Is the water for a radient floor contained in a closed system or is additional water necessary to maintain full efficiency?

typically it's closed, but makeup water is plumbed to it using backflow preventer and a pressure reducing fill valve to make sure that during air elimination, and long term slow rate evaporation do not cause the system to become airbound.

We just built a new house with a walk out basement with floor radiant heat and the upper level has floor radiant heat, however, it is the stapled up. We have 3 zones for the basement and 1 Zone for the garage, 4 zones for the upper level. We keep the basement off most of the time unless we have quests. The basement will remain at 64 or 65 most of the time because of the mechanical room heat. When we turn on the basement heat it will start cycling the boiler to domestic water heating and back to space heating, but never allowing it to reach boiler temp. for space heating, and then start calling for domestic water heating. We have talked to our plumbing contractor and he doesn't know what the cause is but is checking it out.

sounds like you have a zone wired into a domestic demand. Does domestic work the rest of the time?

yes the domestic water works except when it gets around 0 or below outside, then it will start cycling through the domestic water and space heating.