"Don't worry about heat loss calculations and btu mumbo jumbo at this stage. That's for sizing the boiler."
This creates a lot of work for us, but we prefer to do a heat load upfront as this is and has been the accepted practice for a hundred years or so.
We have installed truck loads of barrier PEX tubing. Non-barrier has its place but not in this forum. That does lead us back to the heat load however, as the heat source often dictates whether an oxygen barrier PEX is appropriate or not. The heat source - presumably a boiler - sized for the heat load.
Generally, 12" on center spacing will do the job, but the heat load will also dictate tube spacing, as more tube generally lowers the design water temperature. The lower the temperature the lower the operating cost especially if your design will benefit from lower design temperatures such as condensing boilers, ground source heat pumps and even properly controlled wood boilers.
Suspending the PEX in the slab will also lower the heat load, increase potential output and response time, but we still don't do it in most applications, following the principle of diminishing returns.
Variable tube spacing is hardly ever necessary and should not be followed as a rule of thumb. When we do use tighter tube spacing, it is for comfort (in residential spaces near large windows) or to overcome an unusually high load (such as a large garage door).
In the typical garage found in this forum, no variable tube spacing is needed or even beneficial, I am much more interested in the pattern of tube and the flow therein.
A proper heat load performed on dedicated software will produce accurate data for PEX tube size, length, pattern and boiler output.
Even loop lengths are desirable if you have an open floor plan and especially is you are using an inexpensive copper manifold without the benefit of flow control/balancing, but you hardly "need" to keep lengths at a certain length if for instance, you have an office to cover with 100' loop. In space heating applications where people are not sitting or even present tube loop length and space may vary considerably for a variety of reasons.
We generally use 25# XPS, stapling 1/2" barrier PEX on top and wire or bar on top of all to finish. The heat load and application dictate the details.
Perimeter insulation is the most important, but whether it goes down, out or in, is irrelevant. If you think of the frost line (normally thought of as down) you get a good idea of where the insulation needs to be. Better to double the insulation at the perimeter than to over-insulate the middle of the slab floor instance. Sometimes the the insulation is run down the inside or outside to "protect" the foundation from frost, but it is by design sitting below the line.
We prefer shallow frost protected foundations that answer most of the concerns for heat loss, radiant performance and foundation integrity.
