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Hip ceiling, kneewall insulation help

wineyarders | Posted in Energy Efficiency and Durability on


First off thanks for the extremely informative site. I’m picking up lots of information, but looking for advice.

I have a 1930 Bungalow, Zone 5 (coastal Massachusetts). Vented roof with 8/12 pitch, asphalt shingles (good condition), felt and plank sheathing, 2×6 rafters.

The home has a 12×25 unfinished room above a screened porch, spanning the width of one side of the house. Both sides of the room have hip ceilings, and the facing wall is also vaulted, but includes a gabled dormer. The peak of the house starts at the back wall of the room, so not hard to seal off the vented attic space, but I was planning to keep the vaulted space open as much as possible. I was also planning to put in knee walls that would allow access to the perimeter.

I want to finish this space as a 3rd bed, but after starting to research insulation, seems I have some challenges given the hip ceilings, cold floor, and knee walls. The other 2nd floor rooms have similar layout with hip ceiling and knee walls. An energy company had blown in cellulose several years back for previous owners between the rafters from the attic but I don’t think there is any venting. The knee walls also have blown in on the exterior walls with a heavy felt vapor barrier, again I don’t think there is venting. I don’t think the same approach as the other rooms would meet code and from what I’ve read not sure it’s the best approach.

Looking for recommendations on how tackle insulating the walls/ceiling of this room. This project will be DIY, although if closed cell is needed I would go out for that. Was going to try and take advantage of a Roxul r23 bulk sale, but not sure if that material is well suited to anything but the cold floor. Any thoughts?

Thank you for any advice,


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  1. GBA Editor
    Martin Holladay | | #1

    If you want to insulate your sloped roof assembly, you can choose a vented or an unvented approach. Either way can work, as long as you get the details right. Here is a link to an article that explains everything you need to know: How to Build an Insulated Cathedral Ceiling.

    Your insulation should follow the slope of your roof, right down to your eaves. That way you'll be able to build kneewalls wherever you want, without having to worry about insulating the kneewalls. For an explanation of why this approach makes more sense than trying to insulate the kneewalls, see this article: “Two Ways to Insulate Attic Kneewalls.”

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