by James Dulley
Engineering Consultant

Tips to keep the a/c cost down without being ‘terribly uncomfortable’

Q: I want to keep my air-conditioning costs as low as possible without being terribly uncomfortable. What can I do to improve natural ventilation in my house while reducing heat through windows and doors?

A: Air conditioning can account for a significant proportion of a family’s utility bills even in some so-called “cool” regions of the country. Not every region has below-zero temperatures during winter, but most areas easily reaches above 90 F for many days during the summer.

I am glad you mentioned not being “terribly uncomfortable.” No matter how much heat you can block from your windows and doors and how effective the ventilation is in a house without air conditioning, an air-conditioned house with a reasonable humidity level will be more comfortable. With proper ventilation, I have to air-condition very little to stay reasonable comfortable during hot, humid Cincinnati summers.

Direct and indirect heat coming in windows and doors does increase the temperature inside your house significantly. Even the most energy efficient windows and doors have an insulation R-value much less than the typical house wall. There also are gaps around doors and windows which, even though sealed with weatherstripping and caulk, allow in some hot, humid outdoor air.

Since you can’t stop all the energy gain at the windows and doors, you might as well make maximum use of ventilation through them. This ventilation can be cross-ventilation from window to window on the same floor or through-ventilation from a window to an upper vent. The air coming in will be warm and perhaps humid, but it will make you feel cooler as it flows over your skin.

The wind blowing over your house creates higher pressure on the upwind side and lower pressure on the downwind side. Open your windows completely on the downwind side. Open them only halfway or less on the upwind side. This combination may reduce the total air flow somewhat, but it increases the speed of the air coming in. In the summer, the higher speed of the breeze over your skin creates more cooling effect. If you have casement windows, crank them out at just a partial angle to catch more breezes.

For through-ventilation, open windows on the upwind side of your house. This also works reasonably well on a calm day. Lift the attic access opening cover, often in a bedroom closet ceiling, and rotate it over the opening to create gaps. The less-dense warm air will naturally exhaust out the opening. Don’t totally remove the cover because that would allow a direct route down for the radiant heat from the roof into the room. If a room has a vaulted ceiling, consider installing a venting skylight (www.velux.com).

If security is not an issue, open your doors and allow air in through a screen door. For additional security, drill a hole in the floor directly over a joist and just slightly inside the primary door. Slip a short steel rod in the hole so the primary door can be opened only a couple of inches.

If you do not like the appearance of screen doors or cannot afford the expense of installing them, consider installing a retractable screen (www.dreamscreens.com). This is a spring-loaded screen which pulls horizontally across the door to the magnetic strip on the other side. The vertical housing which holds the screen is only a couple of inches wide so it is barely noticeable. The spring holds the screening taut over the door.

Adding awnings is a good method to reduce heat coming in through windows. The proper size of the awning depends upon the latitude angle of your area and its orientation to the sun. An awning contractor should be able to help you determine the proper size.

Installing an awning over a door also helps reduce heat gain in several ways. First, the awning blocks the direct sun’s rays from heating the door or shining in an open screened door. Secondly, the sun’s heat on the outside of an insulated composite door may cause it to bow. When this happens, the weatherstripping may not seal well when you are air-conditioning.

If your windows are exposed to the sun, install SunScreen (www.phifer.com) sun-control window screening. This is often made from strong polymers and has a dense weave to block more than half of the sun’s heat. You can still see through the screening.

If you have Venetian blinds, rotate the slats so they slope to the outdoors from bottom to top. As the air between the window and blind slats gets hot, it tends to flow upward. With the slats rotated properly, this hot air tends to stay between the window and the slats. During the winter, reverse the slat angle so the cold air stays against the window.


James Dulley is a nationally syndicated engineering consultant based in Cincinnati.
If you have a question about energy use or energy-efficient products, send it to: James Dulley, Electric Consumer, 6906 Royalgreen Drive, Cincinnati, OH 45244; or visit www.dulley.com.