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It's Not About the Efficiency

by Don Otto - November 14, 2002

Forget about energy efficiency. You may call me a heretic for saying that, but hang in there for a minute. While the systems approach relates heat, moisture and airflow in all aspects of a home, it's worthwhile to emphasize an even larger triumvirate: ventilation, energy flow, and water management.

First, I believe the most important function of a house is to provide enough fresh air for the people in it. And the only way to provide enough fresh air all the time is with mechanical ventilation.

Leaky envelopes in traditional construction worked so well at providing fresh air that adequate ventilation wasn't even an issue. It was when we started tightening up our homes in response to the high energy prices in the '70s that we realized that we could reduce airflow to levels that made us sick. Data that track 35 years of asthma death rates, from 1960 to 1995, support the relationship (Morbidity and Mortality Weekly Report, MMWR, v 47, No. SS-1, p 27).

Second, if a mechanical ventilation system provides all the fresh air you need, it only makes sense to make the envelope as tight as possible around it. The combination of mechanical ventilation with a tight envelope not only ensures adequate fresh air, but also addresses air leakage, the largest single source of heat loss. And it answers the annoying question once and for all "Can't you build a house too tight?"

A tight envelope provides an additional bonus: it makes insulation work better. The installed performance of air permeable materials, like fiberglass or cellulose, can come closer to their rated R-values. Further, with better control over airflow through the envelope, accumulated moisture, and the mold growth it supports, are much less of a problem.

While a tight envelope and insulation address convective and conductive heat loss, radiant energy is addressed primarily by low-e coated windows. I assume quality builders use the most appropriate windows for the application and climate. In my predominately-heating climate of Iowa, double pane, low-e, gas-filled glazing is standard.

Third and lastly, proper water management is crucial to building durability and occupant health. Water leaking anywhere in the envelope can raise the moisture content high enough (20% by weight) to support mold growth. The higher the moisture, the less time it takes for mold to grow. EEBA has just published The Water Management Guide with extensive and detailed drawings of how to properly flash openings in the building envelope. Written by Dr. Joe Lstiburek, one of the best experts on water management, the guide is available for purchase from the EEBA online bookstore.

Builders who properly address ventilation, energy flow and water management have taken an enormous step to building a high quality home. Molds are no longer an issue, building materials are more durable and the home is far more comfortable. And energy efficiency? It tags along as an inescapable bonus.

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