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Using rigid foam behind Roxul batts in a finished room over garage

kandrews5725 | Posted in Energy Efficiency and Durability on

I have done some searching online and on GBA, but I’m looking for a quick and dirty answer to this question. Would it be adequate to fasten 1″ of EPS rigid foam to the cold side of my attic crawl space behind the knee wall in my over the garage bonus room? The floor is insulated from the outside wall in, and the ceiling and slope is also insulated with Roxul batts. I am not interested in insulating the slope behind the knee wall since I am looking to minimize the total square footage of conditioned space. Insulating the bonus room is only half of the goal, the second half is to make that crawl space behind the knee wall more “user-friendly” so I can store things without having to rub against exposed insulation. Thanks in advance for your help!

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  1. GBA Editor
    Martin Holladay | | #1

    The best approach is always to install the insulation along the roof slope. That's much better than trying to insulate the kneewall. This article explains why: Two Ways to Insulate Attic Kneewalls.

    To answer your question, the required foam thickness depends on your climate zone. The colder your climate, the more important it is to install thick rigid foam in this location (to avoid problems with moisture accumulation in the wall assembly). However, the fact that the back side of the kneewall isn't directly exposed to the weather means that you can bend the rules somewhat and get away with thinner-than-usual foam.

    Here is an article that explains more about minimum foam thickness: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    The real problem with insulating the kneewall is the need to put airtight blocking between the rafters above the kneewall top plate, and airtight blocking between the joists under the kneewall bottom plate. That's a real pain.

  2. kandrews5725 | | #2

    Thanks for you reply Martin! I'm not really looking to completely air seal the room, just a better than average performance. I think the other concern I had was whether or not I would run into any issues with moisture, but I think based on my climate zone being in Maine and using the EPS foam as opposed to XPS foam I should be all set. Thanks again!

  3. GBA Editor
    Martin Holladay | | #3

    You wrote that "I am not interested in insulating the slope behind the knee wall," and also "I'm not really looking to completely air seal the room." But I urge you to pay attention to these issues.

    You need to know where the thermal boundary of your house is. If you have no insulation in the sloped roof assembly behind your kneewall, then your kneewall is part of the thermal boundary of your house. That means that you have to provide blocking between the joists under the kneewall, and blocking between the rafters above the kneewall.

    You may not want to do it -- but you should. Otherwise you are leaving big holes in your home's thermal envelope.

  4. kandrews5725 | | #4

    OK, makes sense. Like I mentioned the floor of the bonus room, or ceiling of the garage, depending on how you look at it, is insulated with the roxul batts as well. It was really just the cold attic space between the bonus room and the outside roof I was concerned with doing properly. Does that give you a better idea with what I was trying to accomplish? I will definitely do my best to block the area where the knee wall top plate meets the slope of the roof.

  5. Expert Member
    Dana Dorsett | | #5

    If the back sides of the kneewalls don't have a continuous air barrier, that " insulation..." is providing at best half it's rated-R, but it doesn't take foam to be that air barrier, though it could be foam, provided it has sufficient R for dew point control for the climate.

    Most of Maine is climate zone 6, some is climate zone 7. In zone 6 in a wall assembly the it would take at least 2" of EPS to meet IRC prescriptives for a 2x4 wall, and it would take 2.5" minimum in zone 7.

    So with the EPS at 1" (R3.85-R4.25 depending on density) you're not exactly "all-set" with R15 rock wool batts. You'd need a class-II vapor retarder on the interior to run with foam that thin.

    Also, if you intend to use it for storage you'll have to install a thermal barrier between the foam and the kneewall-attic from a fire-safety code point of view. In some instances you'd be able to use fire-rated polyisocyanurate, but that's also placing a true vapor barrier on the exterior side of the kneewall, and if there happens to be a polyethylene vapor barrier on the interior side it becomes a moisture trap.

    Retrofit air sealing of kneewall spaces is something of a fool's errand (speaking as one who has played the starring role of "fool" several times in that movie.) It's generally easier /better /cheaper /more reliable overall to insulate & air seal at the roof deck side rather than at the kneewall & floor. That way anything you happen to store is inside of the warmer/drier conditioned space too.

  6. Andrew207 | | #6

    I'm want to create an air barrier or at least reduce the air flow on the attic side of my 2 x 4 knee wall that currently has R13 fiberglass. I was thinking about using 2 inch polyiso, but I'm unsure if anything cheaper than Thermax can be left uncovered because of national fire code. I also am not a fan of foil facing because of how vapor impermeable it is. Can rigid foam be left exposed in a knee wall were to be sealed off? Alternatively, could I use a Roxul ComfortBoard mineral wool? Would the Roxul reduce air flow enough without being a vapor barrier like the polyiso would?

    Also, I tried to update my name, but it still displays as scifijock... I'm Andrew.

  7. GBA Editor
    Martin Holladay | | #7

    Don't worry about the vapor permeance of the rigid foam. Even if it is foil-faced, you won't have problems. You didn't mention your climate zone, but climate zone isn't as important in this application, because the rigid foam won't face outdoor conditions. (The attic isn't quite as cold as the outdoors.) The idea is that the rigid foam keeps the stud spaces warm, so there won't be any cold surfaces to allow condensation.

    When rigid foam faces an inaccessible attic (or a difficult-to-access attic), most code officials will allow the rigid foam to be exposed -- no drywall is necessary.

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