Djomidlands

Hello, I’m decorating the master bedroom of the 30s semi. The bay window is currently not insulated. It looks to have the original plaster on laffs and nothing behind. Externally, I don’t believe there to be a brick wall. The house is pebbledashed from the front with felting behind. How’s best to insultate? Should I remove the plaster and laffs and insulate behind with Kingspan or celetex or build a timber frame in front of the existing plasterboard and insulate behind that? if insulating the void with kingspan, do I need to leave an air gap or foam any remaining gaps? also, do I need to insulate the floor beneath the window? thanks