I need to insulate under the slate roof in our 1925 Tudor. We have hot water heating pipes throughout the attic, so we have to insulate the slope. The house is brick with terra-cotta walls all the way to the attic and there is no soffit ventilation. In speaking with several contractors, nobody is really familiar with insulating under slate so I am looking for guidance. The contractors have suggested using fiberglass batts with stryofoam baffles, spray foam directly on the tongue and groove under the slate and a combination of furring strips, rigid insulation and then spray foam (by far the most expensive). The house originally had mineral wool up the slope covered by a fiberboard up and over a flat section that was created about 3 feet below the peak. I want the best insulation possible without compromising the integrity of the slate roof or causing moisture issues. Thanks.

It might be cheaper to move those hot water pipes. Perhaps still in the attic but covered by insulation.

Thanks for the suggestion, but the pipes are actually part of the gravity fed hot water heating system (about 180 degrees). The main pipe coming into the attic from the basement is 4 inches and runs across the length of the house near the peak of the roof and then diameter drops down to 2 inches by the time the pipes start to drop down to the second floor radiators at the walls. As such, moving the pipes isn't an option without major changes to the house. They will be insulated in any event.

I don't think that slate changes anything. So you can search this forum for lots of discussions about insulating attics without removing the roofing.