http://peplers.blogspot.com/p/eco-refurbishment-of-our-home.html This video shows how I insulated the cavity in our dormer roof. The cavity was formed between the downstairs ceiling, the upstairs wall and the sloping roof. There was no access to it, so I had to make a hole and then build a hatch and door there once I was done. I used 50mm Ecotherm (like Celotex) insulation boards, and 200mm Knauf glass fibre insulation, made from recycled glass. After completing the job, there was an immediate difference in the lounge temperature, I suspect mainly from stopping the draught that was blowing between the lounge ceiling and the bedroom floors.