dewalty

After some advice please. New build single floor annexe. 40sqm floor. Walls -Thermalite - 90mm pir - thermalite. Flat Roof single ply with 150mm pir Current floor build up, 150mm pir below 100mm slab. 90mm to play with between slab and final floor finish (engineered flooring 15mm) floor temp 27c. 150mm pipe spacing. Just had a visit from liquid screeder who I've used before. He has suggested 20mm pir with 50mm screed. He is saying heat up times for slab and screed etc will be excessive. Should I stay with the original plan of using slab as thermal store? And input would be gratefully received. Thanks