I have been looking at potential constructions of masonry cavity walls and getting a decent U value (circa 0.17).   I have been going on the basic wall with a 125mm cavity as below;   Plasterboard on dot and dab 100mm thermalite block 75mm Kooltherm K108 Cavity Board 50mm cavity Rendered 100mm dense block    This gives a U value of circa 0.167, however the Kooltherm K108 is expensive (75mm £21/m2 ex VAT https://insulationhub.co.uk/pr