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How to handle slanted wall insulation with kneewall and top attic sprayfoam

Therealbobvilla | Posted in General Questions on

I have a 1920’s house in the high-desert of climate zone 5-dry, with a hodge-podge of different types of insulation: vermiculite, fiberglass, ridged foam, etc… I’d like to spray foam the attic and remove all this old stuff. The kneewall attic has a hardwood floor that is continuous with the living space. All the walls are still lathe and plaster. 

I asked a spray foam contractor to take a look and he recommends spraying inside the roof of the top attic and kneewall attic, then blowing fiberglass in the slanted ceiling between the two. My concern is this fiberglass portion against the roof without a vent. Is there a moisture problem there? 

I would love to not take the lathe and plaster down on the slanted ceiling, but if that has to happen to prevent future problems I’ll just have to do it. I’ll have to take the wife to a fancy restaurant.

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  1. GBA Editor
    Martin Holladay | | #1

    Bob Vila,
    You can't blow fiberglass in the sloped section without first installing ventilation baffles. For more information on this issue, see this article: "Sliding Insulation Between Rafters From Above."

    For an overview of these issues, see these two articles:

    "How to Build an Insulated Cathedral Ceiling"

    "Insulating a Cape Cod House"

  2. Therealbobvilla | | #2

    Thanks Martin,

    This does make me nervous because the contractor intends to seal the rafters at the top of the kneewall and in the top attic, leaving the slanted part a cave with no airflow in or out.

    1. GBA Editor
      Martin Holladay | | #4

      It's OK to use an unvented approach for you sloped ceiling assembly. But you'll need to use closed-cell spray foam, not fiberglass insulation, against the underside of the roof sheathing if you want to avoid moisture problems.

  3. kjmass1 | | #3

    This sounds similar to my finished attic in my 1940s home. We spray foamed the roof but since we had torn down the ceiling from the master below, we had access to get up and behind the knee wall. I'd cut out a section to allow them access for the spray foam. Patching up a wall is the easy part.


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