Understanding Common Moisture Problems
Three case studies illustrate the importance of controlling humidity and air leakage.
Synopsis: An energy specialist details three case studies, each in a house with different heating, humidity, ventilation, or insulation problems. The studies offer general guidelines on steps that builders can take to make houses comfortable and trouble-free for their owners.
About a dozen years ago, a couple called me to complain about serious water stains on the kitchen ceiling of their new home. The builder and the architect were at each other’s throats: The builder blamed the stains on the polyethylene vapor retarder the architect had insisted he install in the ceiling, and the architect disagreed but had no alternative explanation. The confused homeowners hoped that I could offer some help.
It took less than a minute to identify the source of the water causing the stains. Six recessed lights punched holes in the ceiling, and they acted like little chimneys, transferring moist kitchen air into the attic, where the vapor condensed on the cold roof sheathing and dripped back down to the ceiling drywall.
Once everybody could see the evidence, they understood what was occurring and could agree that the solution was to seal the recessed lights. What I learned that day was how much myth and dogma exist in the design and construction professions about simple, common building failures that have straightforward physical explanations.
In the three case studies that follow, I describe the nature of the problems encountered, the diagnostic methods and tools used to determine the causes, and the recommended fixes. All of these homes are in the Northeast, but the construction practices that caused the failures are common throughout the country. What varies is the type of problem that results. In the end, most residential failures are caused by uncontrolled movement of air and/or moisture, whether the building is in Mississippi or Minnesota.
First, find out exactly what’s wrong
Occasionally, I may be able to diagnose a straightforward problem over the phone. But if I visit the house, first I get a thorough description of the phenomena. I want to understand what is happening, where in the house it occurs, in which seasons and how long it has been going on. This last point is important; many problems are the result of a chain reaction that follows a change in a building, like a new kitchen, furnace or windows. Any of these things might alter the humidity level or create new pathways for air.
The next step is a thorough walk-through. The homeowner may have called me because of something obvious like severe condensation on windows. But a trouble shooter might find other failures, indicating a wider problem. So my rule is to start the examination at the footing and end at the ridge.
Case #1: Frost, water stains and ice dams
In the first of the three case studies, the builder of a 3-year-old house called me because he had been unable to solve wintertime problems of severe frost buildup in the attic, stains on the second-floor ceiling below and recurrent ice dams.
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