1927 house in the Boston area. Just added a small room off the back with a sealed and insulated crawl space underneath (has a window we keep open to that leads into the basement of the house). The addittion (10x16) consists of a laundry room and 1/2 bath. The plumber we hired did superb work. He ran coper tubbing and gas lines out to the new area working in a very challenging space. We also put some baseboard heat out along both sides of the room. For this the plumber ran water through flexible plastic piping (hooks into copper once it gets inside the base board radiator). Note - it's not PVC - but clear, 3/4 - 1" plastic. Although the crawlspace is insulated and will have some warmer basement air coming in - is this standard to use plastic piping? Is there a danger of freezing and cracking?
It's very possible your plumber ran Wirsbo tubing in the crawlspace, because he had no joints to solder and was in a tight area like you mentioned. Wirsbo is just a single name brand that is commonly used for radiant under floor heating. I wouldn't worry as long as you maintain a temp in the crawl above freezing. Best of luck with the new space!