Vapour barrier failure

Q: We had our sunroom painted last summer, yet mysteriously the wall paint is now peeling in an area 12 inches from the floor. I cut an inspection hole in the drywall and find that it's wet in there. Can you help solve the mystery?

A: Your problem is more common that most people realize. Somewhere near your patio door there's a flaw in the insulation, vapour barrier or both. That's not surprising since it takes quite a bit of diligence to avoid the problem during construction. Warm, moist indoor air has been seeping into the wall cavity during the winter, then cooling and condensing. This is probably the source of the water. Strip the area back and allow it to dry completely. Resealing the vapour barrier involves applying beads of acoustic caulking to seal strips of new polyethylene to the existing vapour barrier, and perhaps the door frame. You can also seal the gap between the drywall and interior air using spray-on foam. Choose the low-expansion type to avoid warping the door frame.