Can a slab be too thick to have UFH pipes in it?

[Thorfun]

I know a large number of folks on here have insulated slabs where generally the slab is 100mm thick and the UFH pipes are cable tied to the rebar. 

 

We're going this route now for our basement UFH but our slab is 250mm thick with 200mm EPS underneath.

 

I was just wondering if a slab ever got to a thickness where putting the pipe within the slab became the wrong thing to do?

[markc]

Thick slab equals more thermal mass, slower to warm up but more stable temperature.

[tonyshouse]

Should work perfectly, you might need heating! What insulation will you have in the walls?

[Thorfun]

6 minutes ago, tonyshouse said:

Should work perfectly, you might need heating! What insulation will you have in the walls?

200mm EPS. I've done the calculations using previously supplied document that @A_L linked here 

taking in to consideration the depth below ground.

 

although it's just occurred to me that the recent change I've made to the insulation levels at ground level has removed 30mm PIR from the slab. s**t. think I need to do some more quick calculations to make sure what I've done hasn't completely messed up my desired U-values. ?‍♂️

 

 

 

Edited April 21, 2021 by Thorfun

[Dan F]

It's not a great if slab/walls aren't very well insulated:

- If slab isn't well insulated below primarily (but also at edges), then you'll loose more heat than if you were using UFH in screed above some insulation.

- Reaction time is slow (the thicker the slab the slower) so if the house isn't well insulated/airtight then this can be an issue, and supplementary heating that reacts quicker may be needed.

 

If everything is well insulated though, (and you have a control system will work with the reaction times ideally) then the collective experience is that it works really well and helps keep house temperature very stable.  You can actually use the slab as a storage heater and use low tarrifs at night to heat/cool slab at night.

 

Disclaimner:  We haven't turned ours on yet.  Our slab is 150mm minumum, 250mm in some areas (45%)

Edited April 21, 2021 by Dan F

[Thorfun]

1 minute ago, Dan F said:

It's not a great if slab/walls aren't very well insulated:

- If slab isn't well insulated below primarily (but also at edges), then you'll loose more heat than if you were using UFH in screed above some insulation.

- Reaction time is slow (the thicker the slab the slower) so if the house isn't well insulated/airtight then this can be an issue, and supplementary heating that reacts quicker may be needed.

 

If everything is well insulated though, (and you have a control system will work with the reaction times ideally) then you can actually use the slab as a storage heater and use low tarrifs at night to heat/cool slab at night.

well insulated and airtight is the name of the game around here. ?

[Dan F]

2 minutes ago, Thorfun said:

well insulated and airtight is the name of the game around here. ?

 

You're good then.  I would just look at depth of EPS though as 200mm doesn't sound like that much. U-value of 0.17 roughly?

[Thorfun]

7 minutes ago, Dan F said:

 

You're good then.  I would just look at depth of EPS though as 200mm doesn't sound like that much. U-value of 0.17 roughly?

but at a depth of 3.5m under ground that U-value is much reduced.

[Dan F]

2 minutes ago, Thorfun said:

but at a depth of 3.5m under ground that U-value is much reduced.

Yes, forgot that.. I think you'd calculated impact before right?  250mm might be bit better (or your 30mm PIR), but doubt you'll have a major issue with 200mm.  Have you recalclulated without PIR?

[Triassic]

Don’t forget the insulation might be structural under your slab, EPS 300 and the like.

[Thorfun]

15 minutes ago, Dan F said:

Yes, forgot that.. I think you'd calculated impact before right?  250mm might be bit better (or your 30mm PIR), but doubt you'll have a major issue with 200mm.  Have you recalclulated without PIR?

 

I was....then dinner was on the table so I had to take a break.....back on it now. ? 

[Thorfun]

8 minutes ago, Triassic said:

Don’t forget the insulation might be structural under your slab, EPS 300 and the like.

yep. it is EPS300 under the basement and other parts of the house. I had confirmation from the SE that the thickness of the EPS300 had no bearing on the structural calculations so could increase or decrease as desired.

[Thorfun]

40 minutes ago, Dan F said:

Have you recalclulated without PIR?

comes out to 0.103W/m²K when adding the 3.5m below ground insulated effect (14mm x 3.5m of 0.04W/mK insulation)

 

which will do me just fine.

 

increasing it to 250mm EPS300 gives 0.089W/m²K

 

 

which, when you consider the extra cost of dig out and muck away over the area and the cost of the extra EPS300 and labour to lay it, I'm not sure it's worth it.

[Gus Potter]

4 hours ago, Thorfun said:

I know a large number of folks on here have insulated slabs where generally the slab is 100mm thick and the UFH pipes are cable tied to the rebar. 

 

We're going this route now for our basement UFH but our slab is 250mm thick with 200mm EPS underneath.

 

I was just wondering if a slab ever got to a thickness where putting the pipe within the slab became the wrong thing to do?

Hi Thorfun.

 

I was wondering that at 250mm thick there may be a fair bit of rebar in the slab. I would have a quick chat with your SE to check how they have designed the slab and if it is ok to introduce repeating weak spots by way of the pipes.

 

Practically I would be cautious here with a basement. It's hard enough if you are a contractor to deal with a concrete pour, the risk of a burst shutter, delayed wagons etc. If they have to also negotiate the UF pipework without denting it, get good concrete compaction.. you need room for a poker or tamper between the rebar (essential for keeping water out) this may attract a premium at least.

 

My feeling is to keep it as simple as you can. Do your structural slab, take a break to see if water comes in then do your UF as a screed laid all level later. If you do get water ingress then if you have pipes in the structural slab then this could make it more difficult to fix any leaks.

 

Yes, you have the extra cost of the screed but structural slabs are not that flat so the initial perceived saving may not fit with the time you'll have to spend laying / levelling the floor finishes, or finding the one dent in a UF pipe loop that stops it working.

 

 

 

[Thorfun]

29 minutes ago, Gus Potter said:

Hi Thorfun.

 

I was wondering that at 250mm thick there may be a fair bit of rebar in the slab. I would have a quick chat with your SE to check how they have designed the slab and if it is ok to introduce repeating weak spots by way of the pipes.

 

Practically I would be cautious here with a basement. It's hard enough if you are a contractor to deal with a concrete pour, the risk of a burst shutter, delayed wagons etc. If they have to also negotiate the UF pipework without denting it, get good concrete compaction.. you need room for a poker or tamper between the rebar (essential for keeping water out) this may attract a premium at least.

 

My feeling is to keep it as simple as you can. Do your structural slab, take a break to see if water comes in then do your UF as a screed laid all level later. If you do get water ingress then if you have pipes in the structural slab then this could make it more difficult to fix any leaks.

 

Yes, you have the extra cost of the screed but structural slabs are not that flat so the initial perceived saving may not fit with the time you'll have to spend laying / levelling the floor finishes, or finding the one dent in a UF pipe loop that stops it working.

 

 

 

Hi Gus,

thanks for the good advice. I have spoken to the SE and they're ok with the proposal with a few caveats as follows:

"Firstly, you should check with the UFH pipe supplier that they are okay with the pipes being tied to the rebar, as some manufacturers are okay with this and some are not due to possible wear in the pipe wall (I’m guessing it has to do with the wall build-up of the pipe). Secondly, just make sure there are no big bunches of pipes together, as these can cause a weakness in the concrete, and can easily happen in the vicinity of the manifold. We usually look for bunches that are no more than 150mm across with a min. gap of 100mm to the next bunch of pipes (hopefully this is clear)."

 

I have also had a response from the basement contractors who have said there will be an extra cost for power floating and protecting / minding the heating strips while the concrete is being placed, so you're spot on there! I have asked what the extra cost will be and am awaiting a response.

 

your advice is wise and it makes a lot of sense and gives a nicely different perspective. The problem at the moment is they've already dug 2.4m down and hit the sandstone and there isn't any surface water so, even if I did take a break and see if the water comes in, I doubt there'd be any to come in especially as we head further in to summer!

 

But I will sleep on it and ponder some more in the morning.

 

[Dan F]

3 hours ago, Thorfun said:

comes out to 0.103W/m²K

 

A very good wall is typically 0.1 at best, so I'd be more than happy with 0.1 too I think.

 

Few comments questions:

- The 49mm "the earth" is interesting?  Got any links that expain how this works out?

- I wonder if the permiter/area ratio applies in the same way when it's a basement foundation?

[Gus Potter]

12 minutes ago, Thorfun said:

Hi Gus,

thanks for the good advice. I have spoken to the SE and they're ok with the proposal with a few caveats as follows:

"Firstly, you should check with the UFH pipe supplier that they are okay with the pipes being tied to the rebar, as some manufacturers are okay with this and some are not due to possible wear in the pipe wall (I’m guessing it has to do with the wall build-up of the pipe). Secondly, just make sure there are no big bunches of pipes together, as these can cause a weakness in the concrete, and can easily happen in the vicinity of the manifold. We usually look for bunches that are no more than 150mm across with a min. gap of 100mm to the next bunch of pipes (hopefully this is clear)."

 

I have also had a response from the basement contractors who have said there will be an extra cost for power floating and protecting / minding the heating strips while the concrete is being placed, so you're spot on there! I have asked what the extra cost will be and am awaiting a response.

 

your advice is wise and it makes a lot of sense and gives a nicely different perspective. The problem at the moment is they've already dug 2.4m down and hit the sandstone and there isn't any surface water so, even if I did take a break and see if the water comes in, I doubt there'd be any to come in especially as we head further in to summer!

 

But I will sleep on it and ponder some more in the morning.

 

Hi Thorfun.

 

Just a thought.. sorry if I seem to be putting a spanner in the works but the rock level has sparked interest.  If you are hitting this material then you could change the game and save a pile of money. It takes a leep of faith if you are a lay person but sandstone is formed under great geological pressures. It can be a very stable platform. Why put a 250mm RC slab on top? I would start by having a look at the excavated level, see how weathered the sandstone is, try and get a handle on the bedding plane and so on. If you are hitting good rock why spend a load on money digging it out just to pour concrete back in?

 

As I'm on a run. Roughly sandstone was formed in warm seas when the UK was south of the equator. As these plates moved north to form the UK they became tilted so the layers are not horizontal. When we deal with rock we look to see (not least) how weathered the rock is, the bedding plane and the size of the fractures between the rock.

 

I would have a design review with your SE.. yes they may well charge you (should not just to have a 15 min chat on the phone) but the potential savings could be very significant and off the back of this you could end up with a good economic UF design.

 

In terms of abrasion of the UF pipes. I have wrestled with this myself and concluded that as the pipes are plastic (elastic) and you pressurise then to 4 -6 -8  bar when pouring then when you run your heating at say 1.0 to 1.5 bar the pipes will not expand as much, even though they become more elastic when you run warm water ( 40 - 60 deg c) through them.  Also, as the concrete cures it shrinks away from the pipes. My view is that the two will provide a few mircons clearance between the concrete / rebar, sufficient to mitigate any effects of abrasion.

 

 

 

 

[Thorfun]

39 minutes ago, Dan F said:

 

A very good wall is typically 0.1 at best, so I'd be more than happy with 0.1 too I think.

 

Few comments questions:

- The 49mm "the earth" is interesting?  Got any links that expain how this works out?

- I wonder if the permiter/area ratio applies in the same way when it's a basement foundation?

sure, that link to another post above has a document that will explain it for you.

 

but basically it boils down to for every metre below ground level you are you add 14mm of 0.04W/mK of insulation due to the effect of the earth below it.

 

I'm 99% sure the perimeter/area ratio does apply. pretty sure other posters with more knowledge than me on the subject have said as much.

Edited April 21, 2021 by Thorfun

[Thorfun]

28 minutes ago, Gus Potter said:

Hi Thorfun.

 

Just a thought.. sorry if I seem to be putting a spanner in the works but the rock level has sparked interest.  If you are hitting this material then you could change the game and save a pile of money. It takes a leep of faith if you are a lay person but sandstone is formed under great geological pressures. It can be a very stable platform. Why put a 250mm RC slab on top? I would start by having a look at the excavated level, see how weathered the sandstone is, try and get a handle on the bedding plane and so on. If you are hitting good rock why spend a load on money digging it out just to pour concrete back in?

 

As I'm on a run. Roughly sandstone was formed in warm seas when the UK was south of the equator. As these plates moved north to form the UK they became tilted so the layers are not horizontal. When we deal with rock we look to see (not least) how weathered the rock is, the bedding plane and the size of the fractures between the rock.

 

I would have a design review with your SE.. yes they may well charge you (should not just to have a 15 min chat on the phone) but the potential savings could be very significant and off the back of this you could end up with a good economic UF design.

 

In terms of abrasion of the UF pipes. I have wrestled with this myself and concluded that as the pipes are plastic (elastic) and you pressurise then to 4 -6 -8  bar when pouring then when you run your heating at say 1.0 to 1.5 bar the pipes will not expand as much, even though they become more elastic when you run warm water ( 40 - 60 deg c) through them.  Also, as the concrete cures it shrinks away from the pipes. My view is that the two will provide a few mircons clearance between the concrete / rebar, sufficient to mitigate any effects of abrasion.

 

 

 

 

our 'normal' trench foundations for the non-basement areas of the build are designed to be dug down to the sandstone. this is required because of the clay we're on. for the basement though, the sandstone is not deep enough at between 2m - 2.4m from ground level and our basement dig is much deeper than that. so, it's either dig through the sandstone or not have a basement.

 

I think we all know which one won on that decision. ?

 

I don't think you put a spanner in the works. I appreciate a different view on it all. and in fact the way the basement guys have come back saying it'll cost more to take the care around the pipes actually gives me more confidence in them that they do actually know what they're doing! everything I've done with regards to the basement and foundations has been run by the structural engineers (TSD). they have been amazing throughout and I'm very grateful for the assistance and quick responses they've given me to my numerous questions.

 

I also phoned Wunda today about the pipes they use and their technical team said no issues with attaching direct to the rebar using cable ties. so I've got the sign off from them if I decide to go this route and to use them. I'm really not worried about abrasion though as there are so many people on here with pipes attached to rebar and we have friends that have just finished an MBC build with their passive foundations and have the same thing so I just don't think it's an issue if you use the right pipe.

 

this has been an interesting thread and I'm very glad I started it. definitely lots of food for thought!

[Gus Potter]

To me the basement slab insulation is continuous mainly with the wall insulation so you don't have a perimeter per say.

 

 

[Gus Potter]

6 minutes ago, Thorfun said:

our 'normal' trench foundations for the non-basement areas of the build are designed to be dug down to the sandstone. this is required because of the clay we're on. for the basement though, the sandstone is not deep enough at between 2m - 2.4m from ground level and our basement dig is much deeper than that. so, it's either dig through the sandstone or not have a basement.

 

I think we all know which one won on that decision. ?

 

I don't think you put a spanner in the works. I appreciate a different view on it all. and in fact the way the basement guys have come back saying it'll cost more to take the care around the pipes actually gives me more confidence in them that they do actually know what they're doing! everything I've done with regards to the basement and foundations has been run by the structural engineers (TSD). they have been amazing throughout and I'm very grateful for the assistance and quick responses they've given me to my numerous questions.

 

I also phoned Wunda today about the pipes they use and their technical team said no issues with attaching direct to the rebar using cable ties. so I've got the sign off from them if I decide to go this route and to use them. I'm really not worried about abrasion though as there are so many people on here with pipes attached to rebar and we have friends that have just finished an MBC build with their passive foundations and have the same thing so I just don't think it's an issue if you use the right pipe.

 

this has been an interesting thread and I'm very glad I started it. definitely lots of food for thought!

Hi Thorfun.

 

Great to see you are getting on with your design team, it's very much a people thing and pays great dividends.

 

How deep do you need to dig into the stone?

 

Of course the scalpings you get out from the dig will make great hardcore! If you have the time you could maybe get some cracking bits of stone for hearths, walls if the bedding planes split off ok. Will cost a bit to explore but using / recycling the stone makes the house a bit special.

 

Oh, and take a few photos if you manage to recover some good stone as if you sell the house you can put this in the brochure and prove the provenance.

[tonyshouse]

Does soil and substrata act as insulation? 

[SteamyTea]

12 minutes ago, tonyshouse said:

Does soil and substrata act as insulation

Yes, but it is at a lower temperature than what you want.

There is also a difference if it is dry, damp or has flowing water in it.

 

From https://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

Ground or soil, very moist area	1.4
Ground or soil, moist area	1.0
Ground or soil, dry area	0.5
Ground or soil, very dry area	0.33

 

 

[Thorfun]

8 hours ago, Gus Potter said:

Hi Thorfun.

 

Great to see you are getting on with your design team, it's very much a people thing and pays great dividends.

 

How deep do you need to dig into the stone?

 

Of course the scalpings you get out from the dig will make great hardcore! If you have the time you could maybe get some cracking bits of stone for hearths, walls if the bedding planes split off ok. Will cost a bit to explore but using / recycling the stone makes the house a bit special.

 

Oh, and take a few photos if you manage to recover some good stone as if you sell the house you can put this in the brochure and prove the provenance.

 

the dig depth from ground level is approx 4m and with the stone at between 2 - 2.4m it's going around 2m of stone that has to come out. We have a certain type of stone that's local to our area and there is a company that deals with it so I will be speaking to them about either selling it or converting it in to paving slabs etc. so we've already thought of that. ?

 

Don't worry, I'll be taking lots of photos and boring everyone on here with blog posts.

[Thorfun]

9 hours ago, Gus Potter said:

To me the basement slab insulation is continuous mainly with the wall insulation so you don't have a perimeter per say.

 

 

maybe. tbh I don't really have a clue. I have done lots of reading on this whole thing but I don't seem to be able to retain as much information as I used to (mis-spent youth probably) so I keep having to go over it all again and again.