Tuesday, July 29, 2008

Home Economics 101 - Envelopes

When food was a greater proportion of household income, and much of it's preparation performed in the home, we were taught to conserve and efficiently use it. Today, energy is the new food, and a modern home economics should address our use of that commodity. The major energy expenditure for most of us is in heating and cooling, and a major determinant of that is how tightly sealed the barrier surrounding the conditioned space we live in is: the so-called building envelope.

There is a simple but little known or appreciated way to measure this precisely: the blower door test. It operates on a very elementary principle: just as we inflate a punctured tire and submerge it in soapy water to find the leaks, the blower door test pressurizes - actually more often de-pressurizes - our homes so we can discover the sources and extent of infiltration. Some air exchange is essential of course, or we would suffocate. Often expressed in ACH - 'air changes per hour', a healthy house should have a natural rate of about 0.35 - i.e. all new outdoor air in a little under every three hours.

It's called a blower door because it is an air-tight, adjustable frame that fits inside a doorway, open only where a powerful fan is mounted to suck air in, or blow air out of the house. Older, more primitive (uncalibrated) systems only allowed you to locate leaks (usually by following puffs from a smoke pencil or just by feeling the air rush on your skin), whereas modern calibrated systems can also quantify the results in a particularly vivid way: as the cumulative area open to the outdoors. The test on my recently purchased, 2200 square foot house yielded 370 square inches, or the equivalent of leaving a 30" sash window open more than 12 inches year-round (this resulted in a 0.62 ACH rate).

Blower door tests are administered by energy auditors: usually local, small-time operators who own the equipment and are skilled in their use. The service is doubtless still too expensive, and as it becomes more pervasive, prices will fall. But even today, it remains one of the single best ways to gather information with which to formulate an energy-efficiency improvement plan for your home.


Post a Comment

Subscribe to Post Comments [Atom]

<< Home