I'm about to have my basement finished. The house is radiant heated by pex in gypcrete under the floors. The entire basement has fiberglass stuffed between the ceiling joists, I assume for reasons of efficiency. Should I drywall right over the fiberglass or remove it before drywalling the ceiling? Would there be any problems with leaving it in place?

I would leave it in place... I would also insulate all that plumbing

The insulation is necessary for zone isolation, to keep the warm gypcrete floor above from also overheating the basement.

Insulating the foundation walls to current IRC code min would also be a good idea- more important (but also more expensive) in energy use terms than insulating the heating system plumbing.

The basement is not heated. I'm guessing I'll leave the fiberglass in place. Thanks. ~Rolan

The basement is not heated directly, but you can bet it's heated by the exposed plumbing and the standby losses of that oversized beast of a cast-iron boiler. If you insulate the foundation those distribution losses still accrue to the house rather than being fully lost. If you haven't air sealed & insulated the foundation it's probably accounting for a double-digit percentage of the fuel bill, even though you're not directly heating that space.

Yeah that is heated for sure. Our old cast iron boiler, sera blanket or not, puts out so much. Trick is utilizing it!

rolan if you put some polyiso foam sheets on that foundation wall, even in lieu of insulating plumbing, you'll save quite a lot in the long term, as Dana suggested. It'll also provide a vapor barrier and dry out the air some down there and make it feel like a much more enjoyable space. In most places the polyiso is rated to be left sans sheetrock as well, though you should check with your locality.