Retrofit underfloor heating

[thehat]

Yes, so far I've failed to find any figures too.

[cornishben]

I thought I'd add to this topic rather than starting a new one as 'retrofit UFH' is exactly what I'm doing..  the difference in this case is I'm doing it on a solid ground floor rather than a suspended floor.

Our ground floor room is ~4m x 6m and is currently an uninsulated concrete slab sat on some kind of concrete beams above an air space (1960's split level house on a slope).

The room will have a woodburner at one end but the intention is to have underfloor heating below an engineered wood floor.  I know a tiled floor is preferable to wood for heat transfer but that's not negotiable

I have 100mm height available (patio doors have already been fitted with the base 100mm above current floor level). 

The heat source currently is an old oil boiler, but the intention in the longer term is probably to replace this with an ASHP (I started a long thread about this option back in January).  However at this stage we just want to get the floor laid so we can progress with other decorative work.

The original intention was insulation board - screed/ufh - floating engineered board.  However with 50mm insulation and 25mm engineered board that only leaves 25mm for the ufh/heat diffuser.

How would you do this with 100mm height available?

[acresswell]

I'd be tempted to look at the grooved polystyrene panels (something like www.underfloorheating.co.uk/floor-systems/floating-floor) with celotex underneath.  It doesn't have a screed to spread the heat, relying on an aluminium sheet instead, but you get extra insulation instead.

Can you get underneath the concrete beams? 
Adding insulation underneath the beams may help to partially compensate for a lack of insulation below them.
(Of course, you'll still have a huge thermal bridge round the edge!)

[cornishben]

I'd be tempted to look at the grooved polystyrene panels (something like www.underfloorheating.co.uk/floor-systems/floating-floor) with celotex underneath.  It doesn't have a screed to spread the heat, relying on an aluminium sheet instead, but you get extra insulation instead.

Thanks for the feedback.  I went for NuHeat's grooved polystyrene board option in the end and have it all laid and plumbed in.  I am in the process of laying the engineered wood floor on top now, so the set up is >

14mm engineered wood boards (clip together)
15mm Nu Heat polystyrene panels with fast-flo tubing and heat spreader plates
70mm Celotex
damp proof membrane
uninsulated concrete floor
 
I've found there is a certain amount of squeak/movement in the floor when walking on it.   I'm trying to work out the best way to deal with this.  Our intention was not to glue the floor down, partly because of concerns about needing to access the distributors/pipework in the future and secondly because we would only be gluing to the small polystyrene panels which themselves are not fixed. If we fixed them to the celotex, then that would need gluing to DPM which didn't seem to make sense.
 
The movement seems to be because the polystyrene can be slightly compressed and obviously there are small holes here and there between the poly panels where the pipework has tight turns, etc.
 
Any ideas of a way to improve this?  One option is to glue the floor down but I'm sure this will improve the situation as the poly panels themselves are not attached and I'm not sure how well we can glue to the polystyrene.
 
Another option I was considering was to take up the half floor I've laid so far and lay a 3-4mm hardboard or ply layer between the poly boards and the engineered boards.  This would give a solid layer to lay the floor on, however I'm concerned about heat transfer if I do this.  Or is there another material I could use here which would serve the same purpose of adding some rigidity but give better conductivity?
 
I 'think' I can get away with adding another 3-4mm in height without encountering issues at the door thresholds.

[dhaslam]

Perhaps some  kind of thin  metal sheet  would be best.  It would protect the  polystyrene a bit and  conduct heat well.   

[cornishben]

Perhaps some  kind of thin  metal sheet  would be best.  It would protect the  polystyrene a bit and  conduct heat well.   

Good idea - but I guess the cost is going to be prohibitive (we have ~35m2 of floor to deal with)

[Baz]

Mybe the opposite if fixing, ie lubrication with talc.