The thermal stack effect is busy robbing heat from your home every winter. And it may be sucking nasty soil gasses such as radon into your home, which creates health risks for you and your family. The thermal stack effect is driven by a basic law of physics - hot air rises. If you have ever seen a hot air balloon floating in the sky you have seen the physical evidence of this. The balloon pilot uses a torch to heat the air just below the opening at the bottom of the balloon. The hot air rises and is trapped within the balloon. Since the warmer air within the balloon is less dense than surrounding ambient air, it causes the balloon to rise up into the sky.
Now think about your house on a winter night when the outdoor temperature is very cold. While a house is not a hot air balloon, it is a warm-air-filled container surrounded by much colder outdoor air. And your furnace or boiler is working very hard to keep you and your family comfortable by keeping the air within this container, your home, at 65 to 70 degrees. The warm air within the house rises up through the building to the ceiling where it gets pushed out into the unheated attic through any cracks and holes in the ceiling plane. House ceilings are full of cracks and holes around electrical boxes, attic access doors, plumbing vent pipes and wires inside partitions, etc.
Along with pushing warm air out of the house at the ceiling plane, the stack effect sucks cold outdoor air into the house through cracks and holes in lower parts of the building. Much of this cold air infiltration occurs where the wood part of the house rests on the concrete foundation walls.
Air leakage driven by the thermal stack effect can account for 40% of the annual heating costs of a typical home.
This air leakage is also responsible for several other negative impacts on the building, including:
• Leakage of warm air into unheated attics driven by the thermal stack effect is the primary cause of ice buildup and ice damming at roof edges
• Air flowing through most insulation products degrades the manufacturer’s stated R-value –the ability to resist heat flow -- by as much as 60%. Most insulation products do not block the flow of air. That means insulating an attic will have little effect on reducing heat loss from your home unless air leaks are first found and sealed.
Can the stack effect be controlled?
We cannot stop hot air from rising. But it is possible to reduce the warm air from quickly escaping out of the upper parts of the house and cold air from being sucked into lower parts. In existing homes, this is done by finding and sealing the many small holes and cracks. This is most important to do at the ceiling plane and at lower levels of the house. Gypsum board or plaster finish on ceilings and walls is relatively impervious to the passage of air. But a typical house has dozens of holes, cracks, and breaks in the wall and ceiling surfaces. Cracks around electrical boxes, light fixtures, access doors into attics, and openings around plumbing vent pipes, as in this attic picture, provide just a few examples.
At lower levels of a house cold air gets drawn in at cracks where wood parts of the house rest on relatively uneven concrete foundation walls. Air is also drawn in at lower levels where holes have been drilled in the lower wall or floor structure to allow pipes and wires to be installed.
You can take some steps on your own that will slightly reduce air leakage related to the thermal stack effect and lower annual heating costs. However, special diagnostic equipment used in a professional energy assessment is needed to significantly mitigate the wasteful stack effect.
Unfortunately, many home builders and home improvement contractors do not know about the thermal stack effect and the negative effect it has on your home. Choose a contractor who can describe what the thermal stack effect is and how it affects heating costs, indoor air quality and roof ice buildup. Learn more about working with contractors.
Contractors accredited by the Building Performance Institute (BPI) have demonstrated that they have the necessary knowledge, qualifications and diagnostic equipment. To obtain at a list of BPI-accredited contractors in your area, select your county from the map on visit NYSERDA’s website.
Adapted from article by Mark Pierce, Cornell University in Housing and Home Environment News, Fall 2010
Last updated July 19, 2018