Hi all

I am sorry if this is a well trod subject but I am finding it difficult to figure it out from what I can find on existing posts.

This a 1908 semi with (mostly) suspended wooden floors with about 350 mm from floor level to the oversite.
We are looking to install underfloor heating with an ASHP down the line.

The plan is to remove the existing floors and "fill in" with a slab / insulation/ ufh and screed.

I would be very grateful for suggestions as to what the build up should be - thickness of insulation, slab etc.

Should the insulation be under the slab to increase thermal mass (rest of house insulation will be internal)?

Builders we have spoken to for a side extension that is planned seem to lean towards 150 PIR insulation on top of the slab with the UFH pipes pinned to the insulation and a screed on top.

We would like a decent level of insulation in the floor to give the ASHP a chance

Thanks

Thanks Tony
Sorry I don't follow the skim / cut part. Where is that removed from?
I guess it has to be PIR to get the insulation levels with the available depth

We put 150mm EPS in 2 x layers of 75mm each
Which was cheaper than 100mm PIR
for the same insulation value

What floor surface are you planning? We have used brick which has also added to the thermal mass.

What Tony means by skim/cut top 25mm off, I would assume is to remove 25mm from oversite to get undisturbed base.
I have put insulation under the slab, as per Dominic above, because I don't see the point of essentially 2 slabs (100 below insulation and 75 above).
Also I would use EPS.
350mm is quite a bit to fill so perhaps 225 EPS with slab over to level .
I would also put 20mm EPS around the periphery between the slab and the walls.

Posted By: DurI guess there is a method of fixing the UFH tubes to the top of the slab and then a thin screed over to cover them.

I think it's more normal to wire the UFH pipe to the reinforcement in the slab and cast the slab around the whole lot. You don't need a screed on top depending on your intended surface finish.

Oops cross-posted

We also put 75mm EPS around the edge of the slab, 100mm PIR on the walls

Posted By: DurSo putting the UFH in the slab saves a bit of space so we can have a little more eps

exactly - EPS is cheaper and lower embodied carbon than PIR - it should also give a constant insulation value even as it ages. Only down side is that you need it to be thicker for the same performance but with 350mm that's not a problem.

2 staggered layers is considered better as less thermal bridging through imperfections (and it can bend a bit more).

Insulation up-stands at the edges of about the same thickness as your internal wall insulation.

Typically polyethene dpm between the insulation and the inner slab.

Caveat: physicist not builder

Posted By: DurIs a vcl needed above the concrete ( once properly cured) or is the concrete a sufficient barrier in its own right?

The DPM acts as a VCL, so adding another would create a vapour-tight 'sandwich', which is generally a bad idea. Floors tend to have more problems dealing with liquids than vapours though - spills and floods etc, so that's usually the most important consideration. Concrete is moderately resistant to water and water vapour but can absorb them and let either of them through slowly. But the short answer is no, you don't need a VCL above the concrete.

Thanks JMS.
Interesting - I was expecting the DPM would have to be under the EPS but I guess the EPS is not too worried by any damp. Is a vcl needed above the concrete ( once properly cured) or is the concrete a sufficient barrier in its own right?

The last floor I did I put the DPM under the EPS (and coming up the sides to the wall DPM) because I didn't want the builder stomping all over it with big boots and wheel barrows which is what would happen if it was between the EPS and the slab
So the floor build up was soil (happens to be very sandy) DPM, ESP, slab then tiles as it was a kitchen but could equally have been any other floor finish.

Posted By: jms452I think this is a simplified version of your floor dgh?

Yes, we have four layers of 100mm graphite EPS with the DPM above the bottom layer. I think the logic is that in that position it is protected from damage by the hardcore etc underneath and from builders boots etc on top. Some people like it right at the top to prevent any concrete leaking down between the sheets of EPS.

Our slab is quite complicated: it's nominally 150 mm but has some 100 mm downstand ribs to give it extra strength, plus of course a separate perimeter beam for the walls.

I expect one of the reasons that insulation-over-the-slab-then-pipes is more common is that it’s quicker to staple the pipes into the insulation or clip it into “egg-boxes” than to cable-tie it to mesh. Doesn’t mean that it’s the best for your situation.

DJH - I just re-read the above and realised you said 4 x 100 EPS. I have now insulation envy, especially given the latest UN report!

DC - I spoke to a builder the other day and he was suggesting insulation with slab on top and then eggboxes and 50mm screed. I can see that is going to reduce the labour bill significantly but then you lose a little insulation space. I don't know whether there would be much difference in the heat transfer into the slab with the pipes in a 50 mm screed - I presume it would be better in the slab. Builder default was 100 mm PIR so some work to do there/.