I couldn't see any tech data for this one..
http://www.ukflooringdirect.co.uk/Accessories/Cushion_Heat_Flow_Underlay.html

The data here..
http://www.elastilon.com/uk/images/stories/pdf/iff_testreport_723_uk-lock_shear_strength-thermal_resistance.pdf
suggests a 10 degree centigrade drop across the underlay but I've no idea what the equivalent drop would be without any underlay.

Bit of googling found..

http://chauncey.co.uk/floors/underfloorheating.php

FLOATING FLOORS:
Although TECTONIC Oak flooring can be laid as a 'floating floor' (ie where the joints are glued together to form a 'skin' over the sub-floor, but is not fixed down directly to the sub-floor) this is not the best choice, due to the possibility of air gaps above the subfloor which would have a detrimental effect on heat conduction. If your subfloor is absolutely true and even then floating is a viable option.

Underlays compared to Adhesives
We always recommend that where possible you should always glue a floor to any concrete screed especially if you have UFH. The reason is that you can get a "spongy" feeling when floating floors on an underlay especially if you use cheap wood flooring. For example our Oak Floor, solid and engineered is the best quality you can buy which means there is less likelihood of this happening. Underlays do not stop moisture they only prevent some vapour which is different to moisture.

That one you mentioned above above has a thermal conductivity of 0.083 W/(mK)
http://www.elastilon.com/uk/images/stories/pdf/iff_testreport_723_uk-lock_shear_strength-thermal_resistance.pdf

Duralay Heatflow seems to be 20% better at 0.101 W/(mK)..
http://www.completelyflooring.co.uk/underlaydata/heat_wood.pdf

Some other makes that claim to be suitable for UFH seem to have thermal conductivity less than HALF those values (eg there would be twice the temperature drop).

This company are active on other forums and seem to give good advice...
http://www.woodyoulike.co.uk/knowwoodufh.html

They seem to recommend glue where possible.

Posted By: CWatters
That one you mentioned above above has a thermal conductivity of 0.083 W/(mK)
http://www.elastilon.com/uk/images/stories/pdf/iff_testreport_723_uk-lock_shear_strength-thermal_resistance.pdf

Duralay Heatflow seems to be 20% better at 0.101 W/(mK)..
http://www.completelyflooring.co.uk/underlaydata/heat_wood.pdf

Just looking at the question of installing wood flooring on a 100mm thick concrete sub-floor with wet UFH and came across this thread.

Looking at the comparison above the test method for the Elastilon underlay shows the total result whilst bonded to 10mm thick multi layer wood flooring whilst the result for the Duralay Heatflow appears to be the underlay on its own.

I can't seem to find two directly comparable results for these products.

It also seems that an important advantage of the Elastilon underlay is that because of the self adhesive bond to the sub floor and the wood flooring the possibility for air pockets which would inhibit heat transfer is greatly reduced.

Jumping the gun a little and assuming that the Elastilon is the best underlay to go for, how does it compare in terms of heat transfer with a wood floor that is glued directly to the sub floor without any underlay?

Has anyone got any directly comparable figures that would help in deciding whether to go for Elastilon or glue down installation?

Posted By: tonyI cant see why we are so worried about air pockets, the heat will warm them up and still come through almost as if they were not there.

Perhaps so Tony, but eliminating air pockets seems to be the number one reason cited for the recommendation to glue down wood flooring over UFH is that all dis-information?

Aren't “air pockets” what insulation is made of, mostly? Any insulation between the UFH pipes and the room will increase the temperature at which the pipes and the screed around them have to operate to get a particular W/m² into the room and hence increase losses downwards.

Posted By: gustyturbineBut if the possibility of lost heat (minimal though) in air pockets is a real concern then how about tiles instead? I am no expert but the heat lost in air pockets would be tiny surely?

It's not a question of heat loss, there is 300mm of eps below the concrete so the heat will not be lost, but presumably the easier the heat can be transferred to the room the lower the flow temperature in the water can be and the more efficiently the system can operate.

Response time will not be an issue as it will be always on in the heating season with a room stat taking care of things.

We will have tiles on a large portion of the ground floor but SWMBO dictates wood in the living room.

I guess I'll just price up Elastilon and glue per m2 and take it from there.

No probs gusty.

I just went googling for a price on Elastilon and I will definitely be using....adhesive direct to the concrete.

Anydrite screens need special treatment to tile on them or it seems to get anything to stick.

FWIW our flow temps are 32 - 36 degrees and warm the floor just right - and I know there are plenty small air gaps here and there.

SWMBO is right to insist on wood in the living room - tiled floors change the acoustics and feel cool (too cool for my taste) to the foot when the heating is off.

Lounge I would agree. I went for the ufh carpet and underlay. It doe keep the temp slightly lower than the tiles though. Around .8deg as a rule but what can you do. Having a slightly lower efficiency heating system in the lounge was cheaper than divorce.

wood, even engineered wood is a great insulator...experience shows that with wood UFH has to be run as hot as radiators(with compensation controls)...so any energy efficiency gain is wiped out...

of course it works...but you don't get any energy saving benefits...

I'm with James Ingram -
However if there are concerns about the heat getting out fast enough (technical speak) then it would be kinder to the wood to space the pipes more closely and keep the water at the lower temp. If you're using a heat pump the lower flow temp will save a lot of money.

One problem I've heard of is people buying 14mm engineered wood then finding out from the installer that the minimium thickness for an unsupported floor is 18mm. So they put down 18mm chipboard then the 14mm engineered wood on top.

Or weaker depending on the directions of the joists,routed tracks and their positions in relation to the joists, centre span being worst but sheer forces could be a problem if a very heavy person stood in the wrong place and dropped right through!

Cheers tony

Routed tracks would be at 90 deg to the joists except for the returns at either end which could be positioned directly over the joists.

I could see the possibility of it caving if the tracks were all running parallel to the joists OK.

I probably won't put in UFH upstairs in any case and instead rely on a duct heater in the HRV ducts, but I am just planning ahead in case that changes.

@ Chris P Bacon

Another option would be to use something like Polypipe Overlay. It's an 18mm thick grooved board that sits on top of the sub floor and you then lay your finished floor on top. From what I can remember I think that it takes a 12mm UFH pipe.

The disadvantages are

1. additional cost
2. you lose 18mm from the room height
3. unless you put it across the whole storey, you will end up with a step at the edge of the room
4. you need to make sure that the floor structure will take the additional weight

Our house is on a slope and so the main living area sits "upstairs" on JJI joists. We want underfloor heating on this floor so I am planning to go down this route.