For a 6" slab, the best thing to do is place the rebar on 12" centers, running both ways, and tie the PEX to it with rebar ties. Around the perimeter hold the PEX back about a foot from the inside of the exterior walls.
If you have dry ground, insulation is only needed around the perimeter under the slab. Approximately two feet in from the perimeter is fine. More than that is OK if you want it, but will do nothing for efficiency. I'm more comfortable with the concrete being supported by compacted ground, than foam insulation. Be sure to have a good moisture barrier. Three layers of 6 mil is good, or some of the more modern barrier materials.
The rebar should not be above the PEX as it will likely damage it. Mesh is an awkward material to use unless you are pouring a thin slab over a structural slab. Rebar is better.
If you insist on pouring a thin slab over a structural slab, shoot down 6" mesh flat sheets from Home Depot and tie to those. This done by fitting all the sheets in place and tying them together with ties. Then tying the PEX to the mesh. Then shooting the mesh down with a powder activated nailer. Again, putting the PEX in the structural slab, on 12" centers is better.
Make each loop of PEX about the same length and no more than 300' for 1/2" PEX which is 5/8" in diameter. Enter and exit the slab through 3/4" PVC conduit 90s. 200' would be even better as a max.
Arrange the tubing to have a complete band of heat around the perimeter. This means the hot supply line will go along the exterior and then serpentine back and forth. The first pass inside the perimeter run should be placed at 6". So the first tube at 12" inside the room and the second pass at 18" inside the room and all other tubes on 12" centers. Different rooms should have different loops, such as bathrooms or bedrooms. Enter each of those areas through doors instead of under wall plates.
Pressurize the system to at least 60 PSI before the pour and leave it under pressure during the pour. This can be done with a gas test gauge. Replace the gauge with a 100 PSI gauge and use a compressor to pressurize it through the schrader valve.
If the radiant area is inside of a forced air heated room, control the system with a slab temperature sensor and controller. This is best done with a short piece of PEX taped off on one end and tied down to the bar near a heating tube. Then the sensor can be slipped into the tube and replaced if needed. Do not bury the sensor in the slab directly. These sensors are typically 10 K sensors and about 3/8" in diameter with along set of wires that will reach up out of the slab. Place this near your manifold or near the water heater and control area. It does not need to be against the heating tube and it does not need any special conducting grease. If the area has no other main source of heat, such as forced air, use a wall thermostat with setback. Allow a minimum of 15 BTUs per sq ft of delivered energy. Much better would be 25-50 BTUs per sq ft. A maximum delivery temperature of 110- 120 degrees is plenty. This is the high limit, not the continuous delivery temperature. The higher your BTU delivery, the lower the high limit will be. A final average floor temperature of 70-74 degrees is very comfortable in a well insulated room. Higher if the insulation is less. Never over 80 degrees. Lower if it's a workshop. 60-65 degrees is plenty if you are doing a lot of physical work. If you know where permanent fixtures will be, such as cabinets, tool boxes, storage shelves, deep sinks and washing machines, leave the heat out of those areas. This will allow a slightly higher temperature where you actually walk and stand and make you more comfortable without adding any more heat to the room. Also keep it out from under any refrigerators.
If you are heating with solar, the heating process will be slower and the delivery temp should be kept as low as possible. With solar, you are not trying to heat the floor, you are trying to cool the collectors. This leads to the best efficiency. You also want the floor to be as close to its useable temp as possible to give the maximum storage of energy and the minimum extra energy to bring it to the final temp. It's very unlikely you'll want to run the radiant water through the collectors and they should drain by gravity, back to a holding tank, whenever the system is not actually collecting.
A several day response time is ridiculous. You should feel heat in no more than one hour and be comfortable gradually after that. Remember, you are not trying to satisfy the wall thermostat, you are trying to make the room comfortable for you to be in. These are two completely different things and you should not look at the thermostat to decide if the room is warm. The setback feature becomes a very useful timer which can give the floor a shot of heat before you arrive and it can limit the heating to prevent either overheating, energy use or heating on days you don't need to. A very useful device. Also, by using a setback thermostat you tend to even out the heat, whereas a simple thermostat will tend to make the room swing up and down in temp.
Use a nice manifold system, like a Watts, or equivalent. This gives to the ability to accurately balance the flow rates and tune the heat to your liking. If you want more than one thermostat, you can use power heads on the individual loops to simplify the zoning. Run 3/4" PEX (7/8" OD) as main supply lines to the manifold. Do all of you air venting back at the water heater and set the operating pressure at about 15 PSI with a 30 PSI relief valve. Add a flushing tee and shutoff so you can force the air out and flush the system with city water when you start it up. Keep the system as a "closed loop" and use oxygen barrier tubing. This allows for cast iron pumps, cast iron boilers, flanges and steel expansion tanks, etc. Typically, systems are filled from a regulator tied to a domestic supply, but that is not necessary. I, for instance, have an expansion tank up in the attic area and filled it from there by simply pouring water in. It operates at about 2 PSI simply from gravity. This eliminates the need for any air vents at all and the expansion tank is a simpler design.
My primary heat source is solar for my 1800 sq ft radiant home and 1,000 sq ft garage/shop. I have 600 gallons of stored solar water. My backup source is heating oil (diesel). Last year a I used about 38 gallons of oil for the entire year of heating. My water heater is electric and stays turned off about 9 months of the year. We supplement the heat in the house, as needed, with a wood stove.
Have fun enjoy the wonderful heat.
