EpiPHany Corner | How is a Passive House building like a glider?

The wings of an airplane are designed to provide lift when pushed or pulled through the air at sufficient speed. The wing itself can be considered a “passive” element because it is the design and shape that enables it to produce lift whenever the airplane moves through the air. As long as the airplane maintains sufficient airspeed the wing will deliver lift and the aircraft will fly.

When an airplane in flight loses power, it will need to descend and give up altitude in order to maintain the airspeed necessary to remain in flight. The “glide ratio” determines how far the airplane can glide before reaching the ground. Obviously, the further it can glide, the higher the probability it can reach a place where it can land safely. The pilot’s task is to glide to that place before running out of airspeed, altitude, and ideas.

Different types of airplanes have very different glide ratios. Let’s do a comparison of how far different airplanes can safely glide after losing power at three miles of altitude above the ground (around 16,000 ft. at sea level). A Cessna 172 has a glide ratio of approximately 7 to 1, so would potentially be able to glide to an airstrip within about a 20-mile radius. An F-16 fighter or Boing 767 airliner each have glide ratios of approximately 12 to 1 so would be able to glide in the neighborhood of 30 miles from 16,000 feet or up to 60 miles from 32,000 feet, which is a more common cruising altitude for both types of planes. A glider however, towed to 16,000 feet and released, with a glide ratio of 70 to 1, could be expected to glide over 200 miles in any direction. It is also true, that with updrafts along mount ranges or coastlines, a glider could potentially stay aloft indefinitely. (Note: We use “about”, “around”, and “approximately” in the paragraph, above, because variables like weight and wind can extend or reduce glide range significantly.)

“Resilience” of buildings is similar to glide ratio for aircraft in that it reflects how long a home will maintain a safe temperature after losing power for heating (electricity and gas) during extremely cold weather, not unlike what many Texans experienced during the cold wave last winter. The chart below from Rocky Mountain Institute (https://rmi.org/insight/hours-of-safety-in-cold-weather/) illustrates how long it will take different homes to fall from a comfortable 70 degrees to below 40 degrees, which is considered “severe” indoor temperature for healthy families. A house built in 1950 would drop from 70 degrees to <40 degrees in about 8 hours, a 1980s home would drop <40 degrees in around 23 hours, a 2009 home for in about 45 hours, and a Passive House over 150 hours.

The occupants of a Passive House, similar to the pilot of the glider, would have a lot longer to make decisions and take actions in their respective situations.

Also, if power to run AC was interrupted on an extremely hot day, a Passive House would warm more slowly as outside temperature increased.

As climate change increases weather extremes, the resilience of a Passive House is becoming more and more important.