Screed or overlay UFH

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Morning everyone,

Just trying to make a final decision on how to do UFH.

Background: Property is 1980s, screed over (I think) uninsulated slab, CWI, decent insulation in the loft, double glazing, new doors etc. Screed is being ripped out due to renovations, an extension and water damage, so we would really like to do wet UFH while we can. Due to thresholds, bottom stair height etc, we have minimal amount of potential height build up (probably c.70mm FFL).

So, there are two options that we have settled on, but just trying to establish which would be most efficient:

1. 30mm insulation on slab with 35mm 'Thermio+' liquid screed on top encasing pipes. Where height is an issue (e.g. hallway with staircase) we would use Karndean/Antico to minimise buildup, other areas we'd be pretty much free to use whatever flooring is compatible.
2. 35mm screed on slab with 20mm insulated overlay panels. Engineered wood finished floor in areas where height is an issue. As the overlay panels are foil backed then not sure if we could use a tile or vinyl floor and would be limited to floating only.


It seems that option 1 would provide more insulation and allow the screed to act, at least in part, as a radiator that you would get with a new build UFH system where the slab is kept warm. But, would the minimal thickness of insulation below mean that we'd be struggling to keep the heat in the screed?

In comparison, option 2 seems like it would provide much more rapid warmup and we'd just turn the heating on when we need it?


I'm leaning towards option 1, but grateful for any thoughts the experts might have.

Thanks in advance.
 
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I think this will fall under building regs: “'renovation of a thermal element” - so the insulation would be determined by building regs rather than your choice.

Personally I wouldn’t recommend underfloor heating being used as a primary heat source if there isn’t sufficient insulation.

can you dig up the slab and reduce levels to accommodate the insulation? - I think the increase in cost would be worth it in the return on increased energy efficiency.
 
So my bathroom now has UFH. This is my experience of the last 4 years.

1960 concrete slab floor/footings. Synthaproofed, 10mm of insulation (was recommended 6mm but I went to 10mm 'cause I could). 15mm Overscreed, heating element, another 15mm of screed/selfleveling/tile adhesive and 10mm tiles. Door trimmed to suit.
Had I been able to add more insulation I would have.

Floor takes around 45mins to come up to temp and 2 to 3 hours to cool down. Only used as a means of a warm floor and take the background 'chill' off the room.
 
I think you are in the middle with your options whereby both solutions could be viable.

If that screed solution is ok with a 30mm top then fair enough, It was always advise that a >50mm top at least but that is with traditional screed and fibre mix, with all the new composites becoming an option then top layers are becoming thinner but these new types need time to see how they perform long term IMO.

Overlay, that's completely different, at least 20mm of HighDensity insulation on top of the slab then an EPS overboard system will be more than enough as all the heat travels up. Will take all types of floor covering though if carpet is a possible choice then a cementitious overboard would be needed.

It's a toss up I guess and price may be the deciding factor, the latter tends to be more expensive per m2 than traditional but not sure with the new topping types
 
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Thanks all for the input.

I think this will fall under building regs: “'renovation of a thermal element” - so the insulation would be determined by building regs rather than your choice.

Personally I wouldn’t recommend underfloor heating being used as a primary heat source if there isn’t sufficient insulation.

can you dig up the slab and reduce levels to accommodate the insulation? - I think the increase in cost would be worth it in the return on increased energy efficiency.

Notch: Our architect advised (and this is my crude understanding of BC) that if an existing element being refurbished (in this case repaired) on an older property then it's a 'best efforts' approach to upgrading the insulation - i.e. fit in what we can without having to break out slab (which we really can't do), raise door thresholds, replace staircase to accommodate a thicker layer of insulation. Does that make sense?

I am concerned about the use of UFH as primary source downstairs and hear very mixed reviews on its use - some say they've fitted it in even very old houses with good results, others say don't go near it. I can understand the logic of it working in the sense that the room needs to be heated regardless - be that radiators on full chat to 'fill' the room top down or UFH at lower temperature but emitting heat across the whole area. It was our intention to use 15/16mm pipes spaced at 150mm. More research to do in this area.

1960 concrete slab floor/footings. Synthaproofed, 10mm of insulation (was recommended 6mm but I went to 10mm 'cause I could). 15mm Overscreed, heating element, another 15mm of screed/selfleveling/tile adhesive and 10mm tiles. Door trimmed to suit.
Had I been able to add more insulation I would have.

Floor takes around 45mins to come up to temp and 2 to 3 hours to cool down. Only used as a means of a warm floor and take the background 'chill' off the room.

Fred: in your example, do you have a secondary source of heating in this room?

I think you are in the middle with your options whereby both solutions could be viable.

If that screed solution is ok with a 30mm top then fair enough, It was always advise that a >50mm top at least but that is with traditional screed and fibre mix, with all the new composites becoming an option then top layers are becoming thinner but these new types need time to see how they perform long term IMO.

Overlay, that's completely different, at least 20mm of HighDensity insulation on top of the slab then an EPS overboard system will be more than enough as all the heat travels up. Will take all types of floor covering though if carpet is a possible choice then a cementitious overboard would be needed.

It's a toss up I guess and price may be the deciding factor, the latter tends to be more expensive per m2 than traditional but not sure with the new topping types

Rob: I was surprised by the screed thickness too but the company have advised it is a liquid screed that requires minimum of 20mm above the pipe, so 35/36mm screed minimum.

This is the difficulty in making the decision though, I can understand the benefit of c.50mm of insulation, heat up times (I think) should be quite rapid but there is no heat 'store' in having a block of screed.

In terms of cost, we've been quoted c.£10k for supply and installation of: the 30mm insulation, UFH pipes, manifold, screed. In all it's about 80-90sqm, so doesn't feel an outrageous cost but obviously it's significant enough to make me nervous of it not working and then having to install radiators later down the line. Most nervous about what my wife will say as she took some persuading anyway!

Any other thoughts gratefully received!
 
I have a 30m² room with UFH, and it's an old property... if I'd have known it's as good as it is, I'd have had the whole ground floor done the same way. It's a different type of heat and heat-up times are different to radiators, so there is some learning to get used to, I don't turn the floor on and off, just temps up and down through the day (if required).

Mine is on 100mm of insulation and 65mm of liquid screed, and it's stone tiles on top

Wood/carpet will not be as efficient in my mind, but if you know this in advance you can research what to do. (tighter pipework etc).

With thinner insulation, your boiler may fire up more as the heat/loss will be greater.

What floor coverings are you thinking of?
 
I have an overlay UFH system downstairs on a mixture of tile > bathroom - engineered wood > kitchen/hall/office - and carpet > living room. It uses a 22mm EPS400 board with 16mm PEX @150mm centres, all individually zoned. Carpet and underlay is specifically made for UFH (low tog) and it heats as effectively as as the other floor coverings. It works especially well, even the carpet covered room ( in fact it's really nice lying on the floor watching the TV with the CH on) especially as there are no heat up times, we manage it's use like radiators and the system doesn't need to be maintained all the time to keep the thermal mass warm.

With a screed there is still a thermal mass that will need to be warmed up so there will be heat up time that will need to be anticipated, albeit it's not as thick as screed directly on top of a slab of course so would react faster.

I mentioned before in other posts around any UFH setup. UFH needs to be used in a completely different way to traditional radiator wet CH, it needs to be tuned very specifically and there is no one setup that fits all, the space it has to heat is key to how it needs to be used to be effective.

As a comparison, we used to have 3 large radiators in our open plan kitchen/diner, it has a bay window, 2 other large windows and the utility space with back door, it's the largest room in the house @ 32m2 with the highest traffic. It used to have cold spots and never got warm enough and now with the UFH under engineered wood it gets too warm and I need to throttle it back.

Even after a year odds of use I still have to tweak it to suit depending on conditions.

I have installed this in a further 6 clients homes and everyone comments on how they wished they had done it years ago. Running cost is still being monitored but to date I have saved around 5% in gas use compared to last year but it's hard to equate actual cost savings given the hike in energy prices.
 
I have a 30m² room with UFH, and it's an old property... if I'd have known it's as good as it is, I'd have had the whole ground floor done the same way. It's a different type of heat and heat-up times are different to radiators, so there is some learning to get used to, I don't turn the floor on and off, just temps up and down through the day (if required).

Mine is on 100mm of insulation and 65mm of liquid screed, and it's stone tiles on top

Wood/carpet will not be as efficient in my mind, but if you know this in advance you can research what to do. (tighter pipework etc).

With thinner insulation, your boiler may fire up more as the heat/loss will be greater.

What floor coverings are you thinking of?
Thank you both for your further insights.

Chips: This is good to know that you have had success in an old property - comparably old to the 80s or 'very old'? I think we have the benefit of cavity wall insulation, decent loft insulation and double glazing. Some parts of the property have been extended so these areas have 100-120mm of insulation. More would be nice of course but we are limited practically by existing features. So the difference we will have is c.1/3 the insulation and 1/2 the screed that you have.

Floor coverings will be a mix of Karndean/Antico, tile and one room carpet (with perforated underlay and low tog carpet, this room is also supplemented with a log burner).

I have an overlay UFH system downstairs on a mixture of tile > bathroom - engineered wood > kitchen/hall/office - and carpet > living room. It uses a 22mm EPS400 board with 16mm PEX @150mm centres, all individually zoned. Carpet and underlay is specifically made for UFH (low tog) and it heats as effectively as as the other floor coverings. It works especially well, even the carpet covered room ( in fact it's really nice lying on the floor watching the TV with the CH on) especially as there are no heat up times, we manage it's use like radiators and the system doesn't need to be maintained all the time to keep the thermal mass warm.

With a screed there is still a thermal mass that will need to be warmed up so there will be heat up time that will need to be anticipated, albeit it's not as thick as screed directly on top of a slab of course so would react faster.

I mentioned before in other posts around any UFH setup. UFH needs to be used in a completely different way to traditional radiator wet CH, it needs to be tuned very specifically and there is no one setup that fits all, the space it has to heat is key to how it needs to be used to be effective.

As a comparison, we used to have 3 large radiators in our open plan kitchen/diner, it has a bay window, 2 other large windows and the utility space with back door, it's the largest room in the house @ 32m2 with the highest traffic. It used to have cold spots and never got warm enough and now with the UFH under engineered wood it gets too warm and I need to throttle it back.

Even after a year odds of use I still have to tweak it to suit depending on conditions.

I have installed this in a further 6 clients homes and everyone comments on how they wished they had done it years ago. Running cost is still being monitored but to date I have saved around 5% in gas use compared to last year but it's hard to equate actual cost savings given the hike in energy prices.

Rob: Good to know that you have also had success and that it works well for you. Can I please ask what your floor make up is? Are the EPS boards on top of uninsulated slab, insulated slab or insulation on top of slab and under panels?

We've always been 'boosters' of heating really, so not really having a schedule and just turn it on when needed and for short periods. Obviously with the screed system we will probably need to change that behaviour but your solution sounds like it is a lot more akin to how we are used to operating (be that a good or bad way of doing things!).


I think the other thing that we need to consider is what to do with upstairs radiators. Our plumber does seem to really know what he is talking about but we haven't really quizzed him on this yet so as to ensure we also have unbiased view of how it will work. He has suggested though that if using UFH downstairs then it would be sensible to replace the upstairs radiators with oversized ones and run the boiler at low temperature (basically below condensing temp, I think?). We currently have access to the ceilings underneath c.80% of upstairs rads but my wife is not keen on making any further changes. I am concerned that if we run the UFH at low temperature then the boiler will be running at it's current higher temperature and just down mixing for the UFH - so we're using more gas for less heat, if that makes sense. I guess having oversized rads upstairs would have us well prepared for possible ASHP in the future? We've also just had a Mixergy tank fitted which is also well prepared for ASHP; so doing the rads feels like a sensible thing to do?
 

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