To follow up and post for the benefit to anyone with a similar issue in the future; the solution has worked wonders. The engineered oak floor has now been laid and were nice and warm.

The installer is making good. This weekend we're having the chipboard removed and a dry biscuit screed filled in between the joists to remove the air gap. On top we are having an 18mm cement board as advised by the company installing the engineered oak flooring, as our joists levels are very mixed!.

The cement board we have chosen has excellent thermal conductivity and is specific for UFH.

- The heat spreader plate is 0.5mm aluminium sheet. Some other manufacturers use microns thick aluminium foil, which looks pretty but has no effect on heat transfer.

- Pipe spacing is 125mm and the plates can be easily cut with a box cutter or just snapped every 125mm. For radiant heating, closely spaced pipes provides better heat transfer at lower temperature.

- Total plate thickness 30mm with XPS foam insulation backing.

I'm having trouble visualizing the way your floor is constructed. Can you make a cutaway drawing with dimensions?

From what I understand you have insulation between joists, pipe simply dropped on top, surrounded by air, and chipboard resting on the joists. Final floor height is determined by the chipboard.

In all cases you need a heat spreading material (screed, aluminium plates...) and the floor boards must make good contact on top without air gap.

- Option 1: do it from below. Remove PIR insulation between joists without destroying it, install pipe in heat spreader plates screwed to the bottom side of the floor panels. This ensures the heat spreaders make good contact with the floor panels. Then reinstall PIR insulation from below. If the floor below allows this, this is the simplest solution, it also allows you to finish your floor and do the UFH this summer.

- Option 2: do it from above: Remove chipboard panels. Then you need the heat spreader to make good contact with the floor panels when you put them back, which means either a) it needs to be exactly the same height as what's available between the top of the PIR insulation and the top of the joists, there is no reason for this height to correspond to off the shelf UFH panels or even be the same everywhere, which means it'll have to be a screed or b) you can move the PIR insulation to adjust the height, but in this case it may be simpler to do it from below...

Also please check your PEX has anti oxygen barrier. If it does not it will let oxygen in and corrode your steel radiators. Normally it should have the barrier, but since this is not a normal installation... who knows.

 - Engineering wood (~20mm thick) is both structurally strong and an excellent conductor of heat. The boards are fitted directly over the joists - no additional chipboard layer is needed. The room with engineered wood now outperforms the solid oak floored rooms.

 - The underfloor pipes are carried by steel brackets that run along the joists, overlaid with thin aluminium sheet spreader plates. The wood flooring is in direct contact with these spreader plates.

From your description it sounds like your installer has used the pipe-in-screed construction method, where the pipes are clipped to insulation and then covered with screed up to the desired floor level, instead of a floating floor installation.

Step 1) Remove the chipboard - this is not needed. The engineered wood will be sufficient flooring.

Step 2) Either back fill the gap between the insulation and the top of the joists with screed Or refit the pipes in a suitable spreader plate solution (insulation with grooves or steel brackets) - whichever solution you use the screed or spreader plate must be touching the underside of the flooring.

Your original installer should make good or you can discuss this approach with a new installer.

Martin

The floor is insulated between the joists with 75mm PIR and is taped too. The current piping is clipped to that.

I’m looking for a company that will advise, quote and install the remedials as we’re not experts in the slightest. 

It would also help to insulate under the floor not to loose the heat down and the deflectors will emit in all directions (although heat rises) but it will help

I even found that for you :)

as you can imagine I am always modifying everything, 36 years of getting round stuff I suppose, best of luck...

https://www.amazon.co.uk/s?k=sheet+metal+nibler&i=diy&crid=2ABVHIW8JYBRT&sprefix=sheet+metal+nibler%2Cdiy%2C108&ref=nb_sb_noss_2