Ventilation

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Everyone knows the importance of fresh air. Just stay cooped up for a few hours in a poorly ventilated room and see how sleepy and generally out of sorts you feel. Engineers design buildings to meet or exceed building code requirements for fresh air. LEED gives an extra point for designs that circulate at least 30% more fresh air than standard approaches to encourage this simple approach to a healthier building.

In many parts of the US, fresh air can also cool buildings for hundreds of hours a year, without using air conditioning. A well-designed and energy-efficient building will make maximum use of the free cooling of outdoor air, so long as humidity is not excessive or heat loss too great (remember that for every cubic foot of fresh air coming into a building, a cubic foot of conditioned air has to leave).

But there's more to the story. Many buildings in windy areas can take advantage of that energy to push air into a building on the windward (upwind) side and cause it to exit on the leeward (downwind) side. Natural ventilation of this type can reduce the fan energy required for moving air inside a building, particularly in a large open-plan office. Restrictions on internal air movement will defeat the purpose of natural ventilation, which requires that engineers and architects make this decision early in the building design process. Because it's so difficult to get just the right wind conditions for natural ventilation, most buildings of this type use a mixed-mode approach, installing fans to help move air in stagnant wind conditions. The figure below shows how natural ventilation might work along with a "stack effect," where heated air rises out the top of a building,

Natural ventilation can complement standard ventilation strategies and reduce fan energy use in larger buildings, as shown fora new engineering building at Portland State University, designed by ZimmerGunsul Frasca Architects.

Natural ventilation can complement standard ventilation strategies and reduce fan energy use in larger buildings, as shown fora new engineering building at Portland State University, designed by ZimmerGunsul Frasca Architects.

with cooler air coming in below. This is a useful approach, since most office buildings require cooling year-round.

In cooler climates ventilation systems need to be coupled with heat-recovery systems, so that heated air leaving the building can give up some of its energy to the incoming air, thus avoiding some of the "energy penalty" of increased ventilation.

Looking out a window at the outdoors is so fundamental to the human experience that we often fail to notice its importance. Some years ago I saw research data that showed that people who sat farthest from the window in office buildings had higher rates of physiological and psychological illness. Think of a modern office building and a "cube farm" stretching from the windows to the interior core. People in the cubicles farthest from the window can't see outdoors without getting up and moving around (which of course most people do, even if they don't know why). In a green building people should be able to see outdoors from 90% of all regularly occupied spaces (this might exclude some interior conference rooms, for example). In recent years architects have designed offices on the perimeter to have

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