The Fundamentals of Natural Ventilation

Oct. 1, 2008

Consider adding naturally ventilated buildings to green your portfolio

At one time, all buildings were naturally ventilated, and many surviving examples provide hints about what strategies are effective in certain climates. Old buildings had articulated plans and courtyards to provide windows in each room; some buildings had more complicated and ingenious features.

For example, ancient Persian architecture included wind scoops called "malqafs" that would even route air over a water fountain to cool it. The knowledge of how to design these systems was purely empirical, and it was lost as designers came to rely exclusively on mechanical ventilation after the electric fan was invented.

The green-building movement has renewed interest in natural ventilation as a means of reducing energy use and cost while maintaining healthy and comfortable indoor conditions. Structures in which strict temperature and humidity limits aren't expected are good candidates for natural ventilation, including bus stations, picnic shelters, restrooms, agricultural barns, warehouses, swimming pools, casual dining areas, and maintenance shops. Even in mechanically cooled buildings, there are some spaces, such as hallways, atria, and lobbies, that can benefit.

Wind-Induced Ventilation
A natural-ventilation system must complete a circuit through the space by use of open plans, transom windows, or louvers, but code requirements related to smoke and fire must be observed. Open area for exhaust usually equals open area for intake. You should minimize obstructions to airflow, and offset openings to avoid stagnant areas.

In warm conditions, wind-induced ventilation supplies as much fresh air as possible; in winter, ventilation is reduced to levels sufficient to remove excess moisture and pollutants. You can maximize wind-induced ventilation by siting the ridge of a building perpendicular to the summer winds.

Buoyancy Ventilation
Ventilation induced by buoyancy due to indoor/outdoor temperature difference (stack ventilation) is effective in cold conditions, but not in warm conditions, because it requires that the indoors be warmer than outdoors. A chimney can increase the required height and can be heated by solar energy to drive the stack effect without increasing room temperature—very common in composting toilets.

Water vapor is lighter than dry air, and the equation for airflow induced by humidity is similar to that for temperature. Buoyancy induced by humidity (cool tower ventilation) is only effective in locations where outdoor humidity is very low, such as in the Southwest. Provide ridge vents at the highest point in the roof as an outlet for both buoyancy and wind-induced ventilation. The total airflow resulting from the combined effects of wind, temperature, and humidity is calculated in a root-square fashion vs. simply adding the airflow rates due to each. Handbook methods are very useful in calculating airflow for simple building geometries. To predict the details, numerical computational fluid dynamics (CFD) computer models can be used.

An expression for airflow induced by wind:

Qwind = K x A x V

Qwind = volume of airflow (m³/h)

A = area of smaller intake or exhaust opening (m²)

V = outdoor wind speed (m/h)

K = coefficient of effectiveness, which ranges from 0.4 (for wind hitting an opening at a 45-degree angle) to 0.8 (for wind hitting at a 90-degree angle)

An expression for the airflow induced by buoyancy (stack effect):

Qstack = Cd*A*[2gh(Ti-To)/Ti]^1/2

Qstack = ventilation rate (m³/s)

Cd = 0.65, a discharge coefficient

A = free area of inlet opening (m²), which equals area of outlet opening

g =9.8 (m/s²), acceleration due to gravity

h = vertical distance between inlet and outlet midpoints (m)

Ti = temperature of indoor air (K)

To = temperature of outdoor air (K)


Andy Walker is a senior engineer with the Golden, CO-based National Renewable Energy Laboratory's Energy Management and Federal Markets Group in the Strategic Energy Analysis and Applications Center.

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