150mm or 125mm concrete screed over insulation for ufh?

[JayDee]

Looking for some advice please.

 

Doing a 8.5m x 4.4m concrete slab for a house extension.  Substrate to slab floor height is a total of 250mm.  Dpm and 100mm insulation going in first, then pour a 150mm concrete slab over the 100mm insulation.  Reason for the 150mm thick concrete is for polished concrete floor finish, to help prevent the slab cracking.

 

I'm aware the overall max recommendation for screed on ufh pipes is 100mm .  Although with 150mm screed thickness over the ufh pipes that are at the bottom of screed, and on top of the insulation.  Will it still work ok? Or should this be avoided?  I guess it would take longer to heat up, but if its set and maintained temp it should still store the heat and work ok?

 

I could possibly add 25mm insulation to increase to 125mm and decrease concrete thickness to 125mm if needed.  Although not ideal, as can increase the risk of slab cracking.

 

ufh would be best as the existing house already has a heat pump ufh system, and adding two zones in the new extension is fine for the current system to cope.  However, due to the 150mm thick concrete screed, should another heating method be considered instead?  Like radiators etc... instead of ufh?

 

Picture attached.

 

Thanks in advance.

 

 

[Conor]

150mm insulation and 100mm concrete with a layer of A193 mesh to prevent cracking. Tie the UFH pipes to the mesh, set 50mm below the top.  Does 100mm floor insulation even pass the BC requirements?

[saveasteading]

4 hours ago, JayDee said:

can increase the risk of slab cracking.

The ufh pipes are creating multiple thin points which may increase cracking...but having  multiple tiny cracks is not a bad thing

 

When you say polished, I hope you're not expecting a beautiful self- finish. Cracks are normal, but can be controlled

 

125 slab on 125 pir for me.

Edited September 7, 2023 by saveasteading

[saveasteading]

4 hours ago, JayDee said:

radiators etc... instead of ufh?

they have to be double the size of normal rads because the water is cooler than gas ch. 

they get in the way of furniture etc , and send heat up the the ceiling.

ufh warms the feet and gives a good feeling of warmth for lower air temperatures.

 

what is the plan for floor finish?

[Adsibob]

I wouldn’t go too thick with the slab. Anything thicker than about 100mm or 120mm will have a very poor response time. It will take you 3hr or between switching the UFH on and feeling the warmth. Although if It’sa heat pump system, maybe your plan is to run it almost all day at a low temp.

[JayDee]

On 07/09/2023 at 21:56, Adsibob said:

I wouldn’t go too thick with the slab. Anything thicker than about 100mm or 120mm will have a very poor response time. It will take you 3hr or between switching the UFH on and feeling the warmth. Although if It’sa heat pump system, maybe your plan is to run it almost all day at a low temp.

Cheers. Yes its an air source hp .  So the idea would be to run at low temp continuously through cool months.  the slow response time is not too much an issue.  My main concern is at 150mm is it still going to work, or will it use too much energy and not work very well?

 

So long as it does work and not take too much energy from the hp system to maintain, or be very inefficient.  If its worth going for then I'm keen to have ufh installed.  

 

Or happy to take other recommendations for alternative heating should this be a no go.  I know rads aren't great for hp systems.  What about wall heating systems like variotherm? Or the skirt heating systems?

 

Looking for the best advice under the circumstances really.  Should I go for the ufh? Or is there a better solution based on this particular set up?

 

I can throw in another 25mm pir boards ontop of the 100mm I already have.  If this will make a huge beneficial difference.  However not ideal due to the polished concrete/exposed agg finish.

 

Answers from all commentors welcome.

 

Thanks again.

[PeterW]

14 minutes ago, JayDee said:

My main concern is at 150mm is it still going to work, or will it use too much energy and not work very well?

Heat usage will be broadly the same, the decrement / response will be your issue although 150mm of concrete will have 20% more heat capacity than 125mm, so you could run it as a big storage heater overnight if calculated correctly. This is what @TerryE does with his setup.

[saveasteading]

14 minutes ago, JayDee said:

 will it use too much energy 

No. Energy in equals energy out. Assuming there is plenty of insulation, the energy will all end up as heat in the room.

It will just be a bit slow to affect the room after turning up or down.

[PeterW]

17 minutes ago, JayDee said:

I can throw in another 25mm pir boards ontop of the 100mm I already have.  If this will make a huge beneficial difference. 

Yes absolutely as 100mm is barely BRegs these days !!! More the merrier ..!!

[saveasteading]

1 minute ago, PeterW said:

decrement / response will be your issue 

That's probably what I meant. The night storage point is good...if you can store cheaper energy off-peak.

[saveasteading]

20 minutes ago, JayDee said:

polished concrete/exposed agg finish.

I've said what I think on this before. 

Some people like it.

Are you meaning that you will have the surface ground back to remove the surface and reveal the stones?

[JayDee]

5 minutes ago, saveasteading said:

I've said what I think on this before. 

Some people like it.

Are you meaning that you will have the surface ground back to remove the surface and reveal the stones?

Yes that's right.  exposed agg polish then sealed. small surface cracks are always expected.  It's the slab cracking all the way through the thickness that I'm trying to avoid.  This can be controlled with cut contraction joints, 150mm screed thickness , miothene foam around the block dpc and A193 mesh will be going in to reinforce.  Due to the size of slab I'm just on the limit to avoid cutting a contraction joint.  So wanted to do everything else to limit any big cracking to occur.

[saveasteading]

1 hour ago, JayDee said:

the slab cracking all the way through the thickness that I'm trying to avoid.

Cracks do go right through. It can't be avoided. 

That isn't  a problem structurally as there is aggregate interlock. (If they don't go right through then there really are problems with curling slabs, so let's not go there)

ut it is the visible part of cracks on the surface that matter aesthetically, so seal them to prevent spillages or mop water seeping in, and you are probably going to be happy.

 

However with careful preparation at the perimeter, crack inducers under partition walls and quality control of the concrete, the cracks will be minimal.

The use of steel mesh, or fibres,  will make the concrete crack infinitesimally all over and avoid any bigger cracks.

 

But most importantly, keep the concrete as it comes. Give instructions to workers and the delivery driver that no water must be added. Surplus water evaporates and leaves the equivalent space within your slab. It does other stuff too, so never add water. 

 

But then, after the concrete is just hard, cover it and/or or spray it to keep it wet for at least a week. With your expectations I think do this for as long as possible, 2, 3 4 weeks.

[JayDee]

22 minutes ago, saveasteading said:

However with careful preparation at the perimeter, crack inducers under partition walls and quality control of the concrete, the cracks will be minimal.

All very helpful information.  There is an internal wall going in, splitting the extension equally into two rooms/zones with a door way on the left hand side (the existing building side).  Internal wall extending from the centre column to the outside wall. (see picture)

 

I was considering cutting a joint 1/3 depth of slab, beneath the internal wall however was wondering I might getaway without it as the joint will then be exposed in the door way.  Not heard of the inducers before, is this a better way of doing it and what type of inducer and where to get?

 

The last conundrum I'm battling is, do I need to add another 25mm pir boards on top of the 100mm or will the 100mm pir be ok?  One ufh guy i spoke to said don't worry, another ufh guy said add it, but mainly to decrease the slab thickness.  After reading the comments above, I'm ok with the slab thickness heating slower if there's no significant increase in energy consumption. 

 

Though I will add 25mm pir if 100mm pir isn't enough.  If it's not a big deal, so long as the joins are taped properly I'll save the faff adding the additional pir... thoughts?

[JayDee]

Another conflict I've come across in ufh industry advice with this job (not just on this forum).  Is some say tie the pipes to the mesh halfway to 50mm from the surface of the screed.  Others say don't, and install the pipes directly over the pir at the bottom of the screed.  Even Wunda recommended to install on the pir and not on the mesh.  What's the general consensus on that one?

[saveasteading]

2 minutes ago, JayDee said:

will the 100mm pir be ok? 

It's ok but 125 is better.

 

the sawcut is the tidiest as it is exactly where you want it. Inducers are plastic things placed below that spot to make the slab weaker at that precise spot, but the cut should be enough . It would have to go full width though including the doorway, and all the way to the walls. i don't know if that works for your intended slab.

 

[saveasteading]

If you are having 125mm concrete, i think mesh supported on 50mm packers is about right, then the pipes will be mid slab.

The mid height mesh is best for the slab strength and allows concrete to flow under it and around the pipes.

mesh at the bottom of 125mm concrete isn't great. Normal but not great, especially if you want a good finish.

 

It's all different with flowing screeds.

[JayDee]

4 minutes ago, saveasteading said:

If you are having 125mm concrete, i think mesh supported on 50mm packers is about right, then the pipes will be mid slab.

The mid height mesh is best for the slab strength and allows concrete to flow under it and around the pipes.

mesh at the bottom of 125mm concrete isn't great. Normal but not great, especially if you want a good finish.

 

It's all different with flowing screeds.

Yes the A193 mesh will definetly be supported on 70 or 90mm height chairs anyway.  So the the mesh is about 50mm below the surface no matter what.  Mesh should usually always be on chairs to reinforce it correctly, especially at this thickness.

 

I've just heard different things about placing the pipes.  Some say install pipes directly on the pir at the very bottom of the screed (beneath the mesh that's on support chairs).  Others have said put the pipes on top of the mesh so it is halfway or 50mm from the surface.  Wunda advised not to do this and to put the pipes directly on the pir at the bottom of screed, even if it were 125 to 150mm of screed.  

 

It's interesting though because this exercise, I've come across this a few times.  ufh installers, some say put it in the middle of the screed tied on the mesh so they're suspended in the upper half of the screed.  Some say not to and put the pipes at the bottom of screed directly on the insulation. 

[saveasteading]

32 minutes ago, JayDee said:

Mesh should usually always be on chairs to reinforce it correctly,

My apologies for teaching you how to suck eggs.  your question therefore is only about what will heat the room best.

 

it won't make a huge mount of difference i don't think. But for your immaculate floor surface the pipes would best be tied to the mesh in mid slab, where concrete gets all around it. also no risk of the chairs sitting on pipes.

 

[JayDee]

1 hour ago, saveasteading said:

My apologies for teaching you how to suck eggs.  your question therefore is only about what will heat the room best.

 

it won't make a huge mount of difference i don't think. But for your immaculate floor surface the pipes would best be tied to the mesh in mid slab, where concrete gets all around it. also no risk of the chairs sitting on pipes.

 

Thanks dude, your advice and others on this forum is immeasurable!

[FuerteStu]

Would be interesting for someone to do an experiment regarding ufh Pipes stapled to the PIR vs halfway on the mesh.. Checking surface temperatures over time etc.. 

 

Logic says that you'll feel it faster if its near the surface, but also 50% of the initial heat will be going downwards towards the PIR as well as up, so would it really be that noticeable? 

 

It's still the same mass of concrete to absorb the heat before dissipating it out to the room.. 

 

Thoughts? 

[JohnMo]

14 minutes ago, FuerteStu said:

Would be interesting for someone to do an experiment regarding ufh Pipes stapled to the PIR vs halfway on the mesh.. Checking surface temperatures over time etc.

I would say if the pipe is at the bottom of concrete, compared to middle the heat would be felt at the surface quicker if mounted in the middle. The logic is the heat radiates from the pipe, almost evenly (almost as it will always prefer to travel towards the lowest temperature). The heat as it spreads would have lost some heat to concrete each mm it moves from the heat source, the further way from the heat the more time it takes to get to the surface.

 

However saying all the above, thick screed requires long slow heating regime, either 24/7 or use as a storage heater over 7 plus hours. Hoping to put on the heating when you get home or an hour or so before you get home will lead to a cold house. 

 

Thick screeds make a simple heating system, one thermostat for the whole house, no need for so called smart controls, run on weather compensation or a modified curve to allow storage heating over 7 hours.

 

 

3 hours ago, JayDee said:

Mesh should usually always be on chairs to reinforce it correctly

It actually 50mm from the surface and bottom of concrete, minimum, to prevent corrosion, not really much to do with reinforcement properties.

[saveasteading]

3 hours ago, JohnMo said:

to prevent corrosion, not really much to do with reinforcement properties.

Prevention against corrosion can be relevant for an outdoor slab, but doesn't apply to an indoor slab as it is totally dry.

Mesh is used primarily for strength and secondly for crack control. If it wasn't needed, and wasn't present, then there wouldn't be any reason to protect it.

Putting a single layer approximately centrally ensures that the slab stresses are balanced and it doesn't curl.

[JayDee]

Is it common practice to put dpm (joins taped) beneath and above the pir or just beneath?

[saveasteading]

58 minutes ago, JayDee said:

dpm (joins taped) beneath and above the pir or just beneath?

Underneath as a dpm. Also on top, to prevent screed getting under the pir and floating it....which is not a pretty sight.