Situation: Basement floor, 175mm reinforced concrete over hardcore on
clay but ~2m underground. I want to put travertine tiles (12mm) over
electric underfloor heating for comfort in the winter and primary
heating in the summer when the rest of the house is warm anyway.

The U-value calculator say's given the area and perimeter that U-value
excluding floor coverings is 0.67. Manufacturers of insulated tile
backing board (marmox etc.) say that even with 6mm of insulation under
the heating elements I will feel the benefit of quicker warm-up and
lower running costs. The U-value calculator says that with 6mm of
insulation the overall U-value is ~0.63 i.e. not very different.

Since the heat-loss from the room is largely unchanged I can't believe
the argument about reduced running costs is true? Certainly a gain of
0.04 in U-value doesn't justify the £400 cost of the insulation. Is
there something I'm missing?

On warm-up time surely 12mm thick tiles will still heat up faster than
the 175mm concrete so even in the warm-up phase the effect of the 6mm
insulation will be pretty much nothing. Again am I missing something,
apart from the desire to get the cost down?

I accept that if I had 50mm of insulation it would make a difference,
but this is a basement with finished headroom of 6'6" so I don't have
space to play with.

Should I spend less money (since it's cheaper) on a bit more wall
insulation to keep the overall heat loss down?

Advice welcomed.

Fash