Underfloor heating on concrete floors without insulation

17 Jun 2016
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United Kingdom
Nu Heat claim their underfloor heating can be laid on uninsulated concrete floors without the need for insulation. Is that right? Now I get that the floor will be equally uninsulated with radiators or any other form of heating. I'm willing to believe that if a room needs 1000W of radiator then 1000W of UFH will do the same job. It's just that part of me thinks that if the heating pipes are in the floor, the conduction in to the ground would be more than if the air was being heated. Are Nu Heat right?

Bonus question. Their website says that on a vinyl floor, a 40° flow temperature gives out 71W/m2. With a carpet, a 50°C flow temperature gives 66W/m2, all based on a 10°C differential temperature. Does that mean a 50°C flow into a carpeted living room will flow out at 40°C, which can then circulate under a tiled kitchen and return to the boiler at 30°?

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I wouldn't say that's the accepted norm. There was always the understanding that a thermal barrier was needed between UFH and a retrofit onto a thermally conductive mass like concrete. Yes heat will usually travel upwards but it is understood that it will travel into/through cement/concrete.

In one sentence they say insulation is essential and in another they say it's not needed with concrete as downwards heat loss is minimal?? I wouldn't want any heat loss if possible.

Of course understanding around technologies and how things work can change daily but I would need to see a bit of evidence before I would accept it.
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I've found some of their videos on YouTube. They stick the self adhesive castelated mat straight to the concrete. I agree, I'd rather see some insulation under there. They seem not to bother.

I guess you're heating the sub soil under the slab. The middle bit might be OK after a while, but a couple of metres around the perimeter is going to lose that heat to the outside world.
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I do retrofits with 22mm EPS overlay boards onto concrete, the floor covering can be laid directly on top of that. Floating floors get celotex inlay and alloy spreaders.
I've just run the numbers. 5mm of XPS would reduce the U value of a solid concrete floor from 0.80 to 0.49, which sounds like a lot. Over a six month heating season, it would save about £120. So it would pay for itself in about 2 years. Well worth it unless 5mm of head height is going to cause you a problem.
unless 5mm of head height is going to cause you a problem.
Lets be honest, if the UFH is a retrofit then loss of head height will always be an issue. With their self leveling process, there is at least a 22mm loss of height added to the 5mm so an absolute min of 27mm will be lost.

As suggested, using an EPS board system then 22mm is the max height lost, as far as reducing U values then it may not be 4 times more thermally effective than 5mm as the pipework sits within the board, not on top but it then uses an aluminium spreader layer to conduct the heat upwards.
The other thing I would add is there seems to be more and more statements about these self levelling compounds only needing 20 odd mill depth, I'll be honest I'm not sure I would want such a small amount of compound above the pipework taking day to day loads but again, new chemical technologies and compounds are being invented all the time
I'm looking at wunda floor they allow tiling directly on the 20mm poly/ aluminium sandwich.
They do recommend a 3mm mapae compound for some finishes.
Having read a lot of comments on this website. The lack of mass is appealing on grounds of the reaction time.
I've retrofitted it into a number of homes with excellent results. It does react much faster than traditional slab and screed UFH and can really be treated like radiators in so far as warm up and behaviour is concerned.

It will also take any flooring you want to put onto it, I've even used carpet on one installation by using a 6mm cementitious overboard then a low tog underlay and carpet and that too works very well.
The heat loss into the floor must be close to double a radiator heated room if you do not put a thermal barrier below.
Consider a radiator heated room, top of floor is at ~20C, base below ground will be at 5-15C dependant upon winter to summer, then when you run 40to 50C water through the UFH pipes the delta T will mostly be about double, especially in winter when the heating is working harder.
The EPS 400 poly boards that the pipework sits on have very high insulation properties (0.044 W/m k) and their design allows most of the heat to be conducted upwards through the top aluminium spreader layer into the flooring above rather than soaking downwards.

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