150mm or 125mm concrete screed over insulation for ufh?

[saveasteading]

1 hour ago, saveasteading said:

floating it....which is not a pretty sight

A bit like icebergs bobbing up.   I had a plumber who insisted he was expert  in UFH (before it was common) , and that he had to include the screeding in his package.  An open plan office area of about 200m2.  The PIR floated up, he panicked and my masterful management skills kicked in, amounting to:  stop pouring:   bring in concrete blocks to lay on the pir :  start pouring.   It could have been much worse and fortunately I had felt the urge to be on site.

 

This was with taping of joints but not a plastic sheet as well, which is optional. Tape should work but clearly is fallible*. The cost of the extra sheet is not much compared to the risk.

 

* I think the issues are where tape is loose, or has come off, and at edges where it isn't fixed hard.to anything.

 

The downside of a top sheet might be that it conceal gaps in pir or the use of multiple offcuts.

[SteamyTea]

Can glass fibre chippings be added to a concrete mix to reduce cracking?

 

As for where the pipes go. I would say never next to the PIR, that just heats the insulation more than needed, which increases the temperature delta to the ground.

The closeness to the surface is really set by the pipe spacing. So if your pipes at at 150mm spacing, then 75mm depth, 100mm spacing, 50mm depth.

Depth from surface equals half pipe centre spacing.

 

Edited September 12, 2023 by SteamyTea

[saveasteading]

13 minutes ago, SteamyTea said:

Can glass fibre chippings be added to a concrete mix to reduce cracking?

The concrete or screed supplier can add them to the mix for about £10/m3. Thus much cheaper than steel, esp with thin slabs. For diy it is a small pillow of fibres.

Without steel, these fibres should allow panels up to 4m x 4m.

[saveasteading]

17 minutes ago, SteamyTea said:

never next to the PIR, 

But if the screed is 60mm there isn't a choice, and clips are sold precisely for this.

[SteamyTea]

28 minutes ago, saveasteading said:

But if the screed is 60mm there isn't a choice, and clips are sold precisely for this.

Yes, seems odd to me and lacking in thought.

 

[saveasteading]

35 minutes ago, SteamyTea said:

seems odd to me and lacking in thought.

Doesn't trouble me. Explain why I should be troubled please?

Having recently seen it done, and the (low) level of skill and care, I'd rather the pipes were on the solid bed when they get walked upon, and it means they are all at the same level.

[saveasteading]

1 hour ago, saveasteading said:

add them to the mix for about £10/m3. 

@Jenkiused them i think. I wonder how his slab is looking.

[SteamyTea]

46 minutes ago, saveasteading said:

Explain why I should be troubled please

Because of the potential greater heat losses.

Not considered a problem raising mesh up 50mm or so, and not considered a problem attaching UFH pipe to steel mesh.

Probably not that hard to get different height posts that are suitable to attach UFH pipework to.  May seem extra work and expense, but probably isn't in the scheme of things.

[saveasteading]

Because of the potential greater heat losses. I wonder if there are. I don't know, but my  gut feeling is that it is minimal difference. The easy route for the heat is upwards.

Not considered a problem raising mesh up 50mm or so, and not considered a problem attaching UFH pipe to steel mesh.  Unless the mesh is only for that purpose, so adds cost.

Probably not that hard to get different height posts that are suitable to attach UFH pipework to.  May seem extra work and expense, but probably isn't in the scheme of things. Yes i thought of that too: just a bit more plastic under the peg, as a spacer  to keep the pipe off the pir. But then it is a problem and risk moving around n between dangling pipes.

[SteamyTea]

6 hours ago, saveasteading said:

Because of the potential greater heat losses. I wonder if there are. I don't know, but my  gut feeling is that it is minimal difference.

I think it changes the ratio by 30% if you put the pipework half way up in the concrete. This assumes equal thickness of insulation and screed.

30% sounds a lot, but it is not great, but then it does depend on the temperature differences between the pipe temperature, slab/insulation temperature and ground/air temperature,

If I was not so busy I would try and calculate, been a while since I have looked at differential equations.

 

 

 

[Jenki]

13 hours ago, saveasteading said:

@Jenkiused them i think. I wonder how his slab is looking.

The fibres in our slab were approx 50mm x 2mm x 1 mm.

@saveasteading the slab didn't have any large cracks👍, and the plastic rails/expansion joints seemed to work. I've not got around to "fettle" the edges yet.  We are water tight now, so this will be a rainy day job. Just trying to get the larch cladding fitted before the weather really changes. ( it was 23deg two days ago, and 7deg @ 7 am this morning.( 8 in the static🥶)