Bob and Ann woke up at night to water dripping from the ceiling over their bed. They had recently installed a 50 year roof on their home in northwest Washington. Was the flashing installed poorly? Did the roofers forget to install the shingles? Was it a plumbing leak?
Bob and Ann (not their real names) pointed out the spots that were leaking: one in each bedroom, and one near the front door. There were visible water stains at each location, but they said the dripping only happened when it was cold and windy outside, not when it was rainy. So, scratch the roof leak. And scratch the plumbing, which would have been a constant source of water (besides, there was no plumbing in the attic, which is good, because that is a terrible place to put plumbing). Interestingly, the house was so air tight that closing the front door caused the attic hatch to lift, until Bob created a super tight fitting hatch. They heated with a gas fireplace and electric wall heaters, and had noticed some mold on the walls in previous years.
Water in buildings comes in two forms, vapor and liquid. Warm air can hold more water vapor than cold air. If you cool air enough it will condense leaving liquid water on an adjacent surface. Air that has a lot of of water vapor will condense on warmer surfaces than air with a small amount of water vapor.
Single pane windows are perfect examples: warm air with a large amount of water vapor from inside the house hits the cold glass, condenses, and turns to liquid water which drips down the pane. Condensation can be prevented by either heating up the glass or reducing water vapor in the air. To heat up the glass, raise the temperature of the house or install better windows. To reduce water vapor, use a dehumidifier, a bath fan, a heat recovery ventilator, or stop the source of the water (a vapor barrier on the ground, fix the plumbing leak, etc).
In this case, the water on the ceiling occurred only near the exterior walls, and only in certain areas. Inspecting the attic revealed that these areas were directly below poorly installed low venting baffles that compressed the insulation and left the top plate of the exterior wall exposed. The low venting baffles caused two problems: they reduced insulation levels above the ceiling, and allowed large amounts of cold air to pass over these spots. A double whammy to creating a cold surface. As the cold air roared in through the low venting, it cooled off those spots on the ceiling to the point where they became condensing surfaces.
Bob and Ann’s ceiling was acting just like a single pane window.
My suggestion was to reinstall the low vent baffles to maximize the amount of insulation between the ceiling and baffle and limit the amount of air entering the attic. In addition, I recommended installing a heat recovery ventilator to reduce indoor humidity levels, add fresh air to the living space, and save money.