The Windfall of Wind: Passive Air Circulation

Natural ventilation has become an increasingly attractive technique for reducing energy use, and thereby cost, while still maintaining a healthy, comfortable, and productive indoor climate. Most historic buildings were ventilated naturally, although many of them have been compromised by the addition of partition walls and mechanical systems. In favorable climates, natural ventilation can reduce total energy consumption by 10%-30% by expelling built-up heat.

Natural ventilation systems rely on pressure differences to move fresh air through buildings. Pressure differences can be caused by wind or the buoyancy effect created by temperature differences or differences in humidity. In either case, the amount of ventilation will depend critically on the size and placement of openings in the building. In this situation, it would be prudent to consult an experienced architect or builder, such as Native, who is well-versed in passive ventilation techniques.

Think of passive air circulation systems as circuits, with equal consideration given to supply and exhaust. Openings between rooms such as transom windows, louvers, and grills are techniques that complete the natural air-flow circuit. However, modern code requirements regarding smoke and fire transfer present challenges to the designer of a natural ventilation system. For example, stairways in historic buildings often sensibly doubled as an exhaust stack, a technique now barred by code requirements in many cases. In fact, America is the only nation that does not require some form of natural ventilation in most buildings, such as operable windows, resulting in edifices that perform poorly in terms of occupant comfort and are often detrimental to our health. This is especially important since, according to the Environmental Protection Agency (EPA), Americans spend on average 90% of their time indoors.

3 Basic Things to Keep in Mind

  1. Hot air always, always rises. This drives most passive techniques. Place air-exhausts up high with a corresponding air-supply near the ground. Make sure the supply-air opening is located in a shaded area to maximize cool-air intake.
  2. Cross ventilate. This requires a building plan design that isn’t simply the typical boxed, amalgamation of rooms. An “open plan” works best or one with corridors arranged longitudinally to the direction of prevailing breezes.
  3. Air temperature is not a good indicator of comfort. Human beings are incredibly sensitive to minute movements of the air around them. A comfortable temperature with no air movement or ventilation is perceived as “stuffy” and can cause odors and allergens to build up. Meanwhile, too much air movement could be considered “drafty” and lower the perceived air temperature below the threshold of comfort.

It is crucial to remember that the effectiveness of natural ventilation systems is determined by the homeowner’s behavior and regulation of ventilation systems such as windows and exhausts. When combined with the use of fans and proper exterior shading such as extended overhangs, the energy savings are noteworthy. Money and eco-friendliness aside, sometimes it is just nice to be able to enjoy beautiful weather from within our homes. The simple enjoyment of a cool spring breeze is one of those small pleasures of life too often overlooked in favor of all-encompassing, mechanical, air-distribution systems.