Posted By: dicksterOllied. Two major things I would change now if I had the chance. I didn't realise that you could insulate in/under the slab/ring beam, so would revisit to see whether direct contact with ground or insulated is best.

If you use one of the EPS raft type ringbeam foundations, not only have you separated the floor slab from the ground so that it can work as useful mass, but it's also great for removing the thermal bridge at the soleplate.

I'm not sure whether you are considering a suspended timber floor, but in PHPP, a 20% reduction factor for energy loss is assumed for any external surface in contact with the ground as opposed to air, therefore, the same level of insulation will perform worse in a suspended floor compared to one in direct contact with the ground.