Use more insulation.
I don't know where you live, what are your building regulations and so on.
But here in Ireland the legal MINIMUM insulation for UFH results in 100mm insulation.
Going for a minimum does not mean going for the optimum.
 
So bring in more insulation. The job has to be done anyhow, ordering and installing a bit more material won't cost the world.
I would put in another DPM as well, to be on the safe side.
Check the loadbearing capacity of the finished floor.
 
It usually makes sense to install more pipes than calculated per m2. This allows for a reduced flow temperature, increasing the energy efficiency of the heating system.
More loops might result in a thicker slab, check this out with the manufacturer of the UFH system.
In a cast concrete slab there is no problem if pipes cross, the slab will eliminate any movement due to contraction or expansion.
Provided the slab isn't larger than 4x4m or the like.If so include expansion strips, seperating these zones/slabs. But the manual will tell you this anyhow.
The pipes are installed in one run, no cutting and joining please. And they come in rolls of certain lengths (100m-150m).
Instead of dumping the off-cuts or storing them eternally somewhere lay them into the screed. Go for a maximum of hot water content in the floor, ask your suplier to figure out resistance values of increased pipe runs to make sure the pump is not to weak, the right pressure maintained.
Most UFH sellers have the necessary computer software to guarantee a max. of thermal gain with a min. of primary energy input. Ask for a calculation to be done, tell them you have enough space for a thicker-then-standard slab.
 
The standard installation instructions are trying to keep the slab as thin as possible to guarantee a broader market for the UFH sellers.
 
But in your case where space is no problem go for the ENERGETIC optimum, not for the standard minimum.
 
Good luck!

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If you still have left-overs from the piping material lay some 'blinds' into the slab, allowing you to connect wall heating elements at a later stage. Or to run electric/telephone cables through them.

Thanks very much for the comprehensive reply, Heinbloed. Do you have any recommendations for which insulation to use? - I've been looking at Jabfloor 70 EPS, which is a 1/4 of the price of Kingspan,and mixing the various thicknesses to bring up the level to 70mm below the finished floor level, but any advice would be appreciated.
Simon

Well, in principle the planned loadbearing capacity of the to be build floor should be decisive on the material choice and workmanship.
EPS 70 is very strong, it is used under the foundations of entire buildings.
For a standard home floor with no loadbearing walls or similar heavy objects ( for example masonry stoves) a 'weaker' quality of EPS would do the trick as well. Cheaper and better insulating as well.
 
About the choices of insulants: Gas filled insulants (like most PU or PI boards) lose their better insulation quality over time. Esp. if they are exposed to higher temperatures the gas mitigates, it is exchanged with air and after a while the formerly better PU board is not better insulting then any other air-based insulant. Like EPS for example. Your choice going for EPS is right.
If demanded go for a thicker layer of EPS.
 
A document on the issue had been published by BRE in November 2000 (BRE=Building Research Establishment), titled
"Field investigations of the thermal performance of construction elements as built".
At page 15+16 of this document the calculated U-values of building elements are stated as well as the "adjusted" U-values. "Adjusted" stands for 25 year aging.
The result of aging is that at the latest after 25 years the U-value of EPS is as good as the U-value of PU foam.
 
This document is interesting reading, find it in the www..
 
This effect of gas exchange is speeded up with increased temperature differences between the two faces. And the temperature difference between the cold ground and the heated slab is relativly high.
 
Costs of materials represent the energy input into these. The higher the costs the more energy went into it.
If a high energy containing product would save a lot energy in it's lifetime then it might be worth it to go for it. But if this advantage is eliminated by itself after a while, long before it's life cycle is ended then it propably doesn't make sense to use it in the first place.
 
Expect from an UFH a life time of 50 years.
This is what it states in most literature.Experiences with longer life times do not exist since the UFH systems we use nowadays do not exist that long. It could be well above 50 years.
So prepare for a long future(smiley), put a bit extra insulation of EPS into it. Fuel prices in a few decades might dictate to go for solar thermal heating or whatever low-energy flow temperature.
Run a pipe circle around the perimeters, along the external walls as close as possible. This part of the building will always be a difficult place to keep warm, so running a low temperature along there will keep this part of the wall warm. Around 20 dgrees of flow temperature is enough for this circuit, it is not supposed to heat the house but to keep thermal losses from the rest of the room low.Avoiding cold spots, mould growth in case this part will be blocked with furnitures, carpets or whatever hinders air circulation.
The recommendation frequently heard not to place UFH circuits under fixed furnitures (like kitchens, bath tubs ect..) is non-sense. If a room gets to warm reduce the flow temperature.
When purchasing fridges and freezers (where any heat source would be counter productive) go for a "climate class" higher than usual, 22-23 degrees Celsius. Check the manual of the fridge/freezer, modern ones (A-rated and better) are categorized in climate classes.Not only in energy ratings but also for different climatic conditions.
Liebherr, Bosch and so on have them all categorized, check their home pages. Made for cold environments or warm environments, the one-fits-all type would not be suitable for UFH due to it's higher energy demand and the risk of unwanted defrosting in warmer conditions.
If fridges and freezers come with adjustable footings(they usually do so) bring them up high to go for perfection in energy wise kitchen design. This will also increase the air flow rate at the back where the heatexchanger is positioned.

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Avoid the EPS with flame retardent, the plain standard material is cheaper and less likely to cause recycling problems in the far future. There is no need to use EPS equipped with flame retardent in the planned project as far as I can see.

Thanks again for a very comprehensive reply,
S