Insulating a concrete floor....

[NikNak]

Hi guys, next question on my list to ask re converting our kitchen-dining room is....


Our house is relatively new (mid 80's i think) but it has a solid concrete slab floor. And in the winter it can get rather chilly underfoot.

All rooms in the house are either laminate or engineered flooring (apart from the hall/stairs/landing which have carpet) and even with various mixes of underlay etc they still get cold.

The best 'mix' i've found is to use the green fibre boards directly onto the concrete floor followed by a layer of the very thin (2mm.?) polystyrene foam on a roll, sold as underlay, with the flooring on top of that.

However.... we'd like to put ceramic tiling in the newly converted kitchen diner. I'm keen to explore underfloor heating, probably/possibly run from the central heating system. But the other half isn't.

Sooooo.... i'm now thinking about using the Backer-Boards you can get for insulating the sub floor prior to installing underfloor heating etc. (the ones with a polystyrene middle and a cement covered mesh either side....) and just wondering if that will work/improve the cold from coming up floor the concrete slab.? I'm thinking of using the 12.5mm ones, as that plus the thickness of the tile plus adhesive etc will come to a similar thickness as the engineered flooring/underlay in the hall.

Any thoughts....?


Nick


Oh.... almost a p.s..... have any ove you got underfloor heating (electric or wet) and care to comment on the running costs.?

 

[Fitzroy]

We have electric u/f heating installed on top of new insulated concrete slab so once warm in theory it stays warm. Only a small area c. 4m2 and I recon it cost about £350/yr to run with the floors at a good warm temperature 26 degC it’s a bathroom with tiled floor.

The system is 150w/m2 so at 100% on that would be 150x4x24x365/1000 = 5256 kWhrs per year = £788/yr @ 15p/kWhr. So my system is powering on for c. 40% of the year.

I think this is excessive and am always turning it down but the wife and kids are always trying it back up.

Gas is about 1/3 the price of electric per kWhr and a decent boiler c. 90% efficient so running costs for water system would be less, but install costs are higher.

Upsides of electric is it will keep the floors warm in the summer months when the heating is off. Which in a bathroom is nice.

Fitz

 

[Woody2Shoes]

Hi - my 2d's worth.

I think that, given the age of the house, it's highly unlikely there will be any insulation in the floors (unless you've put some in during work on the kitchen diner). On that basis:

- Underfloor heating, of any type, is likely to be sluggish to respond (as there's probably quite a large thermal mass in the screeded slab) and wasteful of energy (without insulation, as much heat (or more) as moves upwards will move downwards).
- You haven't said what kind of heating system you've got, but it's unlikely to work efficiently feeding wet u/f heating (flow and return temps are generally lower for ufh).
- I'd definitely want something vapour impermeable between the top of the slab and the bottom of any timber put down on the floor (there's a risk of interstitial condensation on the 'warmer' side of the slab otherwise - particularly in kitchens/bathrooms or rooms near them).

Am I right in thinking that the perception of coldness is because people generally pad about the house in socks/barefoot? Ceramic tiles on concrete will feel cold in that case - I'd suggest marmoleum (good old 'lino') as a 'warmer' alternative, or perhaps vinyl (Amtico has some surprisingly good-looking options) - the latter being a little less "environmentally sound"(!), but perhaps more robust to stand up to wear and tear.

Cheers, W2S

https://www.forbo.com/flooring/en-gl/
https://www.amtico.com/

 

[Rorschach]

Underfloor heating is really just a luxury fad that is expensive to install, difficult (impossible sometimes) to repair, expensive to run and adds very little to overall household warmth. You are better off with a good pair of slippers.

£350 just to heat the bathroom floor. I don't think we spend that much to heat our whole flat :shock:

 

[MikeG.]

Underfloor heating is really just a luxury fad that is expensive to install, difficult (impossible sometimes) to repair, expensive to run and adds very little to overall household warmth.......

Electric underfloor heating.....I completely agree. However, warm water underfloor heating set in screed above insulation is truly wonderful. It works extremely well. is more energy- and financially efficient that radiators, and produces a lovely even warmth without the hot spots and drafts associated with radiators. It is extremely difficult to retro-fit, and doesn't work at all well with suspended timber floors, but in a new build with solid floors, there is nothing better.

 

[Fitzroy]

Underfloor heating is really just a luxury fad that is expensive to install, difficult (impossible sometimes) to repair, expensive to run and adds very little to overall household warmth. You are better off with a good pair of slippers.

£350 just to heat the bathroom floor. I don't think we spend that much to heat our whole flat :shock:

Yup, I rue the day we installed it, but now it’s in trying to tell the rest of the family not to use it is about impossible, all I can do is minimise the damage.

F.

 

[Rorschach]

Underfloor heating is really just a luxury fad that is expensive to install, difficult (impossible sometimes) to repair, expensive to run and adds very little to overall household warmth.......

Electric underfloor heating.....I completely agree. However, warm water underfloor heating set in screed above insulation is truly wonderful. It works extremely well. is more energy- and financially efficient that radiators, and produces a lovely even warmth without the hot spots and drafts associated with radiators. It is extremely difficult to retro-fit, and doesn't work at all well with suspended timber floors, but in a new build with solid floors, there is nothing better.

Yes definitely for electric but I would also argue that water based underfloor heating, while more cost effective in terms of running costs, still has major drawbacks. It is still very slow compared to radiators, it is much more expensive than radiators, it is difficult to retrofit as you say and incredibly difficult to repair. Yes if a radiator fails it will make a mess but it is noticed quickly, the leak can be isolated and the radiator or pipe work can be replaced easily the same day. Can't do that if your leak is in an unknown spot underneath a finished floor and a concrete screed.

 

[jimmy_s]

With Underfloor Heating (UFH) you should try to limit the back loss through the slab as the heat loss from UFH is higher than conventional heating systems. For example, UFH typically runs at about 50 deg C flow / 40 return (depends on heat losses/ floor finishes/ pipe spacing etc) so mean water temp is about 45deg C. therefore if the floor U value is not decent then the back loss can be quite high. I can't remember what the max U value was for 80's housing but I think it was something like 0.6w/sqmk. Therefore for standard radiator heating the heat loss through the floor would be something like 15 watts per sq metre as the floor temp would be about 20deg C. For UFH the screed is at about 45 deg C so the heat loss in winter would be circa 30 watts per sqm. Designing UFH in accordance with BS EN 1264 the back loss should be no higher than 10% of the heat output.
If a client wants UFH I normally advise to install plenty of underfloor insulation as you only get one shot at it and if its lacking you are just wasting energy and will end up with a system thats costly to run.

In a normally occupied space (not shower room etc) then the normal max design floor surface temp is 29 deg C. This works out as about 90 watts per sqm output. If your floor U value is about 0.6 w/sqm k then for 90 watts out you are having to put in 120 watts so the back loss is 33% of heat output.

I wouldn't be installing UFH without beefing up the floor insulation considerably.

If you can install it with sufficient floor insulation its a great heating system as its largely radiant, works well with low temperature heating systems and is not always that expensive to install. It depends on the size of the areas being heated.

A lot of the grief with UFH relates to controls. We tend to use OJ waterline controls as if the wiring gets crossed its easily fixed as each thermostat is addressable - just have to swap the thermostat addresses and its fixed.

Hope this helps!

 

[MikeG.]

With Underfloor Heating (UFH) you should try to limit the back loss through the slab as the heat loss from UFH is higher than conventional heating systems. For example, UFH typically runs at about 50 deg C flow / 40 return (depends on heat losses/ floor finishes/ pipe spacing etc) so mean water temp is about 45deg C. ..........

You sound like you know what you're talking about, so I won't argue.....but will simply point out that my UFH runs at 35 degrees. It is also the reason why UFH is a natural partner for ground-source heat pumps, which produce warm rather than hot water.

 

[MikeG.]

........A lot of the grief with UFH relates to controls. We tend to use OJ waterline controls as if the wiring gets crossed its easily fixed as each thermostat is addressable - just have to swap the thermostat addresses and its fixed.....

The low energy houses I get involved with I insist are always dealt with thermostatically as one zone only, although all the rooms/ zones have their own individual pipe run. The simplicity is fantastic (and I've never yet found an UFH system which is more expensive than a radiator system would be). With properly set return temperatures for the individual pipe runs the manifold never need be touched again. Yes, people have to understand that this is a slow-reacting system. Mine reaches maximum temperature some 5 hours after it goes on (and 2 or 3 hours after it goes off). So, the programmable stat is set for 2pm.

 

[jimmy_s]

Mike,

Yes, UFH is ideal for use with heat pumps as they are much more adverseley affected by high flow temperatures compared to boilers as the heat pump has to work much harder to produce higher temperatures. The flow temperature relates to the compressor discharge pressure. The higher the difference between suction and discharge pressure then the more work must be done by the compressor, so more electrical engergy is required so lower efficiency.
35 degC flow temp is pretty good going. Must be well insulated. I said 50/40 as a guide, and 40 deg C return is fine with condensing boilers where the return temperature is more important as it governs the extent of latent heat extracted from the flue gas. For a heat pump the flow temperature governs the compressor discharge pressure largely.
I see quite a lot of heating systems using heat pumps where some poor sod has been sold a heat pump which is not suitable for their property as its just not able to run efficiently as the flow temps needs to be so high to heat the place or its been connected to radiators which generally end up needing a higher flow temp compared to UFH.
With regards to controls and single zone - yes agreed, in our last house I fitted UFH to the ground floor. I operated it as a weather controlled heating zone with no stats and it worked fine.
The biggest problem I get on commercial jobs is where the UFH and controls are installed by different sub contractors. Quite often the UFH installer decides to change the pipe loop arrangement and doesn't let the controls installer know. The issue is that the loops where the wiring is wrong are either stone cold or roasting hot. If the stats are wired individually it can be difficult to fix. The OJ Waterline controls makes this easy to fix.
I've moved house to a bit of a doer upper and am intending to fit UFH once I get the existing floors out and re-installed as insulated ground bearing slabs with a suitable level of floor insulation and screed. Best way to Go I recon. In time I'll get a heat pump when I get the whole place to a state where it can operate efficiently. Thats the plan... but I have a bigger headache at the min with the roof situation and am planning to re-roof the whole place as its dire.