What type of heating system?

Hydronic (water, often around 180F) is lot lower than steam (212F).

some insulation on the radiator pipe might be worth it.

Drains are also nearly zero pressure, so it is not as critical as supply lines

It is steam heat. The plumber insulated the radiator pipe pretty good but I still get a little nervous about the whole thing.

I would insulate the steam line to within about 1/2 inch of the PVC, then put some sheet metal in the gap touching the insulation but not the water pipe.

It might be overkill, but it would take a decent amount of analysis optimize this (and there are probably so many unknowns a precise answer is not going to happen, like how tightly sealed the cavity is).