continuous exterior insulation over stucco retrofit

A homeowner with a 1993 stucco house in Utah is asking a building-science forum about a wall upgrade: adding two layers of 1.5-inch rockwool insulation (a mineral-wool board that lets moisture pass through) over the existing stucco, then furring strips, then new cement-board siding. The idea is to wrap the house in a continuous layer of insulation on the outside, which cuts down on the heat loss that happens through wall studs and framing.

The questions raised are technical ones that matter for anyone considering this kind of retrofit. One is whether the old stucco itself can serve as the water-resistive barrier (the layer that keeps rain out of the wall), since no new barrier is planned. The other is about moisture: the house already has plastic sheeting behind the drywall on the inside, so adding a vapor-blocking rigid foam on the outside too could trap moisture inside the wall with nowhere to dry out. Using a permeable product like rockwool instead is meant to let the wall dry toward the outside.

For homeowners with older stucco homes, this is a reminder that re-insulating the outside of a wall is not just a matter of picking a thicker board. The existing barriers already in the wall — vapor sheeting, old stucco, tar paper — all affect whether moisture can escape once new layers go on. Anyone looking at a similar project on a stucco house would want to look closely at what is already inside their own walls before choosing materials.