Because the joists and space below are shallow, I'm limited to 100mm of insulation so going to have to use 100mm PIR between our ground floor joists to get the best U value I can.   Previously I'd planned to use the Ecological Building Systems approach of loose insulation and 2 membranes:   I've come up with 3 ideas for PIR insulation, from least to most work: Install boards using Gapotape (or home made equivalent as that stuff is seriously expensive. I'm experi