I'm not assuming he's face now but he does change roles alot though.
Don't worry, your little girlie man bong rips aren't going to set it off. Smoke away.
There's no easy answer to this, because it does depend on 1) the size of the pipe 2) the run 3) the application. Underground temperatures relatively near the surface (as expected in normal construction) rarely exceeds 80 degrees Fahrenheit, and hot water supply temperatures tend to be at least 140 degrees. For long runs with small piping, you can expect the water to cool to ground temperatures, so most of the time this is not a wanted outcome. There are standard insulating wraps for this sort of problem. Polypropylene itself is a fairly good insulator, but most propropylene pipes aren't thick enough to provide adequate insulation in a case like this. Remember, heat conducts through ground much more efficiently than through air.
