concrete floor: insulation above or below concrete?

yes.

Makes little odds on its final efficacy, though it does change the thermal mass inside the insulation.

You probably need a minimum of 3" of polystyrene or 2" of celotex insulation to be up to regs so unless you are using 3" joists, or leaving no room for pipes, its probably better done in the slab.

thanks, yes according to the Celotex calculator I will need 70 mm of celotex to get U=3D0.22 W/sqmK ; perimiter/area ratio is quite large (about 0.7)

Robert

Insulation under the slab gives you better thermal capacity and better load ability, since point loads are spread over a wider area.

NT

That can be good or bad. If the space is used infrequently and heated on demand, thermal mass is bad (it makes the space take longer to heat up). If the space is used all the time, then thermal mass is good (it smooths out the temperature fluctuations outside).

I can see the thermal arguments both ways. I suppose one thing I fin hard to believe is that a slab of foam is a strong enough base to put a screen and tile floor onto without it flexing and cracking when people walk on it.

Robert

For that reason I prefer insulation under the slab. However, underfloor heating is generally done with insulation over the slab. The screed should be at least 65mm thick and usually contains fibres to make it stronger. Even with my insulation under the slab, the floor exhibits a *just* discernable wobble factor when the washing machine is running ! Simon.