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Using Spray Foam on the Eaves of a Low-Sloped Hip Roof

David Red | Posted in General Questions on

Hello,

Have an existing house in zone 7a, built 1960 with a hip roof (sloped on 4 sides) that looks like 4/12 slope.  Existing ceiling construction is gypsum/plaster, then wax kraft paper, then 4″ wood shavings then 6″ blown insul.

We have had frost/condensation issues in the roof that this past spring has caused dripping within the home.

One solution and likely the best to solve air leakage issue is to remove all the insulation and spray 2″ of closed cell foam and then fill with blown to R50.

We have had one contractor say that due to the low slope and hip roof that the only way to properly spray the top plates of the exterior walls is to remove a 4ft strip of the roof sheathing & shingles so the plates can be properly sprayed.

Im hoping i could get peoples thoughts on this regarding removing the strip of roof sheathing? is that really necessary?

Thanks

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Replies

  1. mgensler | | #1

    Hi,

    We have a 1960 house in zone 4a with a 3/12 hip roof. We just went through the removal of insulation and air sealing with closed-cell foam similar to what you are doing.

    This study that was done describes the issues with the low sloped roof, especially at the corners. https://web.ornl.gov/sci/buildings/conf-archive/2013%20B12%20papers/007-Parsons.pdf

    We were able to access all of the top plate from inside the attic except for the corners where the framing is very complex and tight. We cut two 2' holes at each corner of the house in the eaves and had the foam guy spray from the outside in. This worked great and was easy to do.

    You might consider having 9" of closed-cell from the exterior of the top plate inboard until you can achieve your desired thickness of cellulose. Otherwise, the outside edges of the house won't have the desired R-value due to the slope of the roof.

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