Floor insulation

Good morning all, I'm about to start the process of laying the concrete flooring, having prepared the base with hardcore and sand. I'm installing underfloor heating to all the ground floor rooms so will need to lay insulation boards either as a sandwich i.e. under the concrete and then again under the heating pipes (before the floor screed); or just under the heating pipes. A quick search of the usual builders merchants in my area (24) revealed that such boards are horrendously expensive. I don't want to go down the 'compare the prices' debate again, but it's true that similar boards in the UK are a fraction of the price so far found in France. Anyway, do any of you kind people know of a reasonably priced supplier in France? I have about 160 sq mtrs to lay in total so need between 50 and 60 1.2x2.5 boards.

Yes, things can be difficult to find sometimes in BD. Leroys is in fact slightly cheaper, 8.7 euros sq m, Castorama, a bit more, but we find (in Perpignan), that we often have to order from Casto, whereas Leroy and BD have more stuff in stock.

Sorry, should have been extruded, not expanded, in the first email - my mistake. I suppose you could you use the expanded sort, but its not waterproof like the extruded. I suppose you could sandwich it between two big sheets of polythene, but then it might not work out much less than the extruded sort. Anybody tried this?

Thats expanded polystyrene (the granular sort). I was looking at extruded polystyrene (firmer, and so suitable for floors). Its called Spaceboard (?) in Wickes, and they do quote by the sq. m, and it is £16 + per sq.m.

Thanks everyone for your responses so far. It's clear that my original internet search did not reveal the true extent of availability or price. As it happens, we were in Leroy Merlin today for something else and discovered a rack full of different sizes and types of insulation material. When we called in at Brico Depot a while ago, we couldn't find any - hence the question.

Melissa, Phil-Decor isn't too far from us, so we'll go and have a look to see what they have.

Peter/Robert, Now we have a price to go on, it's clear that prices are indeed comparable, so there's no need to go through the hassle or expence of having some sent over.

I think you will find @ Wickes it is per board, not usually sold per square meter in the UK.

2.88 sq m to a board...so quite a bit cheaper in the UK...

Here is a 75mm Example

Just had a look on Wickes : 60mm expanded polystrene boards are £16.42 sq.m, the same thickness in Bricodepot is 9 euro 24; half the price!

Yes. http://phil-decor.com/. Perigueux.

116 Rte de Bordeaux
Tél. : 05 53 13 27 57
Mob. : 06 78 23 45 68

We did three hour round trips to buy his insulation boards for the same purpose.

Got about 10su meters it’s suitable for either it’s the extra dense type but not very thick it was recommended by the company I bought my underfloor heating from but I never used it as Plummer laid tiles before I was ready agh

Cheap as chips as long as you don't want to measure above about 400 deg C . The cheapos are usually quite accurate ( try 'em on a known source) although in your case it's the temperature difference that's of interest so accuracy is not too important. You should be able to get a decent one for less than 25 quid. They are great for checking anything from freezer temperatures to jam setting temperatures;-)

LOL Peter, sounds to me like your plumber did a marvelous job under the circumstances...a pity perhaps he didn't install a pump somewhere along the line, would've solved the problem. But added a few hundred euros, no doubt.

Although I follow building regs, they are rarely written in stone and can invariably be "exceeded" to great effect. As a builder/loft converter in the UK we were constantly challenged in this way. I have not yet got my head round many of "the rules" here I hasten to add. The new RT2012 will be of great interest to me when I get a copy that I can understand ;/

Bryan, thanks for the offer, I may just take you up on that. How much have you got, what thickness is it, does it have a foil type backing and is it suitable for a water system rather than electric? Oh and of course, where are you?

Thanks, Vic. I never realized that they're not at all expensive.

Robert - I'm sure that you're right and that the drainage angles aren't sufficient, but our plumber was definitely the worst of all the trades that were constructing the house. In his defence, over this one item, the slope on which the house is situated means that there is probably only about 50 - 60cm of fall from the level of the house floor to the entrance to the public drain, some 50 m. away. That doesn't give a lot to play with, especially allowing for bends, intermediate regards & the like.

OTOH, we've never had a complete blockage, but have had strange gurglings and drainage behaviour in the very cold weather that have never occurred at any other time. Many would say that, given French plumbing generally, if we only get strange drain gurglings occasionally, we should consider ourselves fortunate.

As for putting pipes in loft spaces, the old water bye-laws effectively made this, and locating water-tanks up there, obligatory in the UK. I don't know the current UK situation, but I think that there has been some relaxation of the rules, but the great majority of properties were constructed under the old rules.

Peter. Buy yourself an infra red thermometer & measure the slab surface temperatures above & below. These will tell you what you need to know rather than guesswork.

I would do both a good thick layer under concrete,then a special one particularly for underfloor heating under wires.
Not that I,m touting but I do have a box of underfloor heating insulation over if you are close and interested

I see where you are coming from Peter. But, if your drains are freezing up it sounds to me as if the drainage angles are insufficient and if I were you I'd be more concerned about that. You should have no water hanging around to freeze up. Any traps should be internal or at least get attable?

Personally, I would never put pipes in loft spaces in the first place and would certainly never insulate below them. Above them, yes, of course.

The gap is just high enough to crawl around, and it's ventilated, as is required. The base is just bare earth, and pretty damp, with the Jura escarpment about 2 km away, and apparently even capillary action bringing water up from Lake Geneva. The times when ice has been forming it's been below -10 for days on end, so you'd expect it to be pretty cold down there!

As for its being cold meaning that the heat isn't escaping that way, I'd rather it were even colder! It's like loft insulation. Fuel bills go down as we add more and more insulation in the loft, but the loft does get colder, and the pipes and tanks in UK lofts then also need insulating. If I had to insulate my drain pipes down there, it wouldn't be that big a deal, though it's not a nice place in which to operate.

The issue for the OP is that more heat is lost through floors than is generally realized, as there is the principle that heat rises, which is true, of course. However, heat also moves across any temperature gradient, so I'm suggesting that the sandwich insulation idea sounds helpful & that I wish our architect had suggested it. (He still lives in our lotissement and also has a similar system. He rather had the idea that the heat pump gave such cheap heat that it wasn't really necessary to invest too much money into 'excessive' insulation.)

Peter, if you getting ice down there then that means the insulation you already have is working very well in my opinion. How big is the gap, just out of interest? And is it ventilated?

Yeah but surely if it's cold below it means there is little or no heat transfer from the heat emitting slab to below. Isn't that what you want or have I missed something?