Why Air Leaks Are So Important To A/C Users

Image obtained with thanks from Rachel Kramer via Flickr.

It may be winter in the Northern Hemisphere now, but not in the southern hemisphere, where the seasons are the opposite. You may wonder why people caulk their windows and seal their houses/rooms tightly while their air conditioners are in operation. Could a little crack in the window really be a big deal? Unfortunately, gaps and cracks can have a notable impact on your electric bill. This isn’t only because warm air might blow in through the cracks.

Image obtained with thanks from Rachel Kramer via Flickr.
Image obtained with thanks from Rachel Kramer via Flickr.

It is also caused by two other phenomenon: Pressure gradient force and convective heat transfer. Pressure gradient force is a phenomenon whereby a pressure difference across an area causes fluids (in this case, air), to move to areas of lower pressure. This is why pressurized gases tend to burst out of their containers atmosphere. The pressure of a filled propane gas cylinder, for example, is higher than atmospheric pressure, therefore it will be forced out into the atmosphere if opened.

This tendency to equalize pressures is exactly what happens if you open your window while the air conditioner is on. Cooling the air inside a room lowers its pressure (a little), resulting in a weak vacuum which sucks warm air from outside into the room. This little vacuum is strong enough to force warm air through that seemingly innocuous window gap you were wondering about.

Now for convective heat transfer — This is a phenomenon whereby temperature differences cause fluids to mix. You are probably most familiar with the fact that the coldest air falls and the warmest air rises. This isn’t strictly an up and down phenomenon. In contrast, if you are not using A/C, you’re better off leaving your windows open, otherwise the building will trap the heat entering through the windows (the glass, not the opening) and walls. Wall insulation will help to reduce heat gain through the walls, and triple-paned windows are great insulators as well.

For more tips on staying cool, visit the Heating And Cooling page.