Using window fans the wrong way can make a room warmer. Learn how to get the most out of low-cost and natural methods for cooling your home.
The cooling technology that has brought us air conditioning (AC) has arguably brought much needed comfort to the sick and e
lderly of our society. But has the nearly universal use of AC left us with a void in knowledge of how to cool our homes with low-cost fans even when nighttime temperatures drop low enough?
Some homeowners will leave their windows open during the day even when the outside air temperature is warmer than the inside air temperature. Even at night, a window fan will often draw in air that is warmer than interior air…until just before sunrise when the coolest outdoor temps are reached.
Strategy for Second-Floor Bedrooms
Close windows in the morning before leaving for work. Keeping windows closed throughout the day helps keep warmer air out while conserving cooler interior air. Most glass helps block ultraviolet rays that can also contribute to unwanted heating. You should also close curtains or shades to significantly reduce radiant heating, and updating to more reflective window treatments will be even more effective. Install window fans in the upper sash of each bedroom window. Set the fans so they exhaust to outside (many fans allow either in or out). Note that this may not work as well with casement or awning windows. At bedtime (or as soon as the exterior air temperature is less than the interior air temperature), open your first-floor windows on the north side and east side where air temps are cooler. Turn on fans and leave bedroom doors open.
These steps allow the coolest possible air to travel through the first floor thus cooling the surrounding surfaces (such as plaster, floors, and cabinets). This cooler air is then drawn upstairs, which creates air movement in the bedroom. In addition, warmer air tends to rise and will eventually get blown outside with the help of the exhaust fan. Since surfaces are cooled on both floors, they become a source for keeping the home relatively cool while the house is closed up during the day.
Advanced Approach: The Whole-House Cooling Fan (WHCF)
The WHCF is an advanced method of using a low energy consumption fan to cool a home much more effectively than with window fans. Like the window fan method, cool air is brought in from the lower-level windows at night before then traveling upstairs. However, instead of exhausting out of a bedroom window, the air is exhausted through the upper-floor ceiling and into the attic. This forces hot air out of the attic (see picture on previous page). The WHCF thus offers a double benefit: cooling the home’s interior living space plus cooling the attic. Generally it’s best to locate the fan in the south or west portion of the home to help exhaust the excess heat and bring in cool air. Keep in mind, however, noise from a WHCF may be louder than a window fan since they are generally more powerful. It is crucial to get a fan that is insulated and air sealed so that in the winter it does not allow excessive heat and moisture into the attic. Otherwise, you will have a higher energy bill and possibly moisture damage in the attic. Finally, these fans should never be operated with doors or windows closed due to the risk of combustion appliances backdrafting and causing carbon monoxide poisoning.
Bob Alf is a St. Paul based construction coach and sustainable remodeler found at bobalfconstruction.com.
Bob Alf Construction
St. Paul, Minnesota