Fash wrote:
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.

Sopudns about rioght. You don';t want 6mm of exep-snice crap, you want
60mm of expensive insulation. Polystyrene, or best of all Celotex or
kingspan.


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?

The rugs that you pit down on the tiles for one..the sofa that you sit
on for another..I've got UFH and UNDER THE SOFA its about 10 degrees
warmer than UNDER THE RUG which is 5 degrees warmer than OVER THE
LAMINATE which is 5 degrees cooler than the tiled parts...

The amount of energy that goies down3wards relative to the amount that
gies uopwards and is useful, is entirely down to the relative U values
in each direction. A big sofa on a rug is a pretty good insulator. To
get te heat to sone up you have to stop it going 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.


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

With building regs forcing the walls to be about 0.1-0.3 U valuse, 0.63
is going to soak away about 70% of the heat before it even gets into the
room.

And with electricity at 13p a unit or whatever, thats a damned expensive
way of making the earthworms cosy.



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

No. Either do the job properly and excavate and fit 50mm+ insulation,
or scrap the idea of UFH and tiles, and fit a THICK carpet and some
radiators.

I am the biggest fan of UFH there is, but the underfloor insulation on a
ground touching floor is MANDATORY - its not an option, its part of the
whole job.



Advice welcomed.

Fash