A new, free software tool from Commerce's National Institute of Standards and Technology (NIST) could prove to be a breath of fresh air for architects and designers of ventilation systems for "green" commercial buildings.

With the Climate Suitability Tool, building design teams can evaluate whether the local climate is suitable for cooling a prospective building with natural ventilation or requires a hybrid system that supplies supplemental cooling capacity. The tool is based on a model of the heat-related characteristics of a building configured to take full advantage of ambient climate conditions and natural air movement. It incorporates an algorithm—or problem-solving procedure—that crunches hourly weather data (downloaded from annual datasets for U.S. localities) and uses standardized criteria for rating the comfort of building occupants.

"We think this tool will be useful during the early stages of design, when decisions on the form of a building and its components are being made," explains NIST mechanical engineer Steven Emmerich. "It provides estimates of ventilation rates for preliminary design calculations. You can approximate how many air changes per hour will be necessary to offset heat gains due to the occupants, equipment and lighting so that comfortable conditions are maintained."

The effects of direct natural ventilation and a nighttime cooling procedure are assessed using a method devised by James Axley, Yale University professor of architecture and engineering. When the outdoor temperature is below an accepted threshold, direct ventilation through open windows and by other means can deliver the cooling to maintain the comfort zone. When the outdoor temperature exceeds the threshold during the day but drops below it after sunset, the cooler nightime air can dilute heat gained during the day and build a reserve of cooling potential for the day to come.  Read NIST's Tech Beat story