The best window treatment for building new is the windows themselves. They should have a U Factor of .3 or less and you may consider a low-e coating on the inside to help reflect the heat back inside.
While I recommend that people use heavy drapes in colder climates that is not because of normal heat or energy savings, but rather comfort which leads to those savings. For example if you can feel drafts coming in or your body giving up that heat you will feel cold and you will turn up the heater.
One other item to recall for Radiant Floor Heating is that unlike a forced air system which is trying to heat the entire room -- you are only heating from the floor up to about 6 feet.
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