Looking for advice on thermal insulation, needs to be fairly non messy.
I've a 1930's semi detached house that's got bay windows, the downstairs is fine, cavity wall under the window and the cavity is filled with insulation but upstairs it seems to be a plaster lath construction with the outside being roof tiles nailed to some kind of frame over a waterproof membrame.
Given the horrific rise in energy costs I'm trying to insulate and draught proof as much as possible and wondered if anyone could give some advice on how to insulate this sort of mess?
Preferably without resorting to ripping it all out and starting again if possible so some kind of internal fixing insulation. The interior of the bay isn't really seen but I'd like to be able to decorate over it.
I'm thinking some sort of polystyrene or expanded foam sheet?