UFH in our very old converted stable

[strideredc]

After many wasted months we are starting work on it. it's a slow but almost full refurb...our builder is breaking up the existing 4/5 inch concrete slab and then digging down to give us the depth we need. 

 

the slab was put in above the existing sand and rubble floor with a dpm about 40 years ago.Never been any movement or cracking of the slab as our house sits on a sandstone hill and seems very solid

 

The question I have is our builder wants to not put a new concrete sub slab in but to put scalpings in then wacker them down then bind with sand before laying a DPM then 150mm celotex on top... then pipes then another dpm (screed company want this before they pour)

 

I thought you would need a new concrete slab then sand dpm insulation. 

 

I also spoke to a building contracts manager on a job nothing to do with us today and he said you can do it this way and you dont need a new concrete slab if it's done properly  (our builder comes highly recommended and seems to know what hes doing... I am a controlls electrician so have no idea I just see jobs sometimes with pipe all put it. it's very confusing with lots of different opinions on this....

 

 

Edited November 11, 2020 by strideredc

[Mr Punter]

If the ground is nice and solid it is an ideal way to do it, as long as you don't want to build any heavy partitions.  For some reason on a new build they will insist on a concrete slab.  It is not needed.

[Onoff]

I did:

 

-Compacted Type 2 (well, home graded hard core through a 40mm sieve)

-Sand blind

-25mm EPS

-DPM

-150mm Celotex

-Polypipe panels with 16mm pex-al-pex UFH

-A2 mesh

-100mm concrete+fibres laid with screed rails

-Tiles

 

I'm in two minds about the Polypipe panels as in whether to use them again. Cheap enough and they make running your UFH pipes a doddle. You end up with a lighter slab because of the castellations though. You save on concrete of course. With them of course you don't need a membrane between the top of the Celotex and wet concrete that goes on top. Little video of laying the pipe:

 

 

Edited November 12, 2020 by Onoff

[Dave Jones]

i think quicker and easier to staple the pipes direct to the celotex.

[Gav_P]

6 minutes ago, Dave Jones said:

i think quicker and easier to staple the pipes direct to the celotex.

Certainly cheaper. 

[Onoff]

14 minutes ago, Dave Jones said:

i think quicker and easier to staple the pipes direct to the celotex.

 

Everyone's entitled to their opinion.

[strideredc]

thanks for the replies, there will be no heavy partions on it just a stud work bedroom (put in for the disabled previous owner that we have the option of removing if we need latter) and a stud plasterboard bathroom... this is just slab of screed that will sit on a bed of celotex, 150mm I think we will go for. I dont want to go crazy deep due to footings on one side as the house is on a hill/slope...I could put 500mm+++ of celotex on the other side without hitting the ground on the other side!!

 

what screed do you recommend? sand/cement,  liquid, or concret? I presume the concrete is stronger? we had quotes for sand and liquid and there was 200 quid between them (sand/cement being cheaper)

should we go a bit thicker?

[strideredc]

when my wife spoke to the screed people she recons they said before they pour that they want another dpm over the top of the pipes??? I thought the screed went on top of the pipes that are on the dpm (main dpm is below the celotex ontop of the sand)

what should the exact order be?

my reckoning was

type 1 compacted 

sand binding 

celotex (other brands are available!)

dpm

pipe clipped through dpm into celotex 

screed piped direct onto pipe to xxx level....

[Dave Jones]

dpm under the celotex sitting on the sand, its why you blind it. dont have another above the pipes how would the screed flow into all the gaps ?

 

If you intend to lay flooring on the screed it needs to be put in with a laser, if they spin you a yarn about it being self levelling just know it wont be.

Edited November 12, 2020 by Dave Jones

[Nickfromwales]

The second DPM, is it a slip membrane that we're actually talking about here? That would be employed if you are having a liquid screed vs a dry sand & cement based screed.

If you are having a dry screed, some screeders will have a flap about the screed reacting with the foil layer of the PIR. You can mitigate against this by not having a foil backed product.

Details please? 

[strideredc]

I will phone them next week and speak to the myself see what they are on about!

[Onoff]

On 12/11/2020 at 17:37, Nickfromwales said:

The second DPM, is it a slip membrane that we're actually talking about here? That would be employed if you are having a liquid screed vs a dry sand & cement based screed.

If you are having a dry screed, some screeders will have a flap about the screed reacting with the foil layer of the PIR. You can mitigate against this by not having a foil backed product.

Details please? 

 

I wonder...could you paint a liquid DPM onto foil faced boards to guard against any reaction?

[strideredc]

I read the quote. for 50mm of fast dry thet want a min 500 gauge membrane under the pipes on the top of the celotex.  this is what happens when women relay things to you!!!

 

anyway back to the screed... 50mm doesnt seem a lot

 

what would you guys recommend as it's going to be laid on 150mm celotex on sand and compacted scalpings /type 1

 

what do you guys think??

[PeterW]

12 minutes ago, Onoff said:

 

I wonder...could you paint a liquid DPM onto foil faced boards to guard against any reaction?

No.... just NO..!! You use a very cheap slip membrane and tbh the “reaction” only happens when the cement is wet and there is no evidence of it causing any issues as it is only to protect the front of the boards. 
 

3 minutes ago, strideredc said:

for 50mm of fast dry thet want a min 500 gauge membrane under the pipes on the top of the celotex.

500 gauge is the thickness of a rubble sack at best, and is not a DPM - they are quoting a slip membrane there to stop any of the liquid getting under the Celotex and lifting it. A DPM should be a minimum of 1200 gauge and preferably thicker. 
 

50mm is very thin to have UFH in it.  You may only have 30mm or so over the top of the pipes so it will react very quickly but won’t store much heat, and may be prone to cracking. 

 

[Nickfromwales]

2 hours ago, PeterW said:

500 gauge is the thickness of a rubble sack at best, and is not a DPM - they are quoting a slip membrane there to stop any of the liquid getting under the Celotex and lifting it. A DPM should be a minimum of 1200 gauge and preferably thicker. 

Yup. +1

2 hours ago, PeterW said:

50mm is very thin to have UFH in it.  You may only have 30mm or so over the top of the pipes so it will react very quickly but won’t store much heat, and may be prone to cracking. 

50 in liquid will be ok, not great, but it’ll last and do what it’s supposed to. Floor will be just like a slow radiator at that thickness so no real issue. Will allow the heating to be ‘timed’ as it’ll respond quickly. A benefit in some folks eyes. 

[strideredc]

I am running it off a Ashp ( will be ) so would it be better to go thicker I am not after a fast heat up time particularly. it's an old house but looking for a slow steady background heat. will use log burner for when I really needs extra as have big supplies of wood and want to keep flow temps as low as poss for efficiency...