Underfloor Heating Screed NIGHTMARE

[Rob_1]

Hi All,
I am new user so hopefully am posting this thread in the right place.
I am re-doing the kitchen, and incoporating electric underfloor heating.
I have built the floor up as follows;
1. Levelled the existing concrete sub-floor using Treadfast sx-1.
2. Fixed 20mm floor insulation boards with a flexible tile adheisive.
3. Coated insulation with PVA based primer to make sure the tape to hold wires in place sticks, as supplied by heating company.
4. Laid heating wires.
5. Re-screeded using Treadfast sx-1 level to finish ready for tiling.
I've just learnt today from Tremco / Treadfast that they recommend sx-100 for final layer as its flexible. I understand sx-1 to be a latex compound (part 2) and a cement based powder (part 1), and unlike the sx-100 doesnt include fibres with the cement for flexibility.
I'm now worried that this will not be flexible enough to cope with increasing / decreasing temperatures and will crack / crumble over time under any floor tiles I put down next.
If I tap the surface it sounds hollow, is this just because its laid on insulation boards? I can break the surface with a hammer easily, but it does seem well stuck to the insulation.
Please help, will the sx-1 be ok? Will the surface break with heat and/or if appliances, washing machine dishwasher etc, jump around or vibrate on it.
.....should i take it all back up and start again?
I appreciate any good advice.

 

[Dan]

Welcome to tilersforums.com | Tile Forums | Tiling Forum!

Hello Rob_1, welcome aboard www.tilersforums.com. Stick around, I'm sure you'll like the place. 🙂

 

[suave]

Re: Welcome to tilersforums.com | Tile Forums | Tiling Forum!

:welcome:

 

[Ajax123]

Fibres do not add "flexibility" to the screed. The poly fibres are there to stabilise the material against shrinkage cracking. They generally do nothing more than this. If it is a latex leveller I would guess it will indeed be flexible.

The SX 1 is a 25 N compressive strength so is pretty strong. I would be more worried about dynamic loading than i would about expansion/contraction. Have you put underfloor heating under the washing machine and other appliances.

Who sold you the gear, i.e. who told you it was suitable. Have you asked Tremco technical if they think it will survive even though it has no fibres in it. Not many levellers do.

 

[DHTiling]

Hello Rob and welcome..

The hollow sound could be the insulation boards have not got 100% adhesion to the substrate.
Hitting it with a hammer will crack it as the poly foam core of the insulation board will crush with the impact.

When fixing the boards do you use a notched trowel and press from the centre out on the boards as you fixed... this ensures the middle if fixed before air entrapment..

 

[Ajax123]

Dave - Does Latexplan trade have fibres in. That is a similar compound to the SX-1 I beleive i.e. bag and bottle polymer modified leveller. Mapei sell this one for underfloor heating. would give a reasonable indication if the SX1 is likely to be suitable or not. I am not really familiar with either products.

 

[DHTiling]

Nope...Al..

But latex plan trade is suitable for UFH..

 

[Ajax123]

That's what I thought. On that basis I would hgazard a guess that the tremco gear is probably ok too. I comes down to subfloor prep I think then.

 

[Rob_1]

All,
Thanks for all the quick replies, great stuff!
I no particular order...
My local tile warehouse recommended sx-1 for first screed, then also over wires. He now says he didnt, so somewhere there has been some confusion. Never the less, i just need to get some confidence its either no problem at all, or its definately something to worry about.
Yes, I used a knotched trowel / comb with flexible adheisive on insulation boards. They seem to have stuck well no problem.
I think the hollow sound is because of the boards, and not a full solid base like concrete for example. It just 'sounds' a bit fragile, maybe i am worrying too much. Just wondered if anyone recognises what i'm explaining when screeding over boards.
The screed seems to have bonded to the boards ok, and as Dave says I think the hammer test(!) is becuase its a shock point load crushing the foam, allowing screed to break.
No heating under appliances or kitchen units, just open area.
As built in I could set appliances, dishwasher and washing machine, on a wooden base to spread their load under the cabinets.
If latex Plan is a close match, no fibres, and seems ok then maybe sx-1 is too.
Tremco technical support have told me that all their screeds meet the BS of 27deg surface temp, but would normally recommed the sx-100. They havnt tested any of them it seems, just developed them to meet certain properties, if they can do this without testing.

 

[DHTiling]

To be over safe.. you could use an uncoupling membrane, this will uncouple the tiling from the SLC and no cracks to the tiles if movement occurs.

 

[The Legend; Phil Hobson RIP]

Again it's all been said,

 

[Ajax123]

All,
Thanks for all the quick replies, great stuff!
I no particular order...
My local tile warehouse recommended sx-1 for first screed, then also over wires. He now says he didnt, so somewhere there has been some confusion. Never the less, i just need to get some confidence its either no problem at all, or its definately something to worry about.
Yes, I used a knotched trowel / comb with flexible adheisive on insulation boards. They seem to have stuck well no problem.
I think the hollow sound is because of the boards, and not a full solid base like concrete for example. It just 'sounds' a bit fragile, maybe i am worrying too much. Just wondered if anyone recognises what i'm explaining when screeding over boards.
The screed seems to have bonded to the boards ok, and as Dave says I think the hammer test(!) is becuase its a shock point load crushing the foam, allowing screed to break.
No heating under appliances or kitchen units, just open area.
As built in I could set appliances, dishwasher and washing machine, on a wooden base to spread their load under the cabinets.
If latex Plan is a close match, no fibres, and seems ok then maybe sx-1 is too.
Tremco technical support have told me that all their screeds meet the BS of 27deg surface temp, but would normally recommed the sx-100. They havnt tested any of them it seems, just developed them to meet certain properties, if they can do this without testing.

It will sound "hollow" and is not a suitable substrate to carry out a hammer test on as Dave says it will crush the insulation and cause deflection cracks. Tremco are not saying it is not suitable (I assume) it is just that they would normally recomend the one with fibres. I am not sure the presence of fibres would make a substantial difference but if they have not done any testing then they will no doubt over specify.

I think I agree with Dave that for complete peace of mind on your part as this is always likley to play on your mind and spoil your enjoyment of the floor I think an uncoupling membrane would be a good idea. That way the tile face is protected from the lateral stresses created if the laeveller does begin to crack later. Have you sorted a tiler yet. It may be that one of the members on here would be grateful for the work.

The 27oC surface temperature is a red herring in this case. That is trhe maximum surface temperature advised when using resillient flooring such as vynil and is to take account of the fact that the adhesives used to stick them down begin to break down at that point.

Putting your washing machine on a timber support could be more trouble than it is worth because if it gets wet it will start to mould and swell and rot. If you want to spread the load a peice of hardiebacker type board might be better.

 

[Rob_1]

Thanks for the replies.
I will be looking for a tiler yes, if anyone is in coventry and could give some advise and maybe interested in the job let me know.
I've never come across uncoupling membrame before. I will do some research, is it a membrame (off a roll / or painted on) that goes on before tile adheisive?
I will look at hardiebacker to put under appliances then too.
I have turned the UFH on, set on its timer to come on and off a couple of times every day. I will see what happens with the screed over a week to see if any movement appears. I'd like to think that if it was going to move / fail it would do so over that time anyway.

 

[Rob_1]

Just an update on the floor screed. I have had the UFH on and off since yesterday. The floor now has shown hairline cracks all over it, seem to be in lines where joints of the insulation boards are below, but some run do run diagonally. They seem to have got a touch worse after cooling, though that could just be me seeing more of them the more i look! It does also seem to be de-laminating it from the insulation in places too.
I looked at the ditra product, it looks great. I spoke to them, but they advised it was only as good as the surface it was laid onto, makes sense.
So, i think i'm going to make the decision and have a busy weekend taking the screed and UFH wires back up.
I have tried a patch, it seems to come up and leave insulation surface resonably intact. What are the views on keeping the insulation down and re-laying wires and correct(!) screed over the top? Obviously all primed propoerly?
Thanks for any more advice.

 

[Ajax123]

Just an update on the floor screed. I have had the UFH on and off since yesterday. The floor now has shown hairline cracks all over it, seem to be in lines where joints of the insulation boards are below, but some run do run diagonally. They seem to have got a touch worse after cooling, though that could just be me seeing more of them the more i look! It does also seem to be de-laminating it from the insulation in places too.
I looked at the ditra product, it looks great. I spoke to them, but they advised it was only as good as the surface it was laid onto, makes sense.
So, i think i'm going to make the decision and have a busy weekend taking the screed and UFH wires back up.
I have tried a patch, it seems to come up and leave insulation surface resonably intact. What are the views on keeping the insulation down and re-laying wires and correct(!) screed over the top? Obviously all primed propoerly?
Thanks for any more advice.

Sadly despite trying to think of an alternative I think I have to agree. Bite the bullet, take it up and start again. You do need to make certain the insulation is stuck down solid. If you are getting cracks along the joints this suggest they were either not sealed (taped) or there is movement at the edges. Have you got any pics?