Underfloor heated screed - edge insulation.

[Tim Watts]

I'm sticking some JG Speedfit UFH pipe in the screed of the conservatory.

The floor will be 75mm screed on 75mm celotex on DPM on concrete. The concrete is already there.

I know you need an edge buffer between the walls and the screed to allow for expansion and also to stop heat losses to the walls.

Could I glue 25mm celotex strips to the wall at screed height (except at the doorway). Or must I use the 8mm edging foam:

The main thing I see happening is with too much thickness, the screed might "walk" over time.

OTOH, 25mm celotex forms a nice edge for the plasterer to work to (he's coming first, floor screed is last).

Cheers

Tim

[The Natural Philosopher]

I have 60mm of polystyrene foam..

[Tim Watts]

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

60mm *around the edges*? Seems a lot of tile overhang might result???

[The Natural Philosopher]

yep. lots of tile overhang resulted.

[Tim Watts]

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

*over* the celotext, then fit the pipes then screed.

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.

[steve]

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...

[Tim Watts]

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!

[1] No I am not laying 16m2 of 75mm screed! However, I have 3m2[2] to do later on, so I'll also be watching to see how it's done (properly) :) [2] With compound gradients to meet a central floor drain - so it's actually easier to do this rather than try to specify everything!

[steve]

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 :-)

[Tim Watts]

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?

[steve]

Very much... but more so!

[harryagain]

25mm thickness for the vertical surround is good. You can carry it up the wall behind the skirting too. (You need a batten along the top edge of where the skirting finishes.) This reduces the effect of the thermal bridge /heatloss down into the ground

[spuorgelgoog]

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.

[1] Now disconnected [2] and the feeder cables re-used for the 100V public-address system. [2] Found another conduit box under the plaster this afternoon ;-)

Owain

[steve]

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.

[Tim Watts]

On Sunday 18 August 2013 18:18 [e-mail] wrote in uk.d-i-y:

Splendid - thanks :)

[Piers]

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

[SteveW]

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

SteveW

[steve]

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

[Tim Watts]

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

Cool - thanks.

Will be doing this tomorrow, probably.

[Tim Watts]

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...

[Piers]

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.