Depth needed for UFH on top of garage concrete slab?

[Gooman]

We're planning to convert a garage into a habitable space. It has a concrete slab floor, 100mm down from the concrete slab for the rest of the house.

 

How much depth would we need if we wanted to put UFH in the garage floor? We could raise the floor in the adjoining room (kitchen) to match if it's not too much extra.

[Gooman]

In fact, would it be feasible to use something like this: https://underfloorheating-direct.com/25mm-floating-floor-panels-26519-p.asp with Celotex under and an engineered floor straight on top?

 

Not yet looked into u-value calculations ... that's a can of worms I guess I'm going to need to get into!

[Jeremy Harris]

As a rule of thumb, you need a minimum of about 100mm of PIR insulation under a heated floor, ideally more, as UFH will always create additional heat losses through to the underlying ground (as the floor is being heated to a higher temperature than the room).  Building regs (which will probably apply when converting a garage to a habitable room) only require a fairly poor level of floor insulation, as they don't take account of the additional heat losses from UFH.  IIRC, the maximum allowable U value for a floor is now 0.2 W/m.K for conversions, but best to check as I've not read the regs for a year or two, and they are different for conversions, I believe.