ASK STEVE: Peeling paint mystery

The paint beneath our south facing windows is peeling on the inside walls, but I don’t know why. Some of it’s coming off in large, wide pieces with plenty of plaster attached. The paint that’s still firm on the wall seems to be bubbling. This looks like water damage, but I’m not sure how the water is getting in. The house is 50 years old, with brick on the outside and lath and plaster on the inside. I know from renovations I’ve done elsewhere in the house that there’s a 1-inch gap between brick and lath. We had new windows installed last winter and noticed the problem about 6 months after that. The installers didn’t caulk the gap between the new aluminum flashing and the brick. They said it would be very difficult to achieve a clean look and that the 1/4-inch gap wouldn’t allow water in unless we got a hard, driving rain. Are they right?

Though not urgent, your problem is a sign that bad things are going on. From your description, I’m confident that water is to blame. The real question is the source…

  • Seal the gap left by the installers using polyurethane caulking.
  • You should also seal the brick surface itself to ensure that water isn’t coming through by osmosis.
  • Deteriorating mortar joints should be repaired before applying a commercial-grade masonry waterproofing compound.