Q: What causes a wet spot to appear each winter in one location on my ceiling near an outside wall? You can see it in the photo I sent. This never happens in summer, no matter how much it rains, but it appears every winter, even when it has not rained or melted recently. The problem seems to get worse when the condensation on our windows is heavy when it’s very cold outside.
A: Your issue is caused by the surface of the ceiling in that area getting cold enough to cause condensation of water from the air inside your home. This is a common problem in many modern houses with shallow-slope roofs. There’s very little space between the surface of the roof and the ceiling out there at the edge of the house, so there’s often not enough room for sufficient insulation, or insulation has not been installed there because it’s difficult to do. Problems like this are usually made worse if overall humidity in your home is too high in winter, as suggested by your heavy window condensation. The same dynamic that causes your windows to get wet on the inside when it’s cold is happening on your ceiling.
The higher the indoor humidity levels, the more likely it is for condensation to appear that way it does at your place. If you have a heat recovery ventilator (HRV) run it more and at a higher speed to lower indoor humidity levels to 50% or less. If you don’t have an HRV, run exhaust fans longer and more often, and leave a few windows an inch or two open while monitoring humidity levels as a test. Many modern homes are tight enough that they lead to poor indoor air quality unless mechanical ventilation and exhaust fans or open windows are used more often and for longer.
The idea here is to see if lower humidity levels will solve your ceiling problem. It may (probably will), while also solving excess window condensation problems. Boosting ventilation will certainly make a big difference to the issue, but if it doesn’t solve the problem completely let me know and we can look at options for adding insulation to that troublesome spot above your ceiling. Reducing indoor humidity during winter is the first step to a permanent solution.
Watch the video below to learn more about how cold weather triggers window condensation (and condensation elsewhere, too). If windows had greater insulation values, wintertime window condensation would not happen.