I heard water running in my attached garage. Turns out that a heating
pipe (hot water heat) runs through the uninsulated garage before it
runs upstairs into the house.

Unfortunately, the pipe is up against the outside wall (on top of
cement block), so I can't surround it by pipe insulation.

Is it okay to stuff fiberglass building insulation up against the
heating pipe? Seems wasteful to let any heat escape into a garage.