Underfloor heated breathable (no longer wood) tile floor!

I'm trying to come up with simple flooring system that's concrete free, breathable and heated with good thermal mass. Something I can also get into easily enough if (when) the underfloor heating needs replaced.

Insulative compacted foam glass foundation:




Geotextile membrane on the foam glass. Joists spaced half a metre apart, in filled with compacted dirt/sand/clay. Electric underfloor heating mats rolled out, covered in sand, etc:



Wooden floor:



Any thoughts appreciated! I should say - we're on the Western Isles and are looking to make good use of all the wind here by generating a lot of leccy.

EDIT: Minor oversight, as the boards would be laying across the joists it would be in no way simple to access the heating mats, duh.

Is the following workable? Laying flagstones, (approx 600 x 300mm) over the joists, with sand/dirt packed underneath? Or am I asking for trouble and cracked flags.  

I'm wondering what this floor is for?  Living space, indoor/outdoor entertaining, existing or new building?

How fancy does it need to be?
How consistently will it be heated?
What are the chances that there will be wet conditions?

Laying tile (or hanging any thermal mass) over wooden joists does sound like a bit of potential trouble.  Wood and masonry move differently.  

There's a chance that if the wood bows or sags, your tiles' edges might fall or wedge on one side of the wood, as well as the possible cracking you mentioned.  
Filling the wood with sand/earth may make it more rot-prone, too. "Breathable" suggests you may be drying out any underfloor moisture (or spills) by evaporation through the floor.
I would want careful attention to proper drainage and water exclusion before even attempting a floor of the proposed mixture of materials.
That's not even considering whether your heaters (presumably electric?) would be affected by moisture.

Depending on the context, I'd be tempted to pick one dominant method, and stick with more typical/traditional detailing.
Either go masonry/patio style: skip the wood joists and just lay your heaters and flags directly on sand/fill;

OR go wood style: skip the flags and sand, raise the joists up a bit, and make it accessible through a crawl space below the floor.
You might also consider making the original wooden floor removable in sections - maybe something like 4x8 plywood sheets, but of floor boards or parquet.    

The manufacturer of your floor heating units probably has some well-tested methods for using them.  
If all their methods involve concrete, you might call or write and ask them why, and what alternatives they could see for a more natural and flexible setting.
If they have a method for heating under a wood floor, that might be closest to your original design ideas, and they could give you good advice about heat storage options and proper detailing.

Thanks for your reply Erica, you've raised some great points.

I should have clarified - this is for an indoor living space of a new build - an externally insulated log cabin.

Needn't be that fancy, it will be heated fairly consistently throughout the year (thought temperatures rarely dip below freezing). Wet conditions mainly arising in bathrooms/kitchens.

Probably better to lay the slabs on bricks - not joists right?

I had considered the wooden floor in sections but this proved to be fairly pricey. The use of engineered wood was recommended which I wish to avoid.

I will email around a few manufacturers and see what the reckon to laying the mats in pure sand. I can only find reference to using them in a sand and lime/ or cement screed.

Waiting to hear back from manufacturers, but here's a mock-up of the tiles on bricks I sent:

Your original idea will work, I have used that method to build my own house, with the exception that I used pex tubing heated via propane/wood/coal instead of electric mats.

Travis Johnson wrote:Your original idea will work, I have used that method to build my own house, with the exception that I used pex tubing heated via propane/wood/coal instead of electric mats.

Thanks Travis, that's encouraging to know! What have you used for insulation, and is that with solid or engineered wood? Interested to hear more details.