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Community and Q&A

Insulating a Low-Slope Hip Roof

brianandson | Posted in General Questions on
Wondering if anyone can offer some expertise or personal experience insulating a low slope hip roof.  I have a 1970 mansard style home.  Pictures attached.  The low sloped top roof is separate from the steep slope side roof/wall.  Their attics are separated and do not communicate with each other.  The top roof is low slope hip roof, maybe 3/12 or 4/12 pitch.  There is no roof overhang or soffits.  The rafters terminate and bear on the exterior walls.  The top hip roof is a 30ft by 30ft square.  The attic is very tight inside.   At the roof peak has, at most, 3ft of clearance in the attic.  At the 4 edges/eaves of the roof there is only 6” of attic clearance.  At 4ft inwards from the edges there is only 18” of clearance.  It’s basically inaccessible in the attic from 4ft to the ends of the roof because the space is so tight.  I want to remove all my existing insulation and then air seal the entire floor of the attic.  Maybe add in a plastic vapor barrier between living space ceiling and attic.  And then add all new insulation.
Does anyone have any advice how to air seal/create vapor barrier and insulate from the edges of the roof to 4ft inwards, that has almost no clearance or accessibility?  I’m trying to avoid using foam, unless it’s my only option.  Can I blow cellulose and fill the attic space from 4ft to the edges of the roof?  Or is it bad for the cellulose to be in contact with the bottom of the roof sheathing?  Any guidance or even advice on what NOT to do would great be appreciated.  I really need to create a barrier between living space and attic because I have had moisture issues in my attic in the winter because of air leakage from my living space.  I don’t believe it’s an attic ventilation issue, I have good airflow in the attic.  Thank you   If it’s easier for you to visualize my attic with a cut sheet picture please let me know and I can draw one up.  It is a low slopes hip roof with very minimal accessibility to reach the space from 4ft to the edge of the roof on all four sides.  I live in mid Michigan with cold winters and hot summers.  It’s important that I get a good tight seal between living space and attic as well as good insulation.

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  1. GBA Editor
    Kiley Jacques | | #1

    Hi Brian,

    I remember your question from recently--I could have sworn some folks chimed in. I'm going to bump it to the top of the list. Have you read this article by Martin Holladay: Insulating Low-Slope Residential Roofs?

  2. brianandson | | #2

    Hi Kiley
    Thanks for your reply and for the article link. I got a ton of great info about the moisture in my attic and the consensus is that I need to air seal and insulate. This roof is not your normal attic to insulate so I’m hoping I can get some great advice on how to properly and successfully air seal and insulate this roof. I’d hate to make a mistake and make things worse with the moisture. Thanks again. I’m gonna check out the article by Martin Holladay

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