[hkt]

Not so. If you want high thermal performance in a building, insulation (therefore thickness) is crucial in the entire building envelope. As the beardier building retrofit types say, your house needs a hat (loft insulation) boots (floor insulation) and jacket (wall insulation).

Consider this: if you're raising the temperature of your floor to say, 25C, that won't do much if you're doing so on an uninsulated floor where the temperature of the ground beneath is 10C or so. UFH at that temperature is very efficient and thermally comfortable.

Consider further that you might then have walls which let all that lovely heat out even if your ceiling and floor are well insulated. It makes sense for them to be just as good, not least because your windows will in all likelihood be the worst part of the room in terms of insulation. Having as uniform a u-value across the room (and building) makes for uniform temperatures throughout. It lends itself to using heat pumps, further raising the efficiency of the building.

Further, lifetime emissions of the building are increasingly front loaded in construction as operational emissions drop. Standardising will presumably cut down on costs as well as emissions at that stage.

Building physics is an entire subject of its own and is worth studying. I'd suggest looking at the Association of Environmentally Conscious Builders and their CarbonLite course to learn more.

[pitaj]

They were referring to insulation in internal walls and floors. That is, the floor between the first and second story or the wall between bedrooms.

[persolb]

It's also clear that the floor between levels doesn't have insulation. I'm not sure where the OP got that all the blocks/insulation was the same.

[hkt]

Still good if you want zoning etc, but point taken