UFH pipes in concrete slab, on insulation or tied to mesh

[Chanmenie]

Hi guy’s
I’ve read just about everything I can find on the forum but I’m still not clear on the best method.

 

What is the best way to build up to floor level in a ground bearing slab, as far as the Underfloor heating pipes are concerned.

On the insulation or tied to the mesh ?

I get the impression that using clips or staples directly on the PIR is easier than cable tying to the mesh, but which is more efficient when using an ASHP.

 

I am also concerned about walking on A193 mesh with ufh pipes attached  it’s only 7mm so will it bend.

 

Slab make up as this from bottom up.

100mm compacted type 1

sand blinding enough to fill and smooth type 1

25mm EPS ( should this be 100 rather than 70)

DPM

150mm Celotex XR4000

UFH pipe clips with pipes

40 / 50mm rebar chairs

A193 mesh

120mm Concrete 

The SE specified the A193 

[AliG]

MBC just installed a slab for us and the UFH is in the slab attached to the mesh.

 

There were no issues walking on the mesh to install the pipework. It was 252 which is 8mm. The main issue is keeping your balance.

 

I think if you have mesh the expected way is to attach to the mesh.

 

I haven't seen that kind of makeup before. What's the 25mm of EPS for? It won't provide a lot of insulation. 150mm of Celotex is roughly the same as 220mm of EPS.

 

It only took two guys around 5 hours to install 140 sq metres of UFH pipe tying it to the mesh. They used a gun which applied the ties, MBC said they would have used cable ties by handing taken similar time.

 

Is a PIR strip specified around the edge of the slab? Without it you can get a cold bridge.

 

 

[Chanmenie]

The 25mm EPS is to protect the DPM, better than just using sand blinding.

I have around 145 m2 so similar size 

 

yes there will be a vertical strip around the perimeter  

[AliG]

Thought it might be. Sounds all good to me.

 

Our UFH company didn’t care what way the  pipes were attached. As far as they were concerned it was the same job.

[Nickfromwales]

1 hour ago, Chanmenie said:

The 25mm EPS is to protect the DPM, better than just using sand blinding.

I always did this, seemed a no brainer.

[Olf]

12 hours ago, Chanmenie said:

I get the impression that using clips or staples directly on the PIR is easier than cable tying to the mesh

It may be, but you need a tool and extra spend on clips. Also it would have to be planned carefully to not to collide with the chairs for rebar, so I think tying to the mesh is the way to go.

 

Not sure about your shoe size, but 15cm between the bars is enough to put a foot on 2 adjacent, you can easily go forwards and sideways, bigger issue is reversing and not stepping on the pipes

 

12 hours ago, Chanmenie said:

which is more efficient when using an ASHP

Irrelevant, flow temperatures are the same.

Saying that, my thinking is that rebar would help to transfer heat from the pipes attached to it initially. Mind that with pipes at insulation level, the same will happen once the heat reaches the mesh above, and considering UFH heating is all about slow and steady, it should not matter in the end.

 

Also my thinking is that UFH pipes at mesh level should result with slightly lower losses: at 35°C flow temperature I get 25°C surface temperature. With pipes roughly in the middle of the slab I'd expect bottom of it (where it touches insulation) will also be at ~ 25°C. Pipes attached directly to the insulation means something close to the flow temperature is reached there, and so higher heat flow happens downwards.

 

12 hours ago, Chanmenie said:

DPM

150mm Celotex XR4000

UFH pipe clips with pipes

40 / 50mm rebar chairs

A193 mesh

120mm Concrete 

 

You need another layer of membrane (thinnest DPM you can find) on Celotex to stop reaction of aluminium facing with concrete, also prevent any leaks between the panels (just in case, even if foamed). As mentioned earlier, apart from compression lagging around, do install perimeter insulation to prevent bridging to the walls. Put it between DPM and the 2nd membrane, otherwise it will float away. I've seen opinions that EPS is 'soft' enough to act as both, though PIR will have better properties. 

 

Also all the services going through the slab will benefit from being wrapped in lagging

 

And last but not least make expansion joint in any doorways, otherwise they'll form naturally. Through those places make sure UFH pipes are bent (even forcing S shape) and covered in pipe insulation, so they can move a bit within once the slab is settling. 

 

 

[Bitpipe]

Our build up was

 

150mm compacted type 1

50mm sand blinding

300mm EPS 200 (could have got away with 200mm) - 200 grade specced by SE, you may get away with 150 (which is same compressive strength as your EPS).

DPM

Concrete construction.

 

We did not have UFH in the basement slab, but if we had it would have just been tied to the mesh.

 

Using just EPS would simplify your slab sandwich and remove need for second sheet of DPM.

 

[Chanmenie]

Thanks for the replies and info guys

I'm still undecided on where to fit the pipes, I am considering upping the Celotex XR4000 to 200mm, should give U value of 0.10 W/M2k 

Edited April 5, 2022 by Chanmenie

[Rishard]

Where did you decide to tie your UFH pipes in the end? 

[Chanmenie]

13 hours ago, Rishard said:

Where did you decide to tie your UFH pipes in the end? 

I used fibre reinforced easy flow concrete in the end, so stapled the UFH pipes to the insulation.

much easier and cheaper than using rebar mesh  

[garrymartin]

Apologies for updating a couple of older topics but if you're like me and have been diligently combing through old threads for nuggets of wisdom, the following might be helpful.

 

Short version, out of three scenarios (pipes at 19mm centres below surface of slab, at 50mm centres below surface of slab, and pinned to the insulation);

• the pipes at 19mm give the best performance, but obviously very close to the surface (~79.2 W/m²)
• the pipes at 50mm showed a small decrease in performance against those at 19mm (~75.1 W/m²)
• the pipes pinned to the insulation showed a much larger performance hit (~56.2 W/m²)

So when considering in-slab UFH and choosing between tied to rebar or pinned to the insulation, tied to the rebar will win from a performance perspective every time.

 

Longer version, in Imperial measurements (US-based author) https://www.pmengineer.com/ext/resources/PME/2019/July/014-017-pme-0719_siggy.pdf

[JohnMo]

See my other response. Plus below on the article 

 

From the article 

"This increases the time required

to warm the floor surface to normal operating temperatures following a call for heat. It also lengthens the cool-down time after heat input is interrupted by system controls. A fully “charged” slab can hold several hours worth of heat that will continue to flow into the space as long as the air temperature and / or interior surface temperatures are cooler than the floor surface. This can be a real problem in buildings

with significant internal heat gains from sun-light or other sources."

 

Comments 

With batch heating the long storage time constant is what you want. Heat for 7 hours, live off the heat for next 17 hrs until next heat cycle. Charge with a low hysterisis thermostat ensure little temp overseeing in the rooms. Well insulated ensures heat doesn't escape and also floor surface is only a couple of degrees warmer than the room so the floor output self modulates.

 

One of the FEA models I constructed is shown in Figure 1 (on Page 14). It consists of a 4-inch concrete slab sitting on 1-inch-thick

extruded polystyrene insulation.

 

Comments - 25mm of polystyrene insulation is a joke and undermines the whole premise of the article.

 

 

[gavztheouch]

On 15/05/2024 at 19:58, JohnMo said:

See my other response. Plus below on the article 

 

From the article 

"This increases the time required

to warm the floor surface to normal operating temperatures following a call for heat. It also lengthens the cool-down time after heat input is interrupted by system controls. A fully “charged” slab can hold several hours worth of heat that will continue to flow into the space as long as the air temperature and / or interior surface temperatures are cooler than the floor surface. This can be a real problem in buildings

with significant internal heat gains from sun-light or other sources."

 

Comments 

With batch heating the long storage time constant is what you want. Heat for 7 hours, live off the heat for next 17 hrs until next heat cycle. Charge with a low hysterisis thermostat ensure little temp overseeing in the rooms. Well insulated ensures heat doesn't escape and also floor surface is only a couple of degrees warmer than the room so the floor output self modulates.

 

One of the FEA models I constructed is shown in Figure 1 (on Page 14). It consists of a 4-inch concrete slab sitting on 1-inch-thick

extruded polystyrene insulation.

 

Comments - 25mm of polystyrene insulation is a joke and undermines the whole premise of the article.

 

 

@JohnMo how would I calculate how much extra heat loss I would have if I tied my UFH to the lower layers of mesh rather than the upper?

 

i have 250mm eps and 250mm concrete the lower layer will be at 70mm from the bottom and the upper layer will be 170mm from the bottom.

[JohnMo]

34 minutes ago, gavztheouch said:

@JohnMo how would I calculate how much extra heat loss I would have if I tied my UFH to the lower layers of mesh rather than the upper?

 

i have 250mm eps and 250mm concrete the lower layer will be at 70mm from the bottom and the upper layer will be 170mm from the bottom.

That's a lot of concrete to heat up. My 100mm takes 24 hrs at the start of the heating season to get going and heating the house.

 

I would just put on the top layer of rebar, otherwise you will be heating the floor for days to get heat out. Basically the heat radiates from the pipe in all directions and looses temperature in a informed manner, until it hits a resistance (insulation, heat flux from another pipe, surface). 

[scottishjohn]

sytem used in my house has been 

subfloor with refeforcing mesh -

then dpc whcih stands up the walls to finished floor hieght 

140mm of celotex boarding 

next is UFH piping  then final 75mm screed  to finshed floor height