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?