Underfloor heated screed - edge insulation.

I have 60mm of polystyrene foam..

On Sunday 18 August 2013 13:48 The Natural Philosopher wrote in uk.d-i-y:

yep. lots of tile overhang resulted.

On Sunday 18 August 2013 14:17 The Natural Philosopher wrote in uk.d-i-y:

Just checking :)

I assume therefore you had no problem with the screed walking and cracking the tiles by a doorway?

I cannot see it being willing to move - but then I've never done screed quite like this.

On a side note - the JG Speedfit site says to put an extra light membrane

I cannot see any point in this (there is a proper DPM under the insulation).

But I will tape the celotex joints to make it easier for the screeder.

Am I missing something? Having a thin fliddy bit of plastic on top of the celotext looks like it will make life harder for everyone as it will move about and probably rip as the bloke packs the screed down.

I used the Screwfix stuff around some 20m sq of UFH. No cracks after three years so it must be absorbing expansion forces.

Regarding your later question about a thin membrane over the pipes. I've not heard of this and can't see any benefit. I clipped the pipes to the celotex, pressurised the system and then screeded directly over the pipes.

I used fibres in the screed mix to strengthen against cracking...

On Sunday 18 August 2013 15:21 steve wrote in uk.d-i-y:

Ok - that's a vote either way.

I should have said - I have spare 25mm celotex whereas I'd have to drive to Screwfix to get the foam strip :)

It was under the pipes, over the celotex.

Indeed - the builder[1] said he will do that. I will be watching!

Fair enough!

Sorry, my bad. Probably to prevent screed mix corroding the aluminium foil coating, not that that matters in this application! I just taped all joints prior to laying the pipes and screeding. I didn't bother with aluminium tape and just used black pvc 2" tape.

Nothing hard about the fibres, except cleaning the damn things out of the mixer! Just a handful per mix but the screed does come out a bit "hairy" and is harder to work with :-)

On Sunday 18 August 2013 16:35 steve wrote in uk.d-i-y:

Cool - thanks - Seems like a safe bet then to proceed with 25mm round the edges and not bother with the secondary membrane.

I will cut the 25mm down the middle and make a thin strip to buffer the door to avoid having tiles dangling over unsupported areas - as it happens, that one will line up with the door threshold and I'll use a silicone joint instead of grout.

Bit like goats' hair in plaster then?

Very much... but more so!

I have about 1" wide of polystyrene for my in-screed electric UFH [1]. it goes up to floor level, then there's a gap above the floor, then a batten, then the plaster. The plasterer worked to the batten and the skirting nails to it.

Owain

It probably does help very slightly in reducing any bridge but the main purpose is as an expansion gap. In my case, with a thermalite inner leaf wall with regulation fibreglass batts in the cavity and with the pipes not coming any closer tham 200mm to the wall, it was deemed unnecessary. It obviously depends on the walls that surround an installation and their construction.

On Sunday 18 August 2013 18:18 snipped-for-privacy@gowanhill.com wrote in uk.d-i-y:

Splendid - thanks :)

I think this is to stop the cement in the screed reacting with the foil on the insulation.

I used 25mm polystyrene. No problems at all with it. Been down for nearly two years, heating working for around 20 months.

SteveW

That is my thought too. I can't see any reason to worry about the foil in this application though.

On Sunday 18 August 2013 22:04 SteveW wrote in uk.d-i-y:

Cool - thanks.

Will be doing this tomorrow, probably.

On Sunday 18 August 2013 22:33 steve wrote in uk.d-i-y:

Yes - guess so. Cement is alkaline and ali is eaten by alkalis.

However, once it is dry nothing will happen...

I didn't bother with it (I didn't understand why it was specified either at the time!) and nothing disastrous seems to have occurred!

On the edge insulation question I wasn't happy with the 8mm stuff - thought this would end up leaking lots of heat to the exterior walls, so I used ~60mm kingspan (cut some of the 120mm stuff I was using under floor in half), plus the 8mm stuff in case it was required for the extra compressibility (as it's way more compressible than regular under floor insulation).

Wasn't fussed about how much this "sticks out" in the room, as I'm bringing the external walls in by about 100mm with battening and insulation backed plasterboard.