Energy Principles - The Stack Effect

by Tim Gould

Now that winter is fast approaching, I thought it would be helpful to re-visit air leakage in homes. I have been getting many calls from homeowners who understandably are conveying a sense of urgency want to do something as soon as possible to control heating costs.

Air leaks can constitute up to 40% of a homes heating cost and is often most cost-effective place to start when beginning to evaluate and implement energy conservation measures. A home should have an air change rate of 33% per hour under natural conditions according to the American Society of Heating, Refrigerating, Air-Conditioning Engineers (ASHRAE). Most homes, even newer ones, are leakier than this and it should be a goal to achieve the 33% per hour standard.

Understanding a few basic energy principles can be very helpful to homeowners in reducing their own energy costs. One principle known as the "stack effect" can be very useful for homeowners to know to help prioritize energy savings improvements options.

The stack effect, also referred to as the "chimney effect", is the movement of air into and out of buildings and is driven by a difference in indoor-to-outdoor air density resulting from temperature and moisture differences.

The stack effect is driven by temperature: When it is colder outside, the increased pressure of heated air builds at the top of the house, pushing out with greater-than-usual intensity. As air leaves, the laws of physics dictate that it must be replaced. Thus, cold air enters and it does so at the basement level-the lowest pressure position in the house. Cold air from above or below ground level enters into the house. During cold weather, the stack effect works 24/7 and thus is a significant contributor to increased energy costs.

The best place to start sealing air leaks is in the attic. It is often the easiest in terms of access, should be most cost effective, and may make the house feel more comfortable throughout. Foam insulation for larger gaps and caulking are two commonly used sealing materials. After the attic, the basement should be the next place to start sealing. The graphic image below illustrates the many points that can be looked at for sealing air leaks. The good news is you start taking action yourself and you can start now.



© 2008 Tim Gould. All rights reserved.