Basement floor and ufh

[miles]

Hi

I want to install wet underfloor heating in the basement of my house. I think The floor is a 100mm concrete slab with no insulation under it

- it was laid a long time ago. I want to lay stone tiles for the final floor finish. Headroom is an issue and I don't want to raise the floor height more than 60mm.

I had imagined laying 25mm insulating board then 25mm reinforced screed containing the ufh and tiles on top. Is that possible - the screed is thin but under tiles I hope it is not an issue. Or is there a different system, eg with grooved insulating boards that I could tile directly over? The floor is actually very flat I have been over it with a spirit level and string.

Is there any particular insulating board or ufh system anyone would particularly recommend?

Thanks

Miles

[Tim S]

miles coughed up some electrons that declared:

Me too

On an aside, what insulation would you think of using? And would it be too elastic for 25mm screed (reinforced or not) to cope?

I think Jablite would be too weedy for 25mm screed. Celotex/Kingspan might be better - not sure what product though. Marmox board would work quite well I think as it's got a glass fibre reinforced face and is relatively uncompressible - but this isn't a standard application for Marmox.

Oddly enough,

may have exactly what you want - you'd probably want the "easypanel". It's jablite (polystyrene) but denser than normal, foil coat and grooves for PEX pipe.

In summary: 2 issues:

a) Very elastic - so tiles can crack if you drop things on them. I did test with the thinnest crappiest tiles though.

b) To a degree, you are paying to give the worms under your house a warm bed. I've attempted various thermal calculations, and come up with mixed results because slab-to-earth calculations are difficult. Basically, aim towards as much insulation under the pipes as possible. I think 25mm EasyPanel would be fairly wasteful, but 40mm would be much better.

I got a sample and here it it, in a trial install:

's a thread discussing it, started by me:

read my findings here too:

'd like to hear your thoughts after you look at this lot, because I'm undecided. At least you have flat floors (important) - one of mine, the biggest, is +/- 15mm in random ways.

Cheers

Tim

[Steve Firth]

25mm of insulation isn't going to achieve much, and TBH I can's see the point of insulation in the situation you describe. The heat from your UFH will warm the slab and soil beneath the cellar floor but that will act as a heatbank to an extent and I suspect that the difference in heat loss between skinny insultation and none at all will be minimal.

As far as systems to carry your tube are concerned, I'd consider using Lithotherm clay tiles, which are grooved clay tiles into which the tube is run. These have a finish height of 45mm so they should be ideal for your use.

[Tim S]

Steve Firth coughed up some electrons that declared:

What's your feeling for a ground floor room with 2 exterior sides? From what I understand, most of the heat loss is heat travelling down, sideways and up to the exposed ground outside. Is my understanding correct?

Cheers

Tim

[The Natural Philosopher]

But what a thermal mass tim..

I would be tempted to chip up the floor, dig down, and start again.

[YAPH]

How do you know it's 100mm? That's a hell of a lot of concrete to have been laid a long time ago - not so much nowadays when you can just get it pumped in. Why would they have bothered with so much?

I'm working on a place where they've just dug up the concrete floor of the basement, and it was no more than 2" of weedy concrete over chunky tiles and soil. That's being excavated to be relaid with thin concrete + 100mm polystyrene + screed with wet ufh pipework embedded. It's taken 2 guys about 2-3 days (dunno exactly, I wasn't there all the time) to dig out about 27 sq.metres to a depth of about 200mm, filling 2 and a bit standard sized (4 yard?) skips.

If you decide not to excavate there are thin wet UFH systems (see wiki.diyfaq.org.uk and look for UFH). Since you want to raise the floor temperature to c.25C for UFH I reckon even 25mm of kingspan/celotex/extratherm (Selco own brand) underneath would be worthwhile, but you can do the heatloss to find out for yourself.

[Steve Firth]

With two exterior walls I'd expect the losses to be greater than in a cellar. However I'd also expect to insulate the walls in that case and to extend the insulation down for the depth of the slab. If that wasn't possible then I'd just live with it. If you have a look at the details given for the Lithotherm tiles, they suggest a minimum distance from the wall to the tile of 7cm, which should help to reduce the heat lost via the wall.

[Ian_m]

Helped a mate years ago (1990) to underfloor heat his cellar, actually I was only involved as a hired hand for the clearing and mixing cement bit. Basically dug up the concrete floor and found it was concrete poured over the left over airpocketed rubble from building the house (1930's ?) and a

1960's ? refurb. We found rusted paint cans, large globs of plaster, soil, dexian metalwork etc all used as filler. Managed to fill a skip and more with rubble. He had already had the floor inspeted to ascertain it as not part of the foundations.

Floor was put back as sand, dampproof membrane, polystyrene insulation (100mm ?), more membrane, concrete and steel mesh (50mm ?) and finally underfloor heating pipe held in place using egg box type thing before being covered over with screed. This was all undertaken by a builder as too big for him to tackle. Walls were stripped back and insulated plaster board batonned on with an air gap behind to allow the wall to breathe if necessary.

In the end not too successfull as:

- Require major messy mods to boiler pipe work to include the underfloor.

- Boiler was probably not big enough for house and cellar.

- Room was never greatly warm.

- Made no thought for ventilation, room always appeared musty and stale.

- Should have put plumbing in so could have then used for washing machine & tumble dryer & washing area.

- Ended up using it as a box room, which it was originally, but now not so damp.