I am just about to cast the footings of our extension and time to think about the next phase.

I need to build a cavity wall and was wondering what the most cost effective materials are.

I have been looking at thermalite turbo blocks (0.11W/m2k) and understand these can be used on both the inner and outer skin (walls are being rendered and cedar clad on outside an plastered on the inside. Using this i can acheive 0.28 with 25mm celotex CW4025.

Are there other brands/products that would be cheaper than this? Would a normal concrete block and thicker insulation be cheaper?

thanks