[Vlad]

I think first of all you have to determine your heat source.

If the source is high temperature boiler or DHWT or wood burning boiler higher temperature in the underfloor loop doesn't matter. The temperature range will be just higher, so higher supply higher return. The most important to keep your efficiency will be to keep as much heat inside your house and not outside like crawlspace (I will insulate my as soon as I have extra time and $$)

But if you want to use HP or solar or even condensing boiler you have to fight every degree. The lower the better. This is just because this systems work the best and the most efficient when temperature is low. I spaced pex @ 12" and have no problem at all. I do agree spacing @ 6" would allow to lower my temperature from max 95 to max 92 or so.

I think many people just focus on floor heating and forget about air supply. Air heating/ cooling must be a part of your HVAC system. This heating season I will try to run 2 systems together.

On other hand space pex @6" is a real pain. You are limited 300' with 1/2 pex. If your room is larger then 10x12 (this is a size of my den) you have to put 2 loops in parallel in one room. Balancing is going be a real pain.