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Community and Q&A

Retrofit insulation of Mansard Roof

kilativv | Posted in General Questions on

I have a 2 story house with mansard roof in Boston that currently has no insulation. The first 2 floors are going to get dense pack cellulose, but I’m wondering what are my options for the 3rd floor mansard roof. After reading tons of info here and at BSC site, I understand that my best best is vapor impermeable spray foam on the underside of the whole roof.
Of course I would like to avoid tearing down walls and the cost of spray foam.

Is something like dense pack cellulose in the vertical walls and roxul batts(while paying attention to air sealing penetrations) in the little attic of the mansard roof an option?

Are there any other options?


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  1. GBA Editor
    Martin Holladay | | #1

    In my opinion, both slopes of a mansard roof -- the very steep lower roof, and the shallow upper roof -- are roofs, not walls. If you insulate a roof with cellulose, you either need (a) a ventilation channel between the top of the insulation layer and the underside of the roof sheathing, or (b) an adequately thick layer of rigid foam on the exterior side of the sheathing (hard to do with the steep roof of a mansard).

    If the interior surfaces are finished, you'll need to do some demolition.

    For more information on ways to insulate the steep lower roofs, see this article: How to Build an Insulated Cathedral Ceiling.

    For more information on ways to insulate the shallow-pitched upper roof, see this article: Insulating Low-Slope Residential Roofs.

  2. kilativv | | #2

    Great! Thanks!

    I'm curious, why does the steep lower roof qualify as a roof?

  3. GBA Editor
    Martin Holladay | | #3

    It's a steep-slope roof. If it were vertical, it would be a wall.

    If it's covered on the exterior with vapor-impermeable roofing (something like asphalt shingles), it certainly won't perform like a wall.

    If (a) the slope of the roof at your house is truly vertical, and (b) it has a vapor-permeable cladding like natural slate or cedar shingles, then you are certainly permitted to call it a wall, and insulate it like a wall.

  4. Expert Member
    Dana Dorsett | | #4

    If the exterior is covered with vapor impermeable roofing materials, even a truly vertical "roof" should NOT be insulated like a wall!

    What is the roofing material?

    If it's slate or cedar on skip-sheathing it's exterior drying capacity is orders of magnitude more than asphalt shingles on felt.

  5. Expert Member

    Under our code roofs of over 60 degrees are only considered walls for the purposes of fire separations between occupancies. In every other respect they have to follow the requirements for roofs.

  6. kilativv | | #6

    Thank you for the responses. The vapor permeability bit was the part I didn't understand. Now it all makes sense!

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