Underfloor heating enquiry

[Joe Bloggs]

Looking for advice/comments from anyone has some experience of underfloor heating. I'm shortly going to refurbish my bathroom, and I'd like to install underfloor heating.

The floor is concrete which presently has laminate flooring (which was put in before we bought the property). I intend to remove that and put in a ceramic tile floor with an electric heating mat under the tiles.

I have no idea of the condition of the floor under the present covering, but I guess that some levelling will be required. What I'm unsure about is the type, and amount, of insulation to fit under the heating mat. My chief concern, obviously, is not to lose any heat to the sub-floor. Anyone got any suggestions?

[Brian Gaff]

Well ceramic tiles are not great heat conductors and neither for that matter is concrete, so I'm always dubious about the cost effectiveness of this sort of underfloor heating and if or rather when the elements go pop, its a nightmare. Told to me by a person who used to rent out flats which all started out with underfloor electric heating some years ago, so unless things have improved.... Brian

[Bob Minchin]

1 Dont do it. Put down vinyl with a tile pattern instead. 2 You will have to dig up the concrete, put insulation under fresh concrete to get a firm base on which to lay tiles. UFH needs the heat reservoir provided by the concrete. 3 You will need other heating in the bathroom if the UFH is not to become very very expensive to run. Ideally set it to just take the chill off the tiles. This needs a thermostat sensor to be in the floor not the air in the room.

[Charles F]

I'm inclined to agree with Bob, but it will depend somewhat on how cold the concrete floor will be in the winter. Our original bathroom floor (1970's bungalow) is uninsulated concrete laid directly on the ground, so very cold in the winter. It does have a good DPC, so it is dry, which is important too, and something to check before going any further.

Not wanting to dig up the floor, but wanting to make the floor a little friendlier to bare feet, I bonded 8mm waterproof ply to the concrete, and laid vinyl Karndean tiles over that - the result, while obviously not exactly hot, is much better than the original vinyl tiles stuck directly to the concrete, where your feet seemed to freeze immediately. There are other insulating options available, but this was sensibly priced, and worked.

We do have lovely warm water underfloor heating in our other bathroom in a recent extension, but that has 100mm of PU foam underneath the screed, so a very different story....

Charles F

[The Natural Philosopher]

Apart from steel, ceramic and concrete and brick habe te highest thermal conductivity of any common building materials.

Of course they have improved.

[The Natural Philosopher]

Correct. for best efficiency

Utter bollocks.

Utter bollocks

Utter Bollocks

Complete and utter bollocks

[The Natural Philosopher]

You would be surprised at how long the thermal path is from the center of a floor slab to the frosty air outside, and, how much insulation a couple of yards of subsoil can effect.

I calculated that in house terms 2 meters thick of solid stone castle wall would meet modern heatloss standards.

Really that's what you need to do.

Dig up and lay that down and then screed over the top. Put the DPM whereever it works - under or over the insulation

chipping out screed and subsoil is not the worst job in the world

Oh and use hot water for the heating.

[Charles F]

You may well be right in your theory, but bare feet in the winter tell a very different story.

It may not be the worst job in the world, but here the slab is continuous across the whole house, with the internal block walls built on top. And the slab is about 1.5" of screed over at least 4" of good quality concrete, which means that really it's not remotely a practical suggestion.

Charles F

[The Natural Philosopher]

Ther is a fifference between hesat loss and thernmal mass...

Of course it is!

shouldn't take more than a week with a kanga and some wheelbarrows

[Tim Lamb]

In message , Charles F writes

Hmm.. if you could just get that screed off you could use one of the grooved insulation systems and piped heating.

Fair bit on the net. Try the Floor Heating Warehouse or John Guest for two:-)

Alternatively 25mm insulation, electric heating and one of the rigid panels recommended for over tiling.

[Jeff Layman]

A couple of years ago we had a conservatory built. I wanted electric underfloor heating - mainly as a background heat. The floor was floating (R.C beams), with blocks between the beams. That was covered with 50 or

75 mm Celotex insulation (I can't remember which was finally used). That was screed over (20 mm?). The heating element was thin film (see ) installed as per instructions - 6mm foam over the screed, then the heating element, then 500 micron polythene (there were plants in the conservatory, and I wanted a thicker polythene in case of water spillage). The flooring is 9 mm laminate.

Basically, it works, but there's no way by itself it can keep a conservatory warm. A bathroom floor would be different. But note the power usage of the heating film - It's only 130 watts per sq metre! Most modern bathrooms would have around only 2 or 3 sq metres of floor space for heating use.

Unless the bathroom is in continuous use, I can't see the sense in using underfloor heating. The thermal delay is a least 20 minutes to heat up and cool down. If it was me, I'd use cork tiles to insulate the cold floor, and seal them to protect them from the damp. To heat the bathroom I'd use a bathroom fan heater, for example as and when needed. It's instant heat, and you can turn it off instantly too. An awful lot simpler and cheaper to install than underfloor heating.

[bblaukopf]

I fitted it back in 2007. I didn't have much space for insulation, so I put 10mm of marmox down, which has the u-value of a pretty crap window (though it's better than that when you factor in the subfloor itself), and the 3mm wire on top, with tile adhesive on it to protect it before doing the actua l tiles (natural slate).

It works well, warms up quickly, and I wouldn't do it again.

If I did it again, I would either:

a) Dig out the screed, which I didn't know was an option back then, and fit more insulation, and wet UFH.

or

b) Not bother, and fit some expensive vinyl instead.

With a strong preference for b).

and

c) If I did use electric cable again, no way would I save £50 by using the cable. I'd get one of the mats. We had to lay that cable out, tile ove r a bit of it, pull it all back so we could cook dinner, and rinse and repe at.

While it wasn't, relatively, that expensive to run it back in 2007 (1kW), t he advent of decent LED lightbulbs means that it's now a reasonably substan tial portion of my electricity bill, given I've got 1kW of electric heating there.

[Andrew]

Even removing 1.5 inches of screed will allow its replacement by Knauf dry-screed elements, or a layer of 25mm celotex with

30mm Fermacell on top. Affixe the heating mesh onto the Fermacell and tile over the top with flexible adhesive.

You might have a small step in the threshold, but not enough to be a problem.

I did my lounge like this, but the screed varied from 80mm in one external corner back to 65 mm in the opposite inernal corner, so I laboriously trimmed 75x50 battens to fit the irregularities of the slab and infilled with 70 mm celotex.

The screed came up in huge sections ny hammering a sparkies big wide flat cold chisel into the joint and hitting the screed with a club hammer.

[Andrew]

But if it was built after 1984 but before 2002 then that 'insulation' will be expanded polystyrene, the nasty cheap open-cell white stuff.

Chances are the builder will have used one inch thick sections inside the cavities, just flapping around loose and also over the DPC but not protected from the water and fines when the slab was poured. This will have infiltrated the open-cell contsruction rendering it useless as an insulant !.

[Andrew]

There are a couple of million modern houses built in the 70's with no insulation and they will be typically on a footprint of about

8 mtres by 5. So thats 80 sq metres which would be 4KW using you U/F heating.

My house, in the condition it was in when I bought it in 1991 only managed to maintain about 16 centigrade during a nasty cold spell in 1992 with cold winds coming from the North.

This was with the 45,000 btu BAxi back boiler on full blast with all the rads on and the 6 kw fire front on full blast too.

Simply having cavity wall insulation fitted, killed all the excess air leakage, and then it was possible to keep the house at up to 20 degrees just with the 6Kw radiant fire.

Dry lining those offending upper floors where there was no cavity made another massive improvement to the 2 bedrooms facing north, and now I have done the Lounge floor with 70 mm celotex in place of screed, and another 80 mm on the inside of the front wall (which was cavity insulated anyway).

A 1Kw radiant bar electric fire can now raise the temp of the lounge up to 20 C on its own, unless we have a beast from the east, when it gets to 17 C and no more.

[Vir Campestris]

We have a solid floor with earthenware tiles.

It's amazing the difference a bathmat makes :)

Andy

[Andy Burns]

The screed here comes up by looking at it funny and using a hoover.