Roz Posted February 22, 2019 Share Posted February 22, 2019 On advice from this forum, we're putting our underfloor heating in our slab instead of having a separate screed. Can anyone point me in the direction of any resources I can link them to? They haven't seen it done before, and I don't want to simply say 'I read it in a forum' ?. They know what to do, I think they just would feel better with some external information about it being done. Additionally, does it use a different type of concrete to a normal slab? One of them mentioned being nervous of the pipes being damaged by the stones in the concrete. Finally, if we have a 100mm slab, and we lay reinforcing mesh on it and put the pipes on the mesh, how far into the slab should each of these elements be? One of our builders mentioned he would have guessed the pipes would have just gone onto the insulation as with a screed, but as the slab is100mm I mentioned warm up time.Perhaps either way can be done though. Link to comment Share on other sites More sharing options...
Ferdinand Posted February 22, 2019 Share Posted February 22, 2019 Try this, and assume some replies from people who are feeling less frisky than I am this morning. F (runs and hides) Link to comment Share on other sites More sharing options...
Roz Posted February 22, 2019 Author Share Posted February 22, 2019 ? thanks Ferdinand ? Link to comment Share on other sites More sharing options...
scottishjohn Posted February 22, 2019 Share Posted February 22, 2019 just remember if putting UFH in slab --you got to get it right --you can,t start moving internal walls or things that fix to slab after or pipes will be in wrong places I think this is why builders like to do it in screed when walls are up and roof on customers have a habit of changing their minds on things maybe Link to comment Share on other sites More sharing options...
Roz Posted February 22, 2019 Author Share Posted February 22, 2019 1 minute ago, scottishjohn said: just remember if putting UFH in slab --you got to get it right --you can,t start moving internal walls or things that fix to slab after or pipes will be in wrong places I think this is why builders like to do it in screed when walls are up and roof on customers have a habit of changing their minds on things maybe Fair enough - although our place is so small we don't have any walls downstairs at all! And we can't really change our minds about that ? Link to comment Share on other sites More sharing options...
PeterW Posted February 22, 2019 Share Posted February 22, 2019 @Roz who has specced the reinforcing in the slab...? If its the engineer then it has to go where they say. If its the builder then consider fibre reinforcing ! Pipes can be anywhere but suggest its below half way as it slows the rapid swings of the floor/room temperature and allows the floor to act as a heat buffer. For example I'm in a building where its 100mm slab with 25mm limestone flooring, and the pipes are in the Wunda trays at the bottom of the slab. If you open the back door and leave it open for 20 mins, then close it, the room recovers reasonably quickly as the slab is heated through so probably only drops 0.5c or less even though the room drops by 6-8c. Depending on layout needed, you can install the A146 or whatever reinforcing and zip tie the pipes to the mesh - standard RC25 will not damage this at all, and if its pressurised during the pour there is a school of thought that makes it harder to crush still. If you don't have any spec on the rebar then you could use the Wunda or Polypipe trays, put the pipes in then lay the rebar over the top. Either is fine, one is a little more expensive but easier to DIY. 2 Link to comment Share on other sites More sharing options...
Roz Posted February 22, 2019 Author Share Posted February 22, 2019 1 minute ago, PeterW said: @Roz who has specced the reinforcing in the slab...? If its the engineer then it has to go where they say. If its the builder then consider fibre reinforcing ! Pipes can be anywhere but suggest its below half way as it slows the rapid swings of the floor/room temperature and allows the floor to act as a heat buffer. For example I'm in a building where its 100mm slab with 25mm limestone flooring, and the pipes are in the Wunda trays at the bottom of the slab. If you open the back door and leave it open for 20 mins, then close it, the room recovers reasonably quickly as the slab is heated through so probably only drops 0.5c or less even though the room drops by 6-8c. Depending on layout needed, you can install the A146 or whatever reinforcing and zip tie the pipes to the mesh - standard RC25 will not damage this at all, and if its pressurised during the pour there is a school of thought that makes it harder to crush still. If you don't have any spec on the rebar then you could use the Wunda or Polypipe trays, put the pipes in then lay the rebar over the top. Either is fine, one is a little more expensive but easier to DIY. Our BCO specced that we had to have it at the edges, and I read that often it's run throughout if you're doing this UFH method... although I don't remember where I read that. I haven't heard of fibre reinforcing before. If you were picking between putting the UFH pipes on the insulation under 100mm of slab, or half way in on reinforcing mesh for example, which would you choose? Quick warm up time would be nice, but I'm not totally stuck on the idea. Link to comment Share on other sites More sharing options...
Ferdinand Posted February 22, 2019 Share Posted February 22, 2019 This piece in H&R acknowledges the concept of in-slab ufh, but does not give a howto, which may be what you really want. https://www.homebuilding.co.uk/underfloor-heating-guide/ There are various pages from Installers eg https://www.buildingservicesindex.co.uk/entry/44100/Warmafloor/Underfloor-heating-for-structural-concrete-floors/ F 1 Link to comment Share on other sites More sharing options...
PeterW Posted February 22, 2019 Share Posted February 22, 2019 20 minutes ago, Roz said: Our BCO specced that we had to have it at the edges, Errrr why...?? BCO should not be spec-ing anything !! They should be confirming that the engineers spec is correct and meets building regs. Is the edge of the slab thicker than the rest then..? If it is, it should be engineer designed as the interface between the thicker and thinner is the crack point. Fibre reinforcing is much better as it makes the whole slab reinforced. Ask your local concrete company about it - its tiny 30mm glass fibres included in the mix. Warm up time is irrelevant here ..! Building heat loss is the key- is this new build or refurb..? Link to comment Share on other sites More sharing options...
recoveringbuilder Posted February 22, 2019 Share Posted February 22, 2019 Ours was put in on the mesh and pipes were pressurised at the time, so mesh was sat 50 mm up on little holding things then pipes attached to mesh then the pour 2 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 22, 2019 Share Posted February 22, 2019 Ours is the same: BTW, the "little holding things" are called chairs 2 1 Link to comment Share on other sites More sharing options...
Roz Posted February 22, 2019 Author Share Posted February 22, 2019 7 hours ago, PeterW said: Errrr why...?? BCO should not be spec-ing anything !! They should be confirming that the engineers spec is correct and meets building regs. Is the edge of the slab thicker than the rest then..? If it is, it should be engineer designed as the interface between the thicker and thinner is the crack point. Fibre reinforcing is much better as it makes the whole slab reinforced. Ask your local concrete company about it - its tiny 30mm glass fibres included in the mix. Warm up time is irrelevant here ..! Building heat loss is the key- is this new build or refurb..? Wrong wording then. Reinforcing mesh wasn't in our structural drawings and the BCO said we needed it at least around the edges. In the document he wrote "The concrete floor slab of the extension is required to rest on the foundation block work and have A142 anti crack mesh 600mm wide around the perimeter" Previously the floor slab didn't extend into the block work. Why is warm up time irrelevant? It's a barn conversion, with new extension. Link to comment Share on other sites More sharing options...
Roz Posted February 22, 2019 Author Share Posted February 22, 2019 @Christine Walker and @JSHarris thanks - how thick was your slab in total? Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 22, 2019 Share Posted February 22, 2019 5 minutes ago, Roz said: @Christine Walker and @JSHarris thanks - how thick was your slab in total? Ours is 100mm deep, with a 200mm deep ring beam around the edges and under the two structural internal walls. There are additional steels running around the ring beam, tied in to the fabric in the slab. Link to comment Share on other sites More sharing options...
PeterW Posted February 22, 2019 Share Posted February 22, 2019 Warm up time will mean that your slab will respond quicker if the pipes are in the top 1/3rd but also cool quicker. The deeper the pipes, longer the heat up but also a slower swing to cold due to the slab acting as a big storage heater. This is relevant if you want to use ASHP and off peak electricity as you heat up the slab overnight and the heat is lost to the building during the day. If the slab is going onto the foundation blocks then you may want to consider either changing the design slightly to just have the slab inside the blockwork and insulate at the perimeter and underneath as currently as designed you have a cold bridge into the foundation blockwork. 1 Link to comment Share on other sites More sharing options...
recoveringbuilder Posted February 22, 2019 Share Posted February 22, 2019 Ours was 100mm deep, so the mesh and pipes were at 50mm, 50mm run of concrete below and 50mm over the pipes/mesh Link to comment Share on other sites More sharing options...
Nickfromwales Posted February 22, 2019 Share Posted February 22, 2019 (edited) 4 hours ago, Roz said: Why is warm up time irrelevant? It's a barn conversion, with new extension. Warm up time is easily managed by setback / advance of heating on / off sets points, so don't get hung up on those. The issue is with folk who are not conversant with these 'technologies' stating Chinese whispers and causing undue worry amongst folk who hang on those 'professional opinions', howsoever generalised ( or even flawed ). If you are building to a good standard ( our UK building regs are utter ? so please don't go by those ) then your home will never actually go 'cold' ergo your 'warm up time' is massively reduced because the heating has far less work to do to get you back to your perceived comfort temps. How far the house drops to between comfort events is easily managed by deciding how 'cold' you're willing to let the house drop to and setting the setback ( aka economy ) temp accordingly. Look at it as "how hot and when" as opposed to "when will the heating switch on or off" where you set comfort and economy temps to be maintained rather than times for the heating to come on / go off. This only needs to be strictly observed for ~4 months of the year in reality, so base your decisions on that 12 hours ago, Roz said: One of them mentioned being nervous of the pipes being damaged by the stones in the concrete. If they use proper Pex-Al-Pex pipe then you'll only seriously damage it if you take a hammer to it. My screeders regularly run barrow-fulls of screed over the pipes and then one of them will up-end the barrow on partially visible pipes but still with no ill effect. The stones in the concrete are of absolutely zero concern I assure you ( and I've been here more than once over the last 23 years ). Adding the fibres would be a cheap assurance and will be of no detriment, but they're usually only used in a non-reinforced slab where the mesh isn't present, to avoid cracking. You'll be fine. Edited February 22, 2019 by Nickfromwales typo's / clarity 3 Link to comment Share on other sites More sharing options...
Roz Posted February 23, 2019 Author Share Posted February 23, 2019 Thanks everyone. I notice that @Christine Walker mentioned hers were pressurised at the time. Does this mean they need to be full of water when we put the concrete in? We haven't currently got water on site and don't see it being on site by the time the slab is laid. Thanks! Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 23, 2019 Share Posted February 23, 2019 We didn't have any water on site when our pipes were laid, so we didn't bother pressurising them. It didn't cause any problems. 2 Link to comment Share on other sites More sharing options...
recoveringbuilder Posted February 23, 2019 Share Posted February 23, 2019 The installers air pressurised them @Roz with a compressor 1 Link to comment Share on other sites More sharing options...
Nickfromwales Posted February 23, 2019 Share Posted February 23, 2019 Wunda sell their pipe pre-pressurised and cap ended. I don't know what pressure they do it to, but I cut into one and nearly followed through. Good idea. 3 Link to comment Share on other sites More sharing options...
Roz Posted February 24, 2019 Author Share Posted February 24, 2019 Ours are from Wunda @Nickfromwales ! Link to comment Share on other sites More sharing options...
Nickfromwales Posted February 25, 2019 Share Posted February 25, 2019 16 hours ago, Roz said: Ours are from Wunda @Nickfromwales ! Has it got the black plugs in the ends of the pipes? ( if you have it yet ). Link to comment Share on other sites More sharing options...
Roz Posted June 3, 2019 Author Share Posted June 3, 2019 I swear every single person who visits the site comments on how they haven't seen the underfloor heating directly in the slab before, on the mesh. It makes me so anxious - I keep telling them I'm on a forum where lots of people do it and you see it on TV too. They sort of give me a raised eyebrow like they don't believe me ? But it's too late now as our concrete just went in. Perhaps we'll start setting a trend for cornwall! 1 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted June 3, 2019 Share Posted June 3, 2019 I just got used to getting strange looks and comments from people working on our build. There's a very strong tendency to disapprove of anything new in the building industry, not for any good reason, just a seeming reluctance to accept change. We have had a few of the people who worked on our house back since it was finished, and every single one of them agreed that what we've done works OK, so perhaps that will go some way to try and get people to adapt to new methods of construction. 3 Link to comment Share on other sites More sharing options...
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