That is simply untrue. I've just self built an extension and it would not pass building regs if the inner leaf was made of heavy concrete block or standard bric

That is simply untrue. I've just self built an extension and it would not pass building regs if the inner leaf was made of heavy concrete block or standard brickwork (both of which were desired for stylistic reasons). The specific blocks used were 1.21 W/mK externally and 0.06 W/mK for the internal aircrete blocks. That's an enormous difference. UK government research indicated a 100mm cavity may be 0.6, so the inner leaf is still doing most of the work. Adding insulation to this cavity massively increases its performance, but the materials are better insulators than the air, and the concrete blocks are the worst part of the system. [1] - https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/48187/3146-thermal-transmittance.pdf https://assets.publishing.service.gov.uk/government/uploads/...