Polythene above insulation prior to UFH

[eandg]

Any particular recommendations? From a quick google anything above 500 gauge should be okay? Lapped to the walls? (We have foil face insulation below UFH pipes and on the walls). 

[JohnMo]

It just there to separate the aluminium foil on your insulation from the cement, to stop a chemical reaction. It also helps stop the screed going under the insulation lifting it all up. Anything will do that does mind being walked on.

[Nickfromwales]

4 hours ago, eandg said:

Any particular recommendations? From a quick google anything above 500 gauge should be okay? Lapped to the walls? (We have foil face insulation below UFH pipes and on the walls). 

Is the pipework down already? If so, you'll need a much lighter membrane (1200 or so) otherwise the membrane will tent over the pipes and not envelop them. This will lead to poor heat transfer from pipe > screed / other.

You lay that in lots of sections rather than one large area and tape after it's been tucked in as best as possible.

[ETC]

4 minutes ago, Nickfromwales said:

Is the pipework down already? If so, you'll need a much lighter membrane (1200 or so) otherwise the membrane will tent over the pipes and not envelop them. This will lead to poor heat transfer from pipe > screed / other.

You lay that in lots of sections rather than one large area and tape after it's been tucked in as best as possible.

You need a VCL- it’s normally only 500 gauge - quite light really. Alternatively if you’re using PIR with a foil face tape all the joints. A separate VCL is easier though.

 

I am presuming the DPM is already laid below the insulation.

[eandg]

12 minutes ago, Nickfromwales said:

Is the pipework down already? If so, you'll need a much lighter membrane (1200 or so) otherwise the membrane will tent over the pipes and not envelop them. This will lead to poor heat transfer from pipe > screed / other.

You lay that in lots of sections rather than one large area and tape after it's been tucked in as best as possible.

No, not yet. We've an odd build up - insulated slab (with 375mm EPS) already down but at initial design stage I'd overlooked the need for a thermal break so adding a further bit of insulation (20mm Kooltherm with joints foil taped) and polythene this weekend prior to pipes and screed next week. 

 

What do you tape the polythene with? 

[Nickfromwales]

500 will tent. Use thinner.

3 minutes ago, eandg said:

No, not yet. We've an odd build up - insulated slab (with 375mm EPS) already down but at initial design stage I'd overlooked the need for a thermal break

Could you explain, please?

 

3 minutes ago, eandg said:

No, not yet.

Great. You can use the thinner membrane and the staple gun will shoot the staples through it just fine.

3 minutes ago, eandg said:

What do you tape the polythene with? 

Just gaffa / duct tape tbh. I've never needed anything more than that, and I've been doing these jobs for north of 25 years.

[eandg]

3 hours ago, Nickfromwales said:

500 will tent. Use thinner.

Could you explain, please?

 

So just the cheapest stuff about, 200 or so? 

 

I had thought we'd be fine with the EPS under the (50mm) structural topping on the slab but UFH company advised we'd need a thermal break between screed and slab otherwise we'd waste a lot of of energy heating the concrete below rather than the screed above. 

 

3 hours ago, Nickfromwales said:

Great. You can use the thinner membrane and the staple gun will shoot the staples through it just fine.

Just gaffa / duct tape tbh. I've never needed anything more than that, and I've been doing these jobs for north of 25 years.

 

Lovely, can manage that. 

[Conor]

6 hours ago, eandg said:

 

I had thought we'd be fine with the EPS under the (50mm) structural topping on the slab but UFH company advised we'd need a thermal break between screed and slab otherwise we'd waste a lot of of energy heating the concrete below rather than the screed above. 

 

Lots of people here have a similar setup with UFH embedded / on top of a concrete slab. Works fine, and give you advantage of using it as a thermal store. I'd skip the insulation and polythene and stick the pipes to the slab using self adhesive rails. Nick has posted photos of a job he's done that's basically the same.

[Tom]

The 20mm of Kooltherm is doing absolutely nothing for you here, other than costing more and being a headache to fit. UFH companies tend to think only one way: slab, insulation, pipes clipped to this, screed. Anything that diverges from this and the computer says no.

[Iceverge]

9 hours ago, eandg said:

had thought we'd be fine with the EPS under the (50mm) structural topping on the slab

 

This doesn't seem to make much sense. Did you mean 150mm?

 

 

[saveasteading]

I think that when ufh was new  it was the considered wsdom that a thick concrete slab with ufh pipes in it was the best way. It acted as a thermal store.

But more recently we are looking for more control and quick response, so a thin screed allows that. It also avoids wasting energy in warming a slab when less heat is actually required, or tomorrow is warm.

 

In our project we regarded the underslabbing as a working surface for level control, some strength, and cleanliness. Then pir then screed with ufh.

[eandg]

Thanks all - the Kooltherm is currently sitting on my slab and I'd need to make it up with additional screed (at further cost) in any case. 

[saveasteading]

From someone who has seen pir floating....you don't want the risk..

.the seal is important. Tape the PIR first then sheet with more tape.

500 might rip. 1000 won't.

[Nickfromwales]

40 minutes ago, saveasteading said:

From someone who has seen pir floating....you don't want the risk..

.the seal is important. Tape the PIR first then sheet with more tape.

500 might rip. 1000 won't.

They only float if it is a liquid screed that's seeped under. Dry screeds or Flowcrete won't be anywhere near as problematic.