Insulating mansard roof attic
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?
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