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Community and Q&A

Insulating kneewalls in Michigan

TIMGABRIEL | Posted in General Questions on

I want to remove all drywall to studs and reinsulatate walls and ceilings properly. I’ve read many of the questions and responses. My questions: 1. On the drywall side, is it necessary to add a vapor barrier? 2. Would insulation be faced or unfaced? 3. For additional cost, would rock wool be worth it? 4. On ceiling, batt insulation between joists then blown in cellulose? 5. Between rafters above knee walks, a one or two inch space with osb or foam board be good for ventilation then rock wool or fiberglass batt? 6. Back side of kneewalls. Would 1/2″ osb be adequate. Would it conduct cold to insulation? 7. I would cut 2x material to put between floor joists and seal with spray foam. Thank you for responding. Great site by the way.

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  1. GBA Editor
    Martin Holladay | | #1

    Before I answer your questions, you should know that it's usually not a good idea to insulate kneewalls. It's far better to insulate the sloping roof in the triangular attic behind the kneewall. To learn why, read this article: “Two Ways to Insulate Attic Kneewalls.”

    Assuming you disregard the advice and insulate your kneewalls, I'll answer your questions.

    Q. "On the drywall side, is it necessary to add a vapor barrier?"

    A. No. What you need is a vapor retarder. Vapor retarder paint or the kraft facing on fiberglass batts would work.

    Q. "Would insulation be faced or unfaced?"

    A. A wide variety of insulation types will work, ranging from cellulose to spray foam to mineral wool to fiberglass. The facing on fiberglass batts is one type of vapor retarder that you may choose to use -- but you don't have to.

    Q. "For additional cost, would rock wool be worth it?"

    A. That depends on your R-value goal and your budget.

    Q. "On ceiling, batt insulation between joists then blown in cellulose?"

    A. Why use two types of insulation? Why not just choose one?

    Q. "Between rafters above knee walks, a one or two inch space with osb or foam board be good for ventilation then rock wool or fiberglass batt?"

    A. Is that a statement or a question? More information in these two articles:

    Site-Built Ventilation Baffles for Roofs

    How to Build an Insulated Cathedral Ceiling

    Q. "Back side of kneewalls. Would 1/2" OSB be adequate?"

    A. What you want is an air barrier. OSB can work as an air barrier, as long as it is installed in an airtight manner (with caulk at the perimeter, and taped seams). Rigid foam might be better, though, because rigid foam has an R-value.

    Q. "Would it conduct cold to insulation?"

    A. A half-inch of OSB has an R-value of 0.6. That's not much.

  2. NormanWB | | #2

    As Martin notes and the referenced article explains, insulating the sloped attic is preferred. However, sometimes you only have a portion of the attic area that will have kneewalls, so some consideration needs to be given to how to handle the transition area from the kneewall encased area and the rest of the attic. Of course, you could insulate the sloped roof for the entire attic, but that can be more costly than say blown cellulose on an air sealed attic floor.

  3. JDuchek24 | | #3

    What's underneath the kneewall room? Cold garage or warm living space?

    - Just blow cellulose in the upper attic. Do you have ducts up there? Can you access the upper attic to air seal if needed?
    - I found rock wool to be really helpful in the sloped ceiling ("rafter") section for a super snug fit. High density or mid density fiberglass batts probably work fine for the vertical wall sections if you insulate there.
    - You definitely want foam board on the attic side of the vertical walls if you choose to keep the insulation layer there. Polyiso is easy to cut, easy to tape and easier to move into cramped spaces. 1in minimum. More is better and not much more expensive (if you can find thicker foam easily). Tape all seams.
    - If really taking everything down to the studs, consider tacking an inch of Polyiso on the interior side of the sloped ceiling sections (i.e. directly to the wood rafters) to mitigate thermal bridging.
    - Stopping air flow underneath the kneewall room is very important. If there's a cold garage underneath the room, then dense pack cellulose would be great in that spot. Consider using foam for the blocking. It's easier to handle.

  4. TIMGABRIEL | | #4

    Thank you for all the responses. Tim

  5. TIMGABRIEL | | #5

    I read through some of the links. I thing I left out: I have two dormers in roof. What effect will this have? Also, with regards to vent channel depth, what is optimal? I've read up to 2 inchs. And finally, is it necessary to run vent channel all way up to ridge vent? Thank you for the help. Tim.

  6. FrankFulton | | #6


    I'll only offer that I somewhat painstakingly considered these kneewall issues in a lengthy thread linked below. It might be helpful for you to read my queries (replete with a few redundant questions) and generous responses from Martin, Dana, and others.

    Bottom line: we're electing to move the thermal boundary. We're using a contractor, and the cost is maybe $2k more than insulating the kneewalls themselves. I'm not sure whether we will see an additional 2k in reduced energy bills (hope to be proven wrong!), but I'm trusting the science and expertise of the GBA and building science communities. Even though we've just moved into the house, I do think another factor is increased future resale from the semi-conditioned storage space (in this case 350sqft).

    Thread here.

    Good luck. Keep us posted.

  7. GBA Editor
    Martin Holladay | | #7

    Q. "I thing I left out: I have two dormers in roof. What effect will this have?"

    A. If the dormers are part of the conditioned space of your house (in other words, if they aren't just decorative dormers in a vented unconditioned attic), you need a plan for air sealing and insulating the thermal boundary surrounding these dormers. The work is tricky, which is why I don't like dormers.

    The thermal barrier of your house needs to be as airtight as possible. It also needs to be continuous -- without interruptions or seams that can leak. Adjacent to the air barrier, you need between R-30 and R-60 insulation (depending on your R-value goals and local code requirements). Achieving this with dormers is tricky.

    Remember, the rules that apply to the insulation of cathedral ceilings also apply to dormer roofs.

    Q. "With regards to vent channel depth, what is optimal? I've read up to 2 inches."

    A. Two inches is best, but 1 inch meets code. For more information on this issue, see Site-Built Ventilation Baffles for Roofs.

    Q. "Is it necessary to run vent channel all way up to ridge vent?"

    A. There must be a clear path all the way to the ridge vent. For a cathedral ceiling, that means you need baffles all the way. If you have a vented unconditioned attic under your ridge vent, the baffles just need to reach the attic -- usually near the floor of the attic, above the insulation layer.

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