Electric underfloor heating on suspended timber floor

My daughter and her husband are having a ground floor extension to a terraced house, roughly 5m x 5m. The floor will be suspended timber partly to reduce access problems for delivering concrete (no rear access except via the house) and partly to facilitate possible future installation of services if the existing kitchen is moved out there in future. I am persuading her to install electric underfloor heating to avoid taking up wall space with radiators and for a more 'liveable' room with a toddler.
Are there any extra insulation requirements or preferences for the timber floor eg T&G boards or flooring chipboard? A lot of what I read is concerned with choice of floor finish ie on top of the heating mat rather than what is underneath it.
The project is at trench foundation stage so still time to adopt any specific requirements. Grateful for advice.

monday said:

I am persuading her to install electric underfloor heating to avoid taking up wall space with radiators and for a more 'liveable' room with a toddler.
Are there any extra insulation requirements or preferences for the timber floor eg T&G boards or flooring chipboard? A lot of what I read is concerned with choice of floor finish ie on top of the heating mat rather than what is underneath it.
The project is at trench foundation stage so still time to adopt any specific requirements. Grateful for advice.

Click to expand...

Electric underfloor heating is the most expensive form of heating possible and it does not heat the room.
Electricity is approx four times the cost of gas (look at the kW hour cost on both bills).
Underfloor heating heats your feet only so why not just use comfy slippers.
Hysteresis is dire. From switching on to a warm floor is several hours by which time you may not want it any more. The only solution is to leave it on 24/7 which is very wasteful.
Does your daughter have central heating? If you still think underfloor is the answer why not suggest extending her central heating. Gas is MUCH cheaper.

You could lay a wet system.

Either way it could be an issue if you later want to fit cabinets or appliances on top.

why not block and beam instead of suspended timber - beams can be carried through and services can be chased through walls or come down from ceiling?

As somebody who is slowly renovating an old suspended timber home with lots of rot issues, I would not build a timber floor if I was planning to put a kitchen in. In my previous house we did a kitchen extension, and on removing the old kitchen found 3 leaks we knew nothing about - the whole floor was soaked under the vinyl. Washing machine had a small leak, fridge drip pipe had come loose and dripping on the floor, and there was a plastic fitting weeping under the sink. Got away with it on concrete, would not have on timber!

If the suspended timber floor is there to facilitate access and future rejig if the layout then having underfloor heating will completely stop you doing that.

much as I hate to agree with Winston1, electric underfloor heating is the most inefficient method. It will warm your feet, but not the room space properly. If you want to avoid radiators then a wet underfloor system is your option.

Just to support what has been said; I have recently fitted wet underfloor heating in our sitting room, and it works great. It's a suspended oak floor and I stuffed the whole depth of the joists with insulation. (The insulation makes all the difference.) However - that floor is not coming up again. Electric uf heating would be madness.

[QUOTE="Taylortwocities, post: 4696213, member: 36058]’

much as I hate to agree with Winston1, [/QUOTE]

Why is that.

The problem with underfloor heating is maximum temperature is 27°C, this means very slow reaction times, so it has to be on 24/7, for space saving kick space heaters are a better option, but they are also not without problems.

With gas using water to distribute the heat, the faster the room can heat up in the main the better, as you can turn off heating until required, or at least turn down heating, I use a 16°C Eco mode and 20°C comfort mode as it does not take long to raise a room 4°C. The old house had a gas fire and a Myson fan assisted radiator and a standard radiator in the main room, so from walking in the door all heating off, to being at 20°C took around 10 to 15 minutes, so there was simply no need for geofencing or internet control in any way.

This house there is a wood burning fire, but it takes so long to light it is emergency only. I have two radiators in the main room, so from turning on to being at 20°C can take an hour, so I do have geofencing using internet. It is not at 20°C when I get home but not far off it.

With storage radiators you have to plan the day before, and with under floor heating not much better, switch on to warm floor around an hour, warm floor to warm room another two hours, so from off to usable temperature 3 hours, four hours before it is at 20°C.

Hot air central heating either direct or using fan assisted radiators is fast, however most rooms are not at a single temperature, around the windows it is cooler and around the radiators it is warmer, and being cooler at windows means we loose less heat, because the fan assisted radiator blows the air around the room there are no cool spots, so it costs slightly more to heat the room, but it heats up fast, so a trade off, it also cools down fast. With pre-modulating boilers you would feel a little chilly before it kicked in again, with modulating boilers that is not a problem, the better system like the ivector have multi speed fans so as you want more heat the fan runs faster, this also makes it less noisy once the room has reached temperature, although don't think the kick space type are multi speed, mainly used in kitchens.

One in a house works well, but the water flows be it on or off, the fan does the controlling so it allows warm water back to boiler, this can turn the boiler output down, does not seem to be a problem with one in the kitchen, but too many and looking at building control packages which start to work out expensive. There are two types, those with single matrix and those with twin, the twin matrix type can do cooling as well as heating. Well so can single matrix type, but not cool one side of building while heating up other side.

Clearly a lot more expensive than a simple radiator but so is underfloor heating, I think wet underfloor heating is ideal for an OAP home, as no hot radiators for them to get burnt on when they stumble and grab things to save them selves, but baby crawling on a hot floor clearly not ideal. And intermittently used rooms also not ideal, in fact about the only place it is good is a OAP home.

Thanks to all for suggestions and ideas. I think on consideration we have reverted to a radiator system, using "tall" radiators which will minimise the impact on wall space. I didn't realise that my daughter's comments about "moving the kitchen" were simply to facilitate it being done by some future occupant - she has no intention of doing so!

Make sure she gets something with a decent output. In our house, the previous owner fitted tall rads in the extension - but used rubbish things that look like a very wide towel rail stood on end. Complete rubbish ! You can feel they are on if you stand close, but otherwise completely ineffective - esp in the lounge which has a high vaulted ceiling (I've replaced that one with a real radiator).
As for UFH - all I'll say is that I have plans fof thst in every room I have the opportunity. My wife jokes that I'll be asking for under coffin heting when the time comes But as above, electric is expensive to run, and restricts where you can put stuff - if you cover the floor then you get localised overheating. A wet system doesn't have that restriction.

Point taken, thanks. They’ll be at least double layer tubes and at least one on each side of the room.

Just to add, now I'm at a proper keyboard.
Electric is a "constant power" system - as long as power is applied, a constant power (and that's per each unit area) goes into the floor. This means that if you cover parts of the floor - furniture, rugs, etc., then you create hot spots as the floor needs to be hotter to dissipate the energy.
A wet system is a constant(ish) temperature and the floor cannot get hotter than the water flowing in the pipes. While the surface will get a bit warmer if covered (because there's some resistance between pipes and surface), in principle you can cover bits of the floor and all that will happen is the heat output will be reduced. In that respect, a wet system is fairly safe. And as said, at the moment, mains gas is roughly ¼ of the cost of electricity.

As for how effective UFH is, well that does depend on floor coverings. You are restricted in that thick underlay and carpets would be like lagging radiators with insulating jackets. But assuming you are OK with that (some people like bare wood floors), then having a warm floor does mean you can keep the room cooler. Simple human factors - if your feet are cold through being either on a cold floor or in the layer of cold air just above it, then you'll feel cold even if the room is quite warm. Conversely, if your feet are warm, then you will feel comfortable at a cooler room temperature.

Now, about that warm up time. Again, a lot of "it depends". If you have a low power system relative to the heat requirements of the room (typical of electric systems as they have to be limited power for safety), and if you have a heavy slab, then it's going to take a long time to warm the slab and then to warm the room. But if you have a high power system (which a wet system can be quite safely), then you can heat the floor up a lot quicker (but again, a heavy concrete slab will take longer than a lightweight wooden floor) and so it can be warming the room more quickly. But ultimately, the amount of heat transferred is heavily dependent on the nature of the floor - as above, a bare floor can emit more heat than one that's got a carpet on it.

Ideally you'd have proportional control for the heating - varying the water flow temperature and hence floor temperature with heat demand. I've never heard of this being done as it gets very expensive very quickly de to the need for a separate pump and blending valve (controlled by the room temperature) for each room. The standard setup uses a common pump and blending valve (fixed temperature) with each zone (room) controlled in an on-off fashion by a room stat - so you'll always have a situation where the floor gets hot, goes cool, gets hot, goes cool, as the room stat cycles with changes in air temperature.

SimonH2 - I agree about the "warm floor" effect and that was the driver for my initial thoughts on electric ufh. We are a 'barefoot family'.a hangover from years in the Far East where shoes stay at the door. Our bathrooms all have electric ufh under tiled floors controlled on a timer and we have electric ufh under a tiled kitchen floor, although that has a radiator as well so not the whole heat source. Walking to the utility room which has the same tiled floor but no ufh really emphasizes the difference.
I kick myself for not installing underfloor heating in a conservatory 20-odd years ago. We don't use it in winter normally because it is too expensive to heat but when we do use it occasionally eg at Christmas, shoe are essential no matter how warm the atmosphere in the room is - it has double glazed windows and double glazed K-glass in the roof but the tiled floor is stone cold.
Wet ufh would be nice but my daughter and her husband are on a budget and my influence is limited so I think we are back to radiators!
Thanks for your comments.