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Community and Q&A

Insulating mansard roof attic

kilativv | Posted in Energy Efficiency and Durability on

I have a house with mansard roof in Boston. After having dense pack installed in the building walls, the mansard is the next target. Originally I was toying with the idea of closed cell spray foam for the whole mansard floor, but I do not want to tear down historic plaster. It’s also pricy. So now I’m thinking about just insulating the little attic which is formed by the low sloped part of the mansard. That little attic is currently unvented. After studying all the materials on this site I have the following questions:

1) The attic is currently not vented. I gather that if the ceiling plane is properly air sealed to prevent interior air leakage, venting the attic doesn’t really accomplish all that much. I do not need to please the building inspector so is keeping the space unvented ok? Or should I install a ridge vent + some sort of vents in the space where the steep part of mansard roof meets the low slope part.

2) I’m having hard time deciding on which insulation material to use – blown in cellulose, blow-in fiberglass or rock wool batts? I’m leaning towards cellulose and blowing it to R-50 or better(probably 16-17 inches or so)

3) Only if I’m advised to go non-vented way. How should the area where the attic tapers off should be detailed? Can the insulation touch the roof deck boards or should there still be some sort of separation?

I’m going to keep the steep portion of the mansard uninsulated for now. I feel like insulating just the attic will be a net improvement, and as I learned from my earlier questions here, the steep portion of the roof is still ‘roof’ for all intents and purposes so the only way to really insulate is something vapor impermeable like closed cell foam(which is not doable) Is it ok if this portion of the roof stays uninsulated?

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    It's hard to give good advice without seeing the space.

    1. I would be wary of adopting the unvented approach unless you have access for regular inspections. Is there an attic hatch that allows access? How high is this attic in the center? Will anyone be able to get up there for regular inspections once the insulation is complete?

    2. Assuming that the attic you describe has a low-slope roof, you should follow the advice in this article: Insulating Low-Slope Residential Roofs. If you are insulating with air-permeable insulation material (something like cellulose) on the attic floor, then my recommendation is to vent the attic. The risk with an unvented attic is that you may miss a tricky air leak -- a hidden pathway from your conditioned space to this attic -- that leads to unexpected moisture accumulation in your roof sheathing.

    3. Spaces near eaves with limited access and limited depth (from the "floor" of the attic to the roof sheathing) are tricky. If you can't get the code-required minimum insulation thickness (generally R-49) of insulation plus an air gap in this tight space, risks increase. Closed-cell spray foam is the usual solution.

    -- Martin Holladay

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