On 06/04/2018 20:03, Mathew Newton wrote:
I am currently planning the construction of a 15m2 ground floor extension to our lounge and am torn between having the insulation above or below the concrete slab.

From what I can gather the most significant and real-world difference between the two is the temperature response with insulation-above-slab being quicker to heat up (and cool) down given the effective reduction in thermal mass.

Even considering this factor alone, and acknowledging that thermal performance is a very important criteria for me with this extension, I still don't know which way to go. As the extension is for the lounge I am assuming that floor temperature will likely be an important component affecting comfort. For what's it's worth, the floor covering will likely be engineered wood with rugs.

Any comments or advice that might help me make the decision?

Speaking from experience of both.... The new extension was done by
builders with insulation under slab and the old garage which was
converted into gym (internal/habitable space) had insulation over
original slab.

I fitted the same UFH pipes to both. New room and entrance
hall/reception were done with a 7 layer multi-foil insulation under
egg-crate pipe trays and old garage was egg-tray obviously sat directly
on 4" insualtion.
Both were screeded to the same depth and 2 new rooms are 2
zones/circuits and old room is 2 circuits but 1 zone.

The egg-tray UFH with multi-foil keeps what appears to be a very good
degree of isolation between floor slab and certainly heats up quickly so
although it sounds correct to stick the insulation on the slab I've not
noticed any difference from both variants.