My builders are laying 65mm of screed (over slab + 20mm kingspan + wet UFH pipes), and they're using aggregate (stones) in the mix. I'm comfortable they know what they are doing (having seen the process they are using) - but I don't understand the pro/con of using stones in screed - I thought screed was entirely sand/cement - nor do I know whether this is good or bad in the case of UFH? Any information would be helpful!
What's the purpose of aggregate in screed, esp. on UFH?
- Thread starter mgx
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A polymer aided, fibre reinforced screed would be my choice with 1 part cement to 3 parts sand for a domestic property.
Ready mixed concrete is a bad choice IMO especially if the stone is large 20mm crushed. I could forsee bad coverage around the UF pipes.
If it was 10mm sandstone then its probably OK but not ideal.
Thats just my opinion. The experts may have other views.
20mm insulation is not enough.
I think the building regulations here in NI stipulate 50mm minimum and thats for a normal floor without UFH pipes embedded.
The last new build I seen had 70mm kingspan high density insulation under the UFH pipes specified by the home owner.
Ready mixed concrete is a bad choice IMO especially if the stone is large 20mm crushed. I could forsee bad coverage around the UF pipes.
If it was 10mm sandstone then its probably OK but not ideal.
Thats just my opinion. The experts may have other views.
20mm insulation is not enough.
I think the building regulations here in NI stipulate 50mm minimum and thats for a normal floor without UFH pipes embedded.
The last new build I seen had 70mm kingspan high density insulation under the UFH pipes specified by the home owner.
Thanks, 20mm insulation is fine (and standard) for this application - it's over a pre-existing slab (whether it has insulation underneath or not, a 1972 build). There's no way building regs would require me to put 50mm insulation on my slab, that would not be practical, I'd have to move gas meters, door ways, etc.
The screed is not ready mix but site mix. The aggregate is about 20mm crushed, when I sample a number of stones.
Presumably they put the aggregate in for reinforcement, and probably they are not UFH savvy - I didn't think to ask this ahead of time because I didn't expect anything other than a normal sand/cement mix.
I am concerned about coverage around the UFH pipes.
The job is half way done. I'd be pretty unhappy to have put UFH in and find it's ineffective ... I'm just not sure of what the consequences are.
The screed is not ready mix but site mix. The aggregate is about 20mm crushed, when I sample a number of stones.
Presumably they put the aggregate in for reinforcement, and probably they are not UFH savvy - I didn't think to ask this ahead of time because I didn't expect anything other than a normal sand/cement mix.
I am concerned about coverage around the UFH pipes.
The job is half way done. I'd be pretty unhappy to have put UFH in and find it's ineffective ... I'm just not sure of what the consequences are.
I've never seen 20mm aggregate in a screed. And I don't really see the benefit other than it's probably cheaper. I would ask who specified it and why they chose this rather than a standard screed? Also check with the UFH system supplier to see what they specify.
Problem is, if lack of pipe coverage does cause reduced heat transfer you won't know until the UFH system is sparked up and really then only in the winter.
Problem is, if lack of pipe coverage does cause reduced heat transfer you won't know until the UFH system is sparked up and really then only in the winter.
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My builder says that is to give additional strength to the screed. The UFH company says that it is not recommended (they only say it is OK to have up to 10mm of aggregate). I'm still working on sorting this out, such an annoyance.
I cannot see how having aggregate in a screed can reinforce it, with my cynics cap on I too suspect it is to save them a little cash/lack of knowledge on UFH. Personally I'd contact the UFH manufacturer, they will have clear requirements for the insulation and acceptable screed thicknesses/types and using information gained by contacting them will allow you to make a constructive and convincing argument to your builders rather than saying you've got some concerns after your posted on a DIY forum. Afterall, any warranty with the UFH is only valid if it is installed to their guidelines. As you say get UFH wrong and it'll come back to haunt you.
Edit, insist they do it as per manufacturers recommendations esp if your builders getting a bit cheeky.
There is no large aggregate in screed purely so that a nice smooth surface can be formed with the fine agregate (ie the screed sand)
Screed sand alone is much easier to lay and trowel up smooth
So as long as a suitable surface can be formed, then there is no problem in using large aggregate
Screed sand alone is much easier to lay and trowel up smooth
So as long as a suitable surface can be formed, then there is no problem in using large aggregate
The manufacturer is clear:
"We say up to 10 mm aggregate is OK around the underfloor heating pipes. I am also concerned about the size of 20 mm aggregate and specially when the total thickness over the insulation is only 65 mm. So it is wise not to continue with this mix."
A whole large room (living room) has already been done, and ripping it out would be pretty difficult, so it looks like I am going to live with it for this one room, but certainly will be requiring a different mix for other rooms. Fortunately for this one room, we took a precautionary measure and put in pipes for two wall radiators in case they were ever needed in future.
The problem is, my builder says this is fine, he's done it with UFH before, it doesn't cause a problem, it works (convects okay, probably not as efficiently ...), and the two guys laying the screed seem to understand the issues (without even asking, they showed me how the screed is sufficiently meeting the pipes, and he referred to the double wall construction of the pipes). It's one of those miscommunications that you cover with the builder all the details about what's to be done, but you don't actually ask "so exactly what are you putting in the screed".
To finish the surface, they say they will use a slurry screed and smooth it. Timber boards are going over it eventually.
But I am pretty nervous, as it is clear: the effects won't be obvious until I warm it up a couple of months from now. But I have to live with it for 10 years, they're off to another job in a few days.
Do I get it ripped out?
"We say up to 10 mm aggregate is OK around the underfloor heating pipes. I am also concerned about the size of 20 mm aggregate and specially when the total thickness over the insulation is only 65 mm. So it is wise not to continue with this mix."
A whole large room (living room) has already been done, and ripping it out would be pretty difficult, so it looks like I am going to live with it for this one room, but certainly will be requiring a different mix for other rooms. Fortunately for this one room, we took a precautionary measure and put in pipes for two wall radiators in case they were ever needed in future.
The problem is, my builder says this is fine, he's done it with UFH before, it doesn't cause a problem, it works (convects okay, probably not as efficiently ...), and the two guys laying the screed seem to understand the issues (without even asking, they showed me how the screed is sufficiently meeting the pipes, and he referred to the double wall construction of the pipes). It's one of those miscommunications that you cover with the builder all the details about what's to be done, but you don't actually ask "so exactly what are you putting in the screed".
To finish the surface, they say they will use a slurry screed and smooth it. Timber boards are going over it eventually.
But I am pretty nervous, as it is clear: the effects won't be obvious until I warm it up a couple of months from now. But I have to live with it for 10 years, they're off to another job in a few days.
Do I get it ripped out?
Okay, a follow up.
I was informed by my UFH supplier -- "The large aggregate for that room will not damage the pipe or reduce output from the floor. It may crack in some places in the future. But that can be repaired by filling the cracks."
So that is reassuring.
In any case, for the rest of our property I found where I can buy fibres to put in the mix. It will be a little longer before we do the rest of the property (we need to get one large room done ASAP), and now that I have watched these guys in action (how they use batons, apply the screed, flatten, etc), I think I can probably do some of the screed myself - but not sure yet, sure is tempting
.
I keep telling my wife this is great experience for when we buy our house in the country, since I've learnt so much!
I was informed by my UFH supplier -- "The large aggregate for that room will not damage the pipe or reduce output from the floor. It may crack in some places in the future. But that can be repaired by filling the cracks."
So that is reassuring.
In any case, for the rest of our property I found where I can buy fibres to put in the mix. It will be a little longer before we do the rest of the property (we need to get one large room done ASAP), and now that I have watched these guys in action (how they use batons, apply the screed, flatten, etc), I think I can probably do some of the screed myself - but not sure yet, sure is tempting
.I keep telling my wife this is great experience for when we buy our house in the country, since I've learnt so much!
M
marsaday
i did this myself for our kitchen last yr. 70mm insulation and then 70mm sand / cement screed at 3:1 plus fibers. it works well enough, but i could dig out enough base to get the insulation in.
if i did it again i would put in the side barrier insulation where the kitchen units start and not against the walls. although this may in practice be harder than it seems (hindering screeding maybe).
if i did it again i would put in the side barrier insulation where the kitchen units start and not against the walls. although this may in practice be harder than it seems (hindering screeding maybe).
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