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Wet underfloor heating installed on a suspended timber floor that doesn't raise the floor level too much

(31 Posts)
audrey01 Wed 28-Aug-13 21:12:40

Does anyone out there have any recent experience of having a wet underfloor heating system installed on a suspended timber floor?

We are currently renovating our house (i.e. everything gutted, sound insulation, external wall insulation, side and loft extension, etc.) and we would like to install underfloor heating in the newly open-plan ground floor space. We would remove all the original floorboards, insulate the void between the joists, lay back the floorboards, lay plywood sheet 18mm, then UFH pipes, then finish with engineered wood. But this would require the whole floor level to be raised by approx. 50mm, which would cause problems with the first step of the staircase, I would imagine.

We have searched the internet for a solution that doesn't involve raising the floor that much and still give us the best heat output. We came across things like "UFH panels placed between the joists", or "wood floor directly on a screed with pipes", but we are not sure if the Celotex insulation between the joists would be strong enough to support the weight of a cement layer.

Has anyone got any direct experience of what solutions might work best? Many thanks!

PigletJohn Mon 02-Sep-13 23:49:55

chipboard is weak, and will crack where it is heavily loaded (e.g. under appliance or bed) or frequently loaded (e.g. in front of sink) or impact loaded (e.g. at bottom of stairs or on landing.)

It expands and crumbles if it gets wet

It will not hold a nail or screw.

It is very unlikely that you can lift or move it without irrepairable damage

It is ugly

It is widely used because it is very cheap, and will usually last until the builder has been paid.

A great deal of time is wasted patching it up where it has broken,or cutting it out where it has been damaged by water, especially in bathrooms which often contain some.

AwsomeMrsFox Mon 02-Sep-13 21:30:33

I'm curious PigletJohn what wrong with chipboard? I have to say I never thought about it sludging up audrey01 shock. We went for Myson as their product is Austrian and has been around and very widely installed on the continent for 40 years, but I didn't look too hard into it, so I hope sluding won't be an issue! Why is raising the floor an issue with the stairs? will it make a very short step?

audrey01 Mon 02-Sep-13 17:50:30

@PigletJohn - we will probably go for Vaillant, as easier to service in the long time.

audrey01 Mon 02-Sep-13 16:28:59

Thanks AwesomeMrsFox for sharing your experience. Unfortunately, due to our situation of joists running in the same direction as the floorboards, we will need something strong to compensate, so we're definitely going with an 18mm plywood. This will have to sit above the joists, so in order to maximise the heat output we will have UFH installed between the ply and the wood finish, rather than between the joists.

We've been looking at the Nu-Heat product that only has 15mm height (with 10mm pipes) and another product PolyPipe Overlay (which is slightly bigger, 18mm, but comes with 14mm pipes). Given that the difference is not that huge (3mm) and if we are to raise the floor level anyway, would we be better off to choose the bigger pipes 14mm rather than 10mm (thinking that the 10mm pipe might block more easily due to sludge, the 14mm pipe will hold more water so it will have a bigger thermal mass, etc.)

What are people's opinions on this?

PigletJohn Sun 01-Sep-13 21:18:36

chipboard is an awful material, and best suited for bonfires.

the reason for the ply is to provide extra strength and rigidity. IMO if you had laid 25mm WBP ply instead, it would be a better job and would not have needed overboarding. Tiles need a very solid base as they are not flexible so are liable to crack, which is why your tiler says overboarding is needed.

18mm ply is adequate for a floor that is going to be have carpet or laminate or a wood finish.

AwsomeMrsFox Sun 01-Sep-13 21:04:47

We have just laid wet UFH on our suspended floor. We used Myson as our plumber has lots of experience with it. Like you we cellotexted between the joists. Then we laid the diffuser plates between the battens (the coil sits in grooves on the plates, so to this point the floor is the level of the joists. We then put chipboard over it. We are having tiles throughout and out tiler wants us to put another layer of ply just to ensue absolutely no movement to prevent cracking, but I guess with wood you could just go over the chipboard? Then you would only raise the height of the chipboard and the engineered wood. Would that work?

PigletJohn Sun 01-Sep-13 21:01:36

It is rather supicious that the difference was so great. Print them off and see if you can find why so different. 20kW is unusually high; 12kW is fairly typical for a modern and well-insulated detached house.

Worcester-Bosch and Vaillant are two very popular and sound makers. The quality of the installation makes a huge difference.

I actually have a Viessmann but (1) you need to make sure you have an experienced local installer as they are not very common (2) I don't want to be blamed if you get one and it breaks down.

If you only have one bathroom and go for a combi, it will probably have a max output of about 30kW because you need a lot of power for instant hot water. I am not a combi enthusiast.

audrey01 Sun 01-Sep-13 20:34:28

Thanks Piglet John. I have used the second calculator with my inputs and it gave me a heating requirement for the whole house of 12KW. Hope that's correct!
The previous calculator gave me 20.2KW, so perhaps it's worth investing as you say in a boiler that would cover double the heat requirement.
Which brand would you recommend that matches the bill?

PigletJohn Sat 31-Aug-13 22:03:07

(however in my test, it reckoned cavity wall insulation would cut losses by about 5kW, which is about what it did, and the final figures are about right, so I think it is a fair guide).

I actually installed a boiler and radiators of about twice the power it calculated, so it heats up fast from cold, and ticks over at low power even during very cold weather, which is what I wanted. This is not the most economical way to plan an installation, though

PigletJohn Sat 31-Aug-13 21:57:03

An updated heat loss calculator

this one does allow for insulated floors, but it is very sketchy (25mm or less; or 25mm or more) but you may find it helps a bit.
I just tried it and it knocked off 1kW for a 6m x 6m floor when I added floor insulation to the calculation. they might be assuming the floor is carpeted, I don't know.

Your treatment will also cut floor draughts, which if you currently have bare boards will be quite a lot.

PigletJohn Fri 30-Aug-13 22:38:31

try this heat loss calculator

It allows you, for example, to specify what the ground floor is made of, and if it is insulated, so you can run two calculations and see what the difference is. Save or print the results because you will wantto look again.

* Edit - no that one doesn't seem to include floor construction. I thought it did. I will see if I can find one. *

Actual heat loss will also depend on weather, day or night and lots of other things, so a calculaor can only give you typical or average figures. In my experience it is about right, but does not include the extra capacity for exceptionally cold weather that I prefer.

If you are using UFH for your background heating, you will only need realtively small radiators for your cold-night supplement, so look at the low rads that are less conspicuous. Actual Skirting systems are ludicrously expensive but if you like the look and are willing to pay for appearances, they might suit you. Be sure to include room thermostats for your UFH and TRVs for your radiators in your design and budget. i would always tend to include long low radiators so that if necessary, and you need extra output, you can swap them for taller ones of the same length without the extra disruption and cost of repiping.

audrey01 Fri 30-Aug-13 21:51:28

Yes, tiles would be better, but in our case we want same floor covering throughout and unfortunately wood in the living area combined with tiles in the kitchen/hallway, just doesn't work. I think we will need to consider adding some extra radiators (either on the wall or below the bay windows) to supplement the low heat output from the underfloor heating.
I'm not sure to what extent these heat loss calculators are reliable as they can't take into account the fact that we will insulate quite thoroughly under the suspended timber floor, internal insulation on the external wall, we will install triple glazing for the north-facing windows, dining room and kitchen will benefit from double glazing and the fact that they are facing south, so hopefully lot of light and warmth from the sun.

PigletJohn Fri 30-Aug-13 17:12:04


you can tile on ply, but in this case you will need 25mm WBP to reduce risk of cracking.

PigletJohn Fri 30-Aug-13 17:10:09


If you have a tiled concrete solid floor in the kitchen and hall you would achieve greater heat delivery. You might not want a hot kitchen though. The rooms with most glass will lose most.

IME open plan feels colder because the heat floats away and you get cooling air currents. Worse if it is open plan to the roof or upstairs since the arm air will rush upwards. Low temperatue UFH is not so bad for this.

PigletJohn Fri 30-Aug-13 17:02:59

6500W is 6.5 kW which is about seven metres length of single radiators (or about 5 metres length of doubles).

for example, this 600mm x 1000mm single radiator has a nominal output of about 1kW, and this double has a nominal output of about 1.8kW.
In fact radiator outputs in real life are rather lower than that, due to the idealised way the measurements are done.

To compare, an electric fan heater with three settings would probably have outputs of about 0.8kW, 1.6kW and 2.4kW. A typical well-insulated modern bedrom with keep warm on the lowest setting or less, but a large living room might need the highest, especially to heat up from cold, or if it has lots of glass. A big iron multifuel stove might have twice that power. A modern combi boiler will be capable of producing about 30kW if necessary, because it needs that much to run a bath or good shower.

In my own house all the rooms have radiators of 1.5 to 2 times the calculated heat loss, so that if the house has been empty in winter, it heats up very quickly. Good thermostatic controls then turn the heat down so that it ticks along at low output. In bitterly cold weather, the heat needed will be greater than the calculators suggest. If you let them, builders will often put in minimum sized radiators to cut costs.

audrey01 Fri 30-Aug-13 15:47:34

Sorry, I should've said that we have about 60m2 surface area, so we will definitely need from the underfloor heating a heat output of more than 100W/m2, which we will not achieve if we have the plyeood and the engineered wood as final finish.

audrey01 Fri 30-Aug-13 15:10:01

Well, it will be an open plan space - living room in front, connected to dining room and kitchen at the back, we will have double glazed bifold doors at the back towards the south-facing garden, and triple glazed bay windows in front.
In terms of heat loss, I checked online for heat loss calculators to see what would be the heating requirement, but all I got was quite a big number (around 6,500 for Watts and 25,000 BTUs) and then I was redirected to buy either a traditional or designer type radiator. How does one translate the results for what output we would need if we plan to use underfloor heating?
We were thinking we could do without the need for extra radiators if we use UFH.
Now I'm not too sure.

PigletJohn Fri 30-Aug-13 14:53:15


I was a bit surprised the heat output is so low, I am more familiar with electric systems on concrete floors, which give 150W per m2 (but are more expensive to run if you do not have solar). Is it the wooden floor that reduces it?

PigletJohn Fri 30-Aug-13 14:43:23

I would certainly start by looking at how best to do it, and then calculating if that would meet the heating requirement.

I have known people use UFH for backround heating, it is very slow to respond with a concrete floor due to the thermal mass, but your wooden floors will be quicker.

have you calculated the heat loss from your room?

audrey01 Fri 30-Aug-13 14:34:44

Thanks PigletJohn.
So we should be looking at this from a structural perspective rather than maximising the heat output?

PigletJohn Fri 30-Aug-13 13:39:35


the pipes should go between the joists below floor level. You only put UFH on top of a floor if for some reason it can't be taken up.

PigletJohn Fri 30-Aug-13 13:37:39

go for option 2

The existing floorboard will be about 18mm thick, so putting ply on the joists will bring you to original height, and putting an ornamental floor on top will just add whatever the thickness of the engineered floor is. Usually 12mm/15mm/18mm so that's all the extra height will be. Similar to underfelt and carpet.

Try to have the ply screwed down, not nailed. It takes a bit longer but it is easy to tighten down if it squeaks, and to lift if you ever need to.

audrey01 Fri 30-Aug-13 12:34:26

@ PigletJohn - our situation is that the joists run perpendicular to the main windows. When our builder asked how we want to lay the decorative flooring, we said we would like to run the engineered wood floorboards parallel with the longest wall in the room, and also in line with the light through the main windows to the room. But when we lifted the original floorboards, we found out the joists already run that way, so our engineered wood would have to run parallel with the joists, which is not ideal.

So, our options are to have the 18mm plywood above the insulated joists to provide a good structural floor, then lay UFH pipes (15mm if we go with the Nu-Heat LoPro product) and then finish with the engineered wood. This gives us about 50mm height increase.

Or: try to put the UFH diffusers between the joists, get them flush, then have 18mm plywood above the joists that would run across them, then have the engineered wood to finish. This solution will give us only 35mm height increase, which is better, but what about the plywood acting as a further heat insulator? Our builder thinks we worry for nothing, as it is still a good heat output (around 45-50W/m2).

I would like to have more obviously, but with engineered wood finish, the most we can have apparently is 60-65W/m2.

Adam91 Fri 30-Aug-13 09:10:27

Message deleted by Mumsnet for breaking our Talk Guidelines. Replies may also be deleted.

PigletJohn Fri 30-Aug-13 08:25:49

The cement screed you mention will be intended for a concrete floor.

With the insulation, supports, heating pipes and diffusers between the joists, I can't see a need for anything on top of the joists except a structural floor (e.g. 18mm ply) and your decorative layer (e.g. engineered wood)

I would have thought you could get an engineered floor thick enough, and in longer boards, to form a structural floor, but have not done it.

BTW structural floorboards always go across joists. Decorative flooring can go any way.

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