Guy King wrote:
The message
from The Natural Philosopher contains these words:

Then get digging. For my money, putting in UFH with any less is a waste
of your time and your money.

I'd agree. I honestly think the whole idea of UHF without a really thick
layer of insulation is misguided. I've used 100mm poly under the
conservatory floor.

--
Skipweasel
Pay no attention to that man behind the curtain.

Digging down further is NOT an option. Not because I'm lazy (I already
dug it down 500mm over a 25m2 area. It's an old house and the cellar
walls don't reach floor level. The structural engineer proposed a
reinforced bath-tub as earth retaining but didn't want me to dig down
more than absolutely necessary due to the undermining effect on the
house (which didn't fall down so that was good!).

Anyway I don't expect there's many volunteers to come and dig up 175mm
concrete with A142 mesh in it.

I'm still not convinced by the figures. Google it and you'll see that
large concrete floors don't require under slab insulation as perimeter
effects dominate heat loss. I accept mine's not big enough for that to
be the case but if you do the calculations adding 50mm of underslab
heating reduces U value to 0.3, which given the loss through the walls
isn't significant. I'm not subect to building regs part L a) it's
existing and b) it's listed and they accept that part L standards can't
always be achieved in a manner sympathetic to the aesthetics of the
building and can cause real damage to the fabric by messing around with
moisture equilibrium levels.

To be honest compared to the rest of the house it's the best insulated
room because the ground has an effective insulation value particularly
in cold weather as it's always at ~12C once you get more than a meter
below. The u-value calculator I used had special settings for basement
floors and walls including data on average ground temperatures across
the UK. I accept that warm-up is based on relative u-value above and
below the heating, but that's why I don't think 6mm insulation makes
any difference since it doesn't significantly shift the u-value below
the heating elements

I also stick with the argument that when the room is at a steady
temperature the heat-loss calculations are valid for underfloor heating
as for rads, if not why not?

If we take the finances. If you believe (and feel free to argue, but I
want to know your theory not just an insulation is good mantra!) that
heat loss is the same, and it's just warm-up time, let's work it out:

Additional cost for insulating (excluding the social effect of reduced
headroom) is £400 if I use 6mm Marmox. At the quoted 13p/kWh
(incidentally I'm paying 10p) this represents 3076kWh. Total heat loss
from the room is 1700W (assuming no additional wall insulation) and
output from UFH is 2000W. If it takes an extra hour to warm-up then
this is 1500 days running, which is a minimum 5 yr payback time based
on expected usage. However since heatloss is unaffected and the floor
acts as a better storage heater without the insulation I should be able
to turn the heating off 0.5 hrs earlier increasing the payback time to
10 yrs MINIMUM.

By the way I have radiators in the room anyway as I said in the first
post as primary heating in the winter, the UFH is just to stop the
little ones from getting cold toes since it's their playroom. For the
radiators I'm absolutely convinced that the only consideration is the
total U-value of the surfaces, floor and walls.

What I really want you all to explain is how the extra heat gets out
given that the U-value doesn't change with the addition of the 6mm
insulation.

Come on take up the challenge show (not tell) me how I'm wrong because
I don't see it in the posts so far.

Fash