Chances are, if you are like the majority of folks, stack effect is something that has never crossed your mind. If it has, perhaps it was in the context of the Hasbro game, Jenga, where you carefully remove wooden pieces from a stack and place them on top until it all comes crashing down. Well, in this case, it is something altogether different.
Stack effect is the movement of air into and out of buildings. It is caused by a temperature difference between inside and outside air which creates a pressure variation. This pressure difference promotes the movement of air from the high to the low pressure area. For example, during heating season, the air inside your home is warmer than the air outside. Since warm air rises, as it moves up through your home, it reduces the air pressure inside your home and pulls in the colder outside air to replace it. This is stack effect.
In the cooling season, the reverse conditions and air flow exist. The cooler air inside your home settles and flows outside, drawing in the warmer air through infiltration points at the roof. The Southern tradition of opening the lower window sash on the cool side of the house in the morning and lowering the upper sash on the warm side is a perfect example of stack effect being managed by the home’s occupants.
But, you ask, won’t my efforts at sealing my doors, windows and other openings prevent this from happening? Proper weather stripping and sealing will greatly reduce stack effect and will reduce your utility bills. According to sources used in Wikipedia, about one third of HVAC energy consumption comes from infiltration. Which is why we always encourage our cooperative members to diligently seal as many openings in the exterior of their homes as possible. However, unless your home was built to the newest construction standards, you cannot completely stop stack effect. There are simply too many points in your home where air can infiltrate to seal them all.
The truth is that you don’t want your home to become too airtight. Indoor air quality (IAQ) will suffer when this happens. By the very act of living in our homes, we create elements inside that need to be filtered out or removed. Smoke from cooking, dust from carpets, off-gassing of furniture and other building components all contribute their share. Plus, Mother Nature has some house guests of her own, radon and mold to name two, which need to be exhausted to the outside or otherwise mitigated.
For proper IAQ, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) recommends .6 air changes per hour. This means that your home should have sixty percent of the inside air volume of the house exchanged with fresher outside air each hour. Generally speaking, the air exchange will occur through mechanical ventilation intakes in the heating, ventilation and air conditioning system (HVAC). This allows your filters to remove a lot of outdoor dust and pollen. Opening and closing doors and windows is another major source of air changes. And then there is our new friend, stack effect.
Stack effect seems to be a double edged sword. On one hand it increases HVAC energy bills and on the other contributes to a healthier IAQ. Properly balanced and sealed homes will take care of these issues, providing you with the lowest possible energy bill and highest possible IAQ. Why not check with your HVAC contractor today and see how they can help analyze your home and recommend steps that will allow you to achieve this balance?