I bought some foil backed insulation board from APCO for my house extension. They have a massive variety of insulation board, both foil and non foil backed at very competitive prices. Although the majority of material are seconds they were able to talk me through my project and explain exactly what I needed and what they had in stock. They then delivered the wall insulation, I didn't have to pay a penny until I had approved the insulation on delivery.

Cheap as Chips!
John Doe