Piping in Exterior Basement Walls

A homeowner in Calgary, doing a basement renovation, ran into a common problem: where to run water pipes when insulating basement walls from the inside. They wanted to keep the washer, dryer, and a shower along an exterior wall, with a wall setup of concrete foundation, 2 inches of rigid foam insulation, a 2x6 wood stud wall packed with mineral wool insulation, and drywall. The city told them pipes must stay on the "warm side" of the insulation, so they asked for advice on where exactly that is in a wall built this way, and whether their planned insulation would be enough to prevent frozen pipes.

An experienced builder answered that because the space is below ground, the risk is already lower than it would be above grade. He suggested placing the pipes toward the inner half of the stud wall, roughly 2 inches back from the interior drywall side. That would put about R24 worth of insulation between the pipe and the cold foundation wall, and only about R8 between the pipe and the warm interior, keeping the pipe closer to room temperature. For extra safety, he suggested compressing or notching the insulation right around the pipes so there's no insulation blocking heat from the room side.

For anyone finishing a basement in a cold climate and running plumbing along an exterior wall, the general idea is the same: keep more insulation between the pipe and the outside, and less between the pipe and the heated living space, so the pipe stays warmer than the wall around it.