To improve the floor U-value we decided in the end to go for a free flowing screed that can be laid as shallow as 40mm, this meant that we were able to squeeze another 25mm of PIR insulation into the floor. Therefore achieving an overall insulation depth of 375mm, this will result in a super low floor U-value of 0.057W/(m²K)

375mm depth of insulation in underbuild

Insulation bunded and ready to receive flowing screed

Free flowing screed being placed