"House is pretty well insulated" indicates an existing house, so probably an over pour on a wood subfloor?
"Zone 3.5" gets pretty cold a times.
I would recommend R20 under subfloor. Bond-breaker plastic on top.
1/2" O2 barrier PEX-A on 8" centers (to minimize hot/cold bands), so 1,800' in 6 loops (300' max per loop) with Sioux Chief copper manifolds with 3/4" pex feeds, one manifold of each pair needs to have the 1/2" pex ball valves to allow air to be pushed out by water, 1 loop at a time).
Clamp pex down with 1 screw clamps as it will try to float up.
3x thermostats controlling 3x 3/4" zone valves, 1 Grundfos Alpha pump moving all the water.
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