I’ve talked about the stack effect several times on this blog, but have never written a post dedicated to the subject. Stack effect is present in every home. Older, less air sealed homes (like mine) will usually have a more air exchanges due to stack effect.
So, what is stack effect. During the winter, warmed air inside a home becomes lighter and tends to move higher in the home. The term “hot air rises” is an accurate description. As this heated air rises, it can leave the envelope of the building through any cracks or holes in or near the roof line. As this air escapes the building, a negative pressure is induced within the building which pulls outside air in, typically at the lower areas of the home. Rim joists, sill plates and mud plates are common areas where unheated air enters the home. As this cooler air is heated, it becomes buoyant and the cycle continues. The more difference in temperature between inside and outside, (known as the delta T) the stronger the stack effect.
How do you find the air leaks? The best way to identify where air leaks are in the building envelope is with a blower door test and thermal image scan. These tests will find the high leaks in the roof and low leaks around the foundation. It will also find other air leaks such as around windows and doors. Read more about blower door testing here and here.
Is it better to address the air leaks in the ceiling or lower levels of the home? It depends. Normally I would try to address the upper levels. Air sealing in the attic space is not fun, but many times it makes sense. Adding additional insulation while air sealing the attic is often the determining factor. (Some jurisdictions have code requirements that attics are air sealed prior to any improvements to the insulation.) Sometimes the attic is not accessible, such as a vaulted ceiling. At this point, it may be easier to air seal the rim joist and lower areas of the home, as long as the basement isn’t finished.
How much money can air sealing save? Again, it depends. I don’t like using energy cost savings as the only metric when deciding to “tighten” a home. My home is very leaky and very uncomfortable in both hot and cold weather, but not horribly expensive to heat or cool. For me, comfort trumps energy cost savings. I also like to look at the health of the home. My home has a moisture issue during the winter. The air leaks in the ceiling cause frost to build up in some areas. As the weather warms, this frost melts and water drips through the walls and into the basement. All sorts of bad things can result from water going to places it doesn’t belong.
Air sealing is easiest when the home is being built. I used to contract with a non-profit weatherization organization performing air sealing and home improvements on existing homes. This work was not fun and wasn’t always successful. Many times air sealing was being completed in homes that should have been condemned, or at least completely gutted and remodeled. Limited funding was always an issue. But this type of work is where I learned a lot about where and why homes have comfort, cost and moisture issues.
Want a little more information on stack effect. Here is an interesting post and discussion on the Green Building Advisor’s web site.