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.