit. But simply covering the back side with housewrap would also be a noticeable improvement, and almost as good as radiant barrier.

    Depending on how the kneewall spaces are vented to the exterior (if at all), and the amount of insualation under the floor in that kneewalled mini-attic you may be able to get some benefit from radiant barrier on the under side of the rafters, but if there's at least R30 under the floor, probably not enough to make it worthwhile. If installing RB on the underside of the rafters, use a perforated aluminized fabric type, not a solid foil or aluminized plastic/bubblepack type. It needs to be vapor permeable.

    If there's at least R30 of horizontal insulation in the attic the benefit of RB on top is miniscule, and in your climate a top side air barrier on horizontal fiber insulation is also of limited value. If it's only low density R19- R25 batts or something you'd be better off spending the radiant barrier money on adding 3-5" of blown cellulose on top of the existing fiber to bring it into the mid-30s total. Adding cellulose is preferable to additional fiberglass since it's opaque to infra-red, whereas fiberglass is somewhat translucent. Blown is preferable to batts, since it leaves no gaps.

  2. cmcgillawee | | #2

    Wow thank you for the detailed response.

    The rigid foam is definitely ideal but the issue is access, which is limited. The area is rather small so I'm happy to pay a premium for the radiant barrier if there is even a small benefit.

    You mentioned house wrap, which has some moisture features that differ it from simple plastic. Are there any moisture-related issues with any of the posed solutions? Also, does cellulose have any moisture problems given the high humidity in FL?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "You mentioned housewrap, which has some moisture features that differ it from simple plastic. Are there any moisture-related issues with any of the posed solutions?"

    A. The housewrap is an effective air barrier, as long as it is carefully installed. You definitely need an air barrier on the exterior side of your kneewall, so housewrap is one of many options (although clearly an inferior option to rigid foam). Neither housewrap nor rigid foam will cause any moisture-related issues in your climate, but I would avoid using polyethylene in this location.

    Q. "Does cellulose have any moisture problems given the high humidity in FL?"

    A. No.

    Here is a link to an article with more information on kneewalls: Two ways to insulate attic kneewalls.

    Here is a link to an article with more information on radiant barriers: Radiant Barriers: A Solution in Search of a Problem.

  4. Expert Member
    Dana Dorsett | | #4

    It's often difficult to get rigid foam into kneewall crawlspaces. Sometimes it's possible to cut the facer and foam on one side to fold it up to squeeze it through the hatch spaces and tape the cut seam once it's up on the wall. It's definitely easier to get flexible thin sheet goods in there, and that's good enough.

    The marginal improvement of installing RB on the kneewalls is pretty tiny. If cost isn't an issue, installing it on the underside of the rafters squeezes more benefit out of it than putting it on the back side of the kneewall, since it'll lower the peak air temps in the mini-attic. But even there the benefits are pretty small for an insulated attic, ESPECIALLY if the attic space isn't being used for ductwork, or any ductwork up there is insulated. See:

    If you look at the second table on p.6 under the "No ducts" and "Code-level attic insulation" right-most column you'll see that even in (hotter than Jacksonville) Miami a 1500' attic would only be saving about $15 / year by adding radiant barrier. In (cooler than Jacksonville) Atlanta it would be saving $10. And that's for a much amount bigger area than you're probably looking at.

    The problem with RB isn't that it doesn't work, but that it's very low bang/buck except in extremely low insulation non-retrofittable situations. You'd be better off spending the RB money to offset the cost of shading the roof deck with photovoltaic panels which would simultaneously lower the cooling load, and offset cooling power use for the load that remained.

  5. cmcgillawee | | #5

    Dana & Martin - thanks again to you both. Your time and attention here is greatly appreciated.

    I'm going to look into whether I can squeeze some rigid insulation in there; however, I understand that this system would need to be sealed tight. I'd do my best but there will be some areas that I inevitably can't seal properly due to restricted access. How important is the sealing aspect in my climate?

  6. GBA Editor
    Martin Holladay | | #6

    The better the job you can manage, the better the results you can expect. If you can't install an air barrier on 10% or 20% of the kneewall, you'll still get some benefit from your work -- just not quite as much benefit as you would have gotten if you could have done a perfect job.

  7. cmcgillawee | | #7

    Sorry I should have phrased my question differently. Let's say I'm somewhere between a well-ventilated situation (which I'm currently in) and a perfectly sealed system. For example, this would happen if I couldn't seal the seams of the foam boards. Is there any moisture-related issues to worry about in this scenario?

    FYI, I'm pretty sure there's no issue here. The reason I ask is I've just moved from a northern climate and have residual paranoia about gaps in the building envelope that lead to condensation/mold issues if not well ventilated.

  8. GBA Editor
    Martin Holladay | | #8

    Q. "Is there any moisture-related issues to worry about in this scenario?"

    A. No.

  9. jchas | | #9

    The most common source of hot attics I've fixed does not have to do with knee wall insulation.

    Instead, often the joist cavities under the top floor living space have been left open to the side attics. This lets unconditioned air freely flow beneath the (uninsulated) floor of your attic room.

    Take a look to make sure that your knee walls have been extended right down to the ceiling of the floor below. If not, install plywood or foam blocking between every joist right under the kneewalls and air-seal the edges of the blocking with spray foam.

    On my house, I dropped the summertime temperature of our attic room by 15F by adding blocking and nothing else.

  10. GBA Editor
    Martin Holladay | | #10

    Good advice. The necessary blocking is shown in the illustration accompanying the article I linked to. I'll reproduce the image below.


  11. BobHr | | #11

    I think I would place a house warp over the existing insulation and work to get it to be an air barrier. I would then cut rigid insulation in strips that I could get into the space, If you have nice clean cuts and can butt them tightly and put on more than 1 layer you shouldnt have a problem using cut up sheets.

    It is not where your air barrier is but rather will it truly be an air barrier. But you want an air barrier on the outside of fiberglass insulation In retrofit work you have to compromise and get the best system you can,

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