Exterior Insulation Window Detail

This one's a forum discussion, not really news, but it touches on a detail that matters if you're ever adding rigid foam insulation to the outside of your house: what happens at the windows.

A homeowner wrapping his timber-frame house in two layers of 2-inch polyiso foam board (a rigid insulation panel) ran into a common problem. He'd left a gap below his windows, unsealed, so that any water getting past the window could drain out. But pressing the foam right up against the window would block that escape route. Other forum members pointed out something more basic first: there was no sill pan visible in his photos. A sill pan is a waterproof liner installed under a window before it's set in place, meant to catch any water that leaks in and direct it back outside. Codes and standard practice call for one.

The suggested fix was to swap a strip of mineral wool board (a water-permeable insulation product, sold under names like Rockwool Comfortboard) in place of the foam near the window sill. Because mineral wool lets water drain through it rather than blocking it, it preserves an escape path for moisture while still insulating that area. The recommendation also included running flashing tape over the foam below and tying it into the tape coming down from the sill pan, so any water has a defined route out.

For a homeowner, the takeaway is narrow: if you're adding exterior insulation to your own house, the window sill area needs special handling so water can still get out.