Hey guys I have a 1886 French Second Empire. I've been slowly closing the envelope by installing spring bronze weatherstripping on the windows, making both interior and exterior storm windows, encapsulated the crawl space and added a huge whole house dehu system. I'm getting ready to hopefully get a new roof and I'm thinking about going back to slate like it was originally. I would like to move the insulation envelope to the roof deck. My questions/issues are: 1) The roof is a mansard roof so it has a funny shape. 2) The current roof(asphalt shingle) is nailed to the original boards so there are gaps up there, so no OSB/Plywood sheathing 3) Is there an underlayment that can act as an air barrier to help the slate dry to the back? Thanks!

1) You can cut and piece insulation in place. You'll need to adjust trim pieces a little bit so the additional dimension doesn't make the roof look odd.

2) You could probably encapsulate the existing roof deck with peel-&-stick. Remove the shingles, of course and clean up the nails of course. Or you could replace the roof deck with something like ZIP system roof decking, WeatherLogic, or ForceField. You could then put your foam board or rock wool board over the top in 2 courses, taping the top layer.

3) Instead of relying solely on the roof decking itself, look at laying down a drainage membrane or a peel & stick setup. Properly taped and liquid-flashed ZIP/WL/FF under a couple inches of foam/Rockwool could PROBABLY do it. I'm a paranoid SOB about stuff like this though.