Chipboard laid over wet underfloor heating pipes?

[Alex Carr]

I'm about to gut 46 sq. mts of basement and then tank it. After that I want to install wet underfloor heating but the contractor's idea of how to do the build-up for the UFH does not seem sensible. So far he has been v. helpful, but advice/guidance from a wiser head that would get us both on the same page would really helpful. This is his approach:

1. Lay DPM (reaching up 1.2 meters on all the walls).
2. Lay 100mm thick insulation (Kingspan or Celotex).
3. Lay 150mm concrete slab.
4. Lay 45 x 45 battens on the slab (centres not fixed yet).
5. Lay UFH pipework between battens.
6. Lay 18mm chipboard flooring.
7. Lay 20mm engineered wood floor onto the chipboard.

What I don't get is 1) Why would he put the insulation under the concrete. If there is a concrete slab under the water pipes surely heat will be lost downwards through the concrete? 2) From my perspective 18mm of chipboard between the pipework and the 20mm engineered floor is a NO because of its poor thermal conductivity. I see the chipboard as a barrier to heat transmisssion This is my version of the build-up:

1. Lay DPM etc.
2. Lay 150mm concrete slab on the DPM.
3. Lay 75mm insulation (Kingspan or Celotex) on the slab.
4. Lay vapour barrier layer (Visqueen)
5. Lay pipework - pinned to insulation.
6. Lay screed.
7. Lay engineered wood floor.

Thanks in advance for advice on this.

 

[Mr Chibs]

Here's mine, from ground up.

Hardcore 100mm
Sand blinding 25mm
DPM
Concrete slab 100mm
Insulation PIR 100mm
Liquid screed with heating pipes laid into castellated trays 65mm (can be sand & cement instead, 75mm)

This heats up quicker, than putting the insulation under the slab, but you can do both versions. Under the slab insulation will retain heat longer.

I would make your slab 100mm and increase the insulation.
You can just float your floor on the screed, no need for chipboard.

 

[Alex Carr]

Hello Mr Chibs - Actually it's a cellar space I'm going to be tanking, so the DPM has to be laid over the solum (Scottish term for the ground the property is built on), and then the hardcore over that. Agree w. the sand blinding. Your specs for hardcore + sand binding sound right.

I have a total of 300mm to work with, hence 75mm rather than 100mm insulation laid over the hardcore + sandblinding > Visqueen vapour layer > UFH pipework, secured either w. a concrete screed or castellated trays w. liquid screed poured over that. From the surface of the tray to the top face of the liquid screed - what would its depth be?

 

[Mr Chibs]

You don't have to use trays, you can clip directly to insulation.

Liquid screeds can be thinner too, 45/50mm.

Sand blinding can be reduced, it's purpose is just to protect the DPM, so 10mm may suffice.

The trays, around 30mm in depth and the pipes clip into this space.

I would be tempted to shave 10mm off here and there to get at least 100mm insulation, you could chuck reinforcement in the slab.

Take a look here: it's good reading for UFH

 

[Alex Carr]

In terms of clipping to the insulation I would have thought that if I use a liquid screed, piercing the Visqueen with barbed staples would negate its function as a vapour barrier. I could be wrong about that.

 

[Mr Chibs]

You put a slip sheet (polythene) in between, if doing it this way.

 

[Alex Carr]

I had a look at the Polypipe installatio guide - illuminating and informative. Thanks for that.

 

[OvertonPlumber]

If using liquid screed use trays. Clipping is only any good for normal screeds. The whole trays vs clipping is only a cost decision by the contractor to save money. Clipped pipes tend to float/lift up with a liquid screed.

 

[Alex Carr]

As I understand it, the clips are barbed and designed to go one way but not another. Can the screed exert a particular pressure that over time loosens the barbs and force them upwards. Or maybe even a chemical reaction that degrades the hole surfaces made by the barbs fitted to the insulation that allows the barbs work themselves loose? Your comment is definitely food for thought, but a bit more clarity around the reason why the barbs would lift would be a big help. Also this: I like the idea of castellated trays to hold the pipework, but how to lay an engineered wood floor over the pipework?

 

[Mr Chibs]

Can't speak about barbs/clips, but when using the trays, the pipework clips into the tray. The pipe is not on the surface of the insulation it is lifted off the surface slightly, probably two thirds down the screed depth, when you've finished. You then simply lay your floor on top of the screed.

One factor if you are doing the clips, you need to buy/rent a stapler, this is when I moved to the trays.

If you are looking at spreader plates, they are cutout for the pipe and are flush with the top of the plate, again, lay your flooring on top.