Ensuring the Durability of Energy-Efficient Houses
Why are so many of today's homes plagued by rot, mildew and stale air, and what can we do about it?
Synopsis: Newer building techniques put a premium on tight building envelopes, but trapped moisture can lead to rot and mold. This article looks at ways to protect the building from damage with better ventilation and construction details that allow moisture to escape.
The energy-efficient wood-frame houses of the 1980s and ’90s are the most comfortable, the most expensive and very likely the least durable houses ever built in the United States. Over the past 20 years, the frequency of moisture-related problems in new houses has skyrocketed. Frustrated homeowners complain increasingly of window condensation, mold and mildew indoors; of extractive staining and peeling paint outdoors; and of rotting windows, doors, trim, siding, sheathing and framing. All these problems are occurring within a few years of construction.
Historically, moisture-related problems were uncommon; the architects, builders and occupants of wood-frame houses relied—perhaps unwittingly—on the natural replacement of air to control indoor humidity and to keep walls dry. Differences in temperature and pressure across a house’s envelope provided the driving force for moving air through random leaks in its walls, foundation and attic. As a result, warm, moist indoor air was flushed out and replaced with cooler (and usually drier) outdoor air.
Reliance on the natural replacement of air by way of random leakage worked fine for many centuries until the rapid evolution of technology in the 20th century made it possible to build houses that had tighter and tighter envelopes.
Insulation was first added to walls on a large scale in the 1920s. Functioning as a physical barrier that hindered movement of air and conduction of heat through the wall, the insulation often lowered the temperature of the interior side of the sheathing below the dew point during cold weather. Mildew and mold sometimes appeared on the back of the sheathing…