The 6.6 m²K/W represents the nominal thermal resistance through the centre of the expanded polystyrene pods which provide the concrete formwork and the floor insulation. This is only a nonimal value because the pods form a waffle tray pattern which greatly reduces the insulation thickness in some areas.

The 0.11 W/m²K is the average U value across the whole floor. The heat loss from a ground bearing floor is mainly from the perimeter, so it takes into account the perimeter to area ratio, cold bridging through the relatively thin horizontal edge insulation & the effect of any vertical edge insulation to the slab toe.

Overall it looks like a good system, but the obvious thermal weakness is in the edge detailing. Wing insulation would help; as would adding full-fill insulation to the base of the cavity wall.

David

Posted By: davidfreeboroughThe 6.6 m²K/W represents the nominal thermal resistance through the centre of the expanded polystyrene pods which provide the concrete formwork and the floor insulation. This is only a nonimal value because the pods form a waffle tray pattern which greatly reduces the insulation thickness in some areas.

The 0.11 W/m²K is the average U value across the whole floor.

OK, but 1/6.6 = 0.15 W/m²K is then the nominal through the insulation, but that's worse than the average across the concrete as well. That's what I don't understand. How can the best insulated bit be worse than the average?

Ah, thanks. I hadn't realized quite how specialized the U-value calculation for an on-ground slab was.

I don't like the look of that system at all. I'm told that, especially with UFH, a significant portion of the heat loss it out the edge of the slab and that system appears to have no insulation in that area.

The insulated raft I am using has a thermal break between the main slab and the ring beam and further vertical insulation outside of the ring beam and then 1.5 metres of wing insulation.

My PHPP guy was telling me of a shopping centre in Waterford Ireland which is built to passive house principles, the centre of the slab has little or no insulation under it, the focus is insulating near the edges. As mentioned already the ground under the centre of the slab comes into balance with the heated space above so there is little heat loss to the ground. I think he said the phenomenon is known as a "thermal lake" but Google doesn't seem to know of it!

Posted By: Chris P BaconI don't like the look of that system at all. I'm told that, especially with UFH, a significant portion of the heat loss it out the edge of the slab and that system appears to have no insulation in that area.

The insulated raft I am using has a thermal break between the main slab and the ring beam and further vertical insulation outside of the ring beam and then 1.5 metres of wing insulation.

It seems they've tried to make it look familiar to UK builders, structural engineers & building control officers by making it look as much as possible like a conventional raft foundation, complete with all the usual thermal bridges!

However, their specification does separately call-up horizontal edge insulation below the ring beam & vertical edge insulation around the slab toe. So I'm sure it could be adapted to have 100mm+ in these areas & wing insulation out-board for good measure.

David