A friend is building a house with radiant floor heating, and is considering placing ceiling mounted zwave temperature sensors. From my experience with HVAC, a sensor in the ceiling always gives a much higher temperature than at body height, but he says that it would be different with floor heating.
Any insights?
Well heat rises so it will be higher at ceiling height but all you do is add a -ve offest to the sensor reading and to be honest it’s all relative anyway it’s about what’s comfortable 
Are these sensors being used to actually control the infloor radiant heat or just general space temp monitoring? Is the infloor heat electric or hydronic (hot water)? Most electric systems will have a floor sensor for control.
How tall are the ceilings?
I think this is a bad idea if being used to enable/disable the heat. Obviously heat rises, so you will have that issue, but you could also introduce a lag or delay into the control loop. The system would be slower to satisfy space temp, and then because the hot air would collect at the ceiling it will stay warmer up there longer and delay the system from coming back on. Also why try to heat the entire volume of the room if you only care about the occupant zone?? Taller ceilings will make this even worse.
I’m a mechanical PE and design mostly commercial HVAC systems. We never spec ceiling mounted sensors, but I’m usually dealing with building codes and ADA so we are pretty limited in where a tstat or sensor would go anyways.
He was considering using the for controlling the for, which I was skeptic about. It will be hydronic. The ceiling is about 3m tall.
The delay turning back on is indeed a very good point!