Window, doors and exterior foam

Q: What’s the best way to deal with window openings when adding foam insulation to the outside of a building? I’m planning to add 2 inches of rigid foam to the outside of my cottage before adding vinyl siding. Is it best to move the windows and doors outwards so they’re flush with the siding, then adjust the interior trim?

A: That's a great question and one that's rarely dealt with properly in print. As you suggest, I strongly recommend relocating (or replacing) windows so they're flush with the outside surface of the walls after exterior foam has been added. And the reason has to do with keeping water out. As many homeowners discover all too late, it's quite difficult to create a waterproof joint when the newly insulated wall surface extends several inches beyond existing windows. And the last thing you want is water leaking in between the foam you’ve added and the old exterior wall surface. You're far better off moving the windows outward, then installing an extended window jamb on the interior surface. One way or the other you'll need to extend the window and door jambs. You might as well do it indoors, where the only thing that matters is aesthetics, not keeping water out.