How to most economically lay extension floor

[endecotp]

just an arbitrary figure set by them.

It's not arbitrary, it's chosen to implement government policy on climate change.

Why not double it again?

Diminishing returns.

 

[pilsbury]

My point being, the U value set has little relevance to me in my situation. Sure I don't want a freezing solid floor, and I suppose I'm meant to be bothered about climate change, but in my house situation it really is nothing more than a set figure I have to adhere to. Arbitrarily set by the government.

 

[John D v2.0]

Which figure would you prefer?
Floor insulation is actually more useful because if you don't have ufh, you find the floor is colder due to heat rising.
Ime Insulating the loft saves you money more then floor whereas insulating the floor improves comfort more then insulating the ceiling.

 

[pilsbury]

Which figure would you prefer?
Floor insulation is actually more useful because if you don't have ufh, you find the floor is colder due to heat rising.
Ime Insulating the loft saves you money more then floor whereas insulating the floor improves comfort more then insulating the ceiling.

Which brings me onto the question should I stick the insulation under a 100mm slab or on top with say a 60-70mm screed. Or is the difference inconsequential?

 

[endecotp]

Arbitrarily set by the government.

No, not arbitrarily set.

 

[endecotp]

should I stick the insulation under a 100mm slab or on top with say a 60-70mm screed.

Insulation under the slab increases thermal mass, over the slab reduces it.

Higher thermal mass means the room warms up slowly and cools down slowly.
Lower thermal mass means the room warms up quickly and cools down quickly.
As a first approximation, for the same average temperature the heating costs will be the same.

But to some extent you want the different parts of the house to match. If one part wants the heating to come on earlier in the morning because it's slow to warm up, but another part wants it to come on later because it's quick to warm, and vice-versa when it goes off, how do you program your heating controller? (Combining UFH and radiators has the same issue.)

So in your case I'd probably aim for lower thermal mass, i.e. insulation higher up in the floor construction.

There is also the issue of the perceived warmth of the floor, i.e. a room with a solid floor where you walk around in your socks will feel colder than a room with carpets, even if the air temperature is higher.

Edit: but I should say, the difference between 60mm of screed vs. 100mm of concrete over the insulation is not going to be huge, if that's what you're considering. The other issues, in particular whether you can DIY a concrete floor with a nice smooth finish, and whether you have the depth necessary, and the amount and type (and cost) of insulation that you can fit, are probably more important.