I live in Zone 5a in roughly 800 sq. feet of living space in a duplex, part of a "development" of houses built in 1917 to house workers in war-time industries.
The houses are brick construction, with slate roofs. Ours has a block foundation.The attic is insulated with about 4 inches of rock wool. There's no vapor barrier between the insulation, the ceiling rafters, and the second story's plaster-and-lath ceilings fastened to the rafters. Also, the attic is unvented at the gables. The soffits /appear/ to be vented: there's the standard aluminum grid material one sees on soffits, but I'm not sure the wood soffits beneath the cladding have openings. The roof is very steeply pitched, with deep eaves. The rock wool looks like it has been shoved down the eave shoots. It also looks like there's no air channel from there into the attic.
Can you suggest how to properly insulate the attic? From what I've read, it seems like we should remove the rock wool (it's damp in spots), open up channels in the eave shoots to the soffits (having opened them if needed), and lay a moisture-vapor barrier before laying new insulation. But the literature on masonry buildings seems to suggest that moisture-vapor barriers are not always a good thing -- but maybe that's just in walls?
Also, the bays between the ceiling rafters are unevenly sized (ranging from 13 - 18" apart), so I'm thinking of using loose cellulose to fill between the bays and then laying fiberglass batts perpendicularly to the joists in an attempt to raise the R value to somewhere over 40. The house's electrical service runs through the attic, following the path of the former knob and tube wiring, and we want access to it, so I don't want to use blown-in products. Does this seem like a good approach?