Vapor open roof assembly insulated at the rafter level
I am in the process of replacing my roof on an 1870s brick home in the lower Midwest (Zone 4A, right on the border with 5A). Coldish winters and humid summers. The roof is a mansard, with a sloped section and a flat section. I have attached pictures. It is reasonably complicated with three dormers, a turret, and two octagonal bays. I have read many of the materials on ways to insulate at the rafter level or above.
I have seen less information, particularly in the US, about how vapor open assemblies perform. We will replace the sloped portion of the roof with slate. The flat portion will be standing seam metal, and we plan on using a vapor permeable underlayment (like Solitex Mento).
Insulation for the flat part of the roof is straight forward. We will put 4″ rigid mineral wool directly above the roof deck.
The sloped portion of the mansard is more difficult. It can’t really be vented – there are no eaves, and thus no soffits, so the roof terminates directly at the wall and you’d have to engineer an outlet vent of some kind at the joint between the sloped and flat portion to get airflow. Insulating on top of the roof deck would run afoul of the historical commission.
My thought would just be dense pack the rafter cavities with sheeps’ wool, for its performance while wet and hygroscopic properties, put rigid board across the rafters to address thermal bridging, and then apply a Pro Clima Intello membrane on the interior side of the rigid insulation to limit moisture escaping in the winter without compromising the ability of the rafters to dry to the inside.
This approach seems to be done often in England. The idea is that the roof can dry to both sides, and so any condensation that forms will be able to dry out. The roof decking is true 1″ thick Northern White Pine and quite sturdy stuff.
Anyone have any thoughts or experience with this sort of assembly? We are against CCSPF, which I know is the typical solution in our circumstances, for both environmental and preservationist reasons.
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