I'm currently in the process of digging out and replacing the floors in my 1930's 3 bed semi. In accordance with current building regulations we need to add insulation. We're going to do hardcore, sand, damproof, concrete, then insulation with a hardboard floating floor, on which I'll be adding woodblock. With regards to the insulation, our local building control has advised a minimum of 75mm of insulation. We've been looking at which products to use from Wickes, and on one hand there's products like Celotex PIR at about £45 for a 2400 x 1000 x 100mm, and on the other there's Wickes Polystyrene Insulation Board at £18.50 2400mm x 1200mm x 75mm. The price difference is huge. My concern is the themal value of the materials. All I've found regarding extruded polystyrene in general is here "The thermal conductivity of extruded polystyrene is typically between 0.027 and 0.025 W/m K at 10 degrees C." Can anyone offer any advice as to a suitable product which doesnt cost the earth - or will polystyrene be fine?