Jack, I'm not much of a foil man myself so can't add anything however in their literature they do state that:
"The horizontal joints are overlapped by 50mm and are left open to allow for any moisture accumulation to dissipate" so it wouldn't act as a VCL and the moisture that dissipated would sit in the top layers of your quilt.

One point that I don't understand is that I was told when putting flooring boards into the attic it was a big no-no to notch ceiling joists to allow electric cables to be dropped out of harms way before the flooring went down. This was on structural grounds.
Yet it seems OK to notch floor joists to an even greater degree when installing this type of UFH pipework even though the joists are taking far greater loads. I know they're deeper but the principle remains surely?

There are rules/guides on where you can drill and notch joists..

http://www.eastdevon.gov.uk/bc-guidancenote7-notchingofjoists.pdf

Regarding the SuperFloor..It seem to me there would be big air spaces around the pipes. Not great for conducting the heat to the floor.

We have a mixture of different floor constructions in our house. The two that perform best are:

a) Tiles or stone over pipe in screed and

b) 21mm engineered oak fixed to joists with this system between...
http://thermoboard.wavin.com/master/master.jsp?FOLDER%3C%3Efolder_id=2534374305495207&middleTemplateName=oc_middle_service_sub

They used to be called Osma. We don't seem to get any significant noise from the system as it warms up or cools down. We also have a variant of that which uses battens over beam&block floor.

What doesn't seem to work so well (higher flow temperatures needed to achieve same room temperature) is:

c) Carpet and underlay over screed. Even keeping the total TOG down to 2.5 (some recommend 2.0 max)

and i suspect

d) Carpet and underlay over 18mm chipboard over UFH with air pockets

..would need higher still. It still works but not so good for ASHP or keeping the thermal store or boiler return temperature down.

I can't say I've noticed any variation in temperature.

Posted By: daveyates They say thick rugs are a no-no (carpets/underlay should have a max tog value of 1.5) but our friends have got a large rug in the middle of their floor and it is wonderful- a big thick warm comfy rug. The kids and pets love it. The room certainly isn't cold but I guess the efficiency suffers.

My wife's very keen to put a large rug down in the middle of the floor, occupying perhaps a third of the total floor area. Please could I ask a few quick questions about your friend's rug:

1) Do you know if they had to increase the UFH water temperature to compensate for the rug?

2) A related question: what's their floor construction? Engineered timber floor boards over screed?

3) I don't suppose you know the tog number for their rug?!? Or thickness? Are we talking a 5mm thick rug or a 25mm thick one?

I appreciate you might not know the answers to these questions but I thought I'd ask!

Thanks,
Jack

Posted By: delboy
1. What's the point of the foil? Is it purely a vapour control device?

The point would be to reflect heat upwards towards the living space. But I'm going off the foil product. Top of my shortlist is the ThermoBoard suggested above by CWatters

Posted By: delboy
2. Are you going to hook up the UFH circuit to a boiler which is also supplying radiators? If so, are you going to install a mixer valve? Is that necessary?

Yes to all three! The UFH system will have its own manifold and pump. So yes, the UFH system will get its heat from the boiler but the UFH system will ensure that the temperature in the UFH circuit never exceeds 27 degrees C (which is what our engineered floor boards are rated at).

Posted By: delboy
3. Is there any risk of your damaging the existing joists because the top half of them is likely to become very hot a lot of the time. Can this cause shrinkage and therefore affect the integrity of the joists?

I think the joists would be fine in our situation. We're using new joists which have been left to acclimatise for months before fitting so they should have a pretty low moisture content. Plus the joists wont see much heat from the UFH pipes because we're using ThermoBoards, plus our water temp will never exceed 27 degrees C, which is not particularly hot. But the joists would see heat from the UFH system if you installed a screed.

The thing that's really worrying me now (and may be a deal-breaker) is that there are documented cases where the solid wood wear layer warps and delaminates from the ply in engineered timber floor boards if you put thick rugs or bean bags over an engineered floor with UFH. We would like to put a nice rug and a bean bag in our room! Here's a photo of a delaminated engineered board: http://www.thesolidwoodflooringcompany.com/case-studies.php?page=224

I could put 1-wire temperature sensors across the floor to monitor the floor temps. But still.

Posted By: delboyCan anyone see a way around this?

When the pipe needs to jump between joists, could you run the pipe under the joists? (you'd have to insulate the pipe for the short under-joists hops, of course).

How deep are the joists? If they are oversize for the span then taking out a few extra mm isn't going to hurt. Google Joist span tables.

This comment may be a bit late, but warning bells started ringing when I read the words 'Victorian house'. The only complaint I ever had from lack of heat installing UFH was upstairs (wooden floor) in an old house - can't remember the era, but it was on alloy spreaders. At a typical delta T you are getting around 20w/m of pipe run in screed, (100w/m2 av.) significantly less where air is between the pipe and wooden(!) floor, with possible rugs (!). That's enough for a modern-spec building's heat-loss, Victorian possibly not. Especially as the joists usually preclude increasing pipe density.... sad:

I think the only way I would do that if this was my home would be with an insulation board on supports and kiln-dried sand as the conducting medium, with the ability to raise the flow temps. Best of luck.

Might this work as an alternative to Wavin's ThermoBoard product?....

My thinking is this:

We want to install UFH in a suspended timber floor. We don't want to use screed. So we're left with using either aluminium heat spreaders or ThermoBoard. Alu heat spreaders can be noisy because the alu is in directly contact with the joists and the floor boards.

On ThermoBoard, the metal heat spreaders are embedded into the top of the rigid foam insulation. The insulation is supported by L-shaped brackets screwed into the joists. And there's a layer of plastic between the floorboards and the ThermoBoard. The end result is that the heat spreaders are not in direct contact with either the floor boards or the joists, so noises are kept to a minimum. But Wavin's prices are high and they insist on charging for a "bespoke design service" even though I don't need them to do any design; I just need them to supply me with enough product for 25 square meters (and a sure-fire way to annoy me is to force me to buy something I don't need).

So I'm wondering if I can make my own "ThermoBoard" for half the price using alu heat spreaders, some tin snips, some Celotex and some plastic sheeting. The ultimate goal is to have heat spreaders which are not in mechanical contact with joists or floorboards but are firmly held against the bottom of the floorboards. Any thoughts?