August 15, 2016

Now that we have tackled the concept of the building envelope, let us turn our attention to a key factor inherent to all buildings: stack effect. Stack effect is a scientific principle that, when coupled with improper, missing or inferior air sealing techniques, leads to one of the greatest causes of increased heating and cooling costs and subsequent health and comfort issues.

What is stack effect?

Let’s use a practical example of the idea before we get into more detail: hot air balloons. Those wonderfully large and relatively silent methods of aviation that either inspire awe or make us think people are crazy. How do they work? The flame at the basket is used to heat up the volume of air inside the balloon. As air warms up, it becomes less dense and begins to rise.

This is the same thing that happens in a house. Warmer air is less dense (fewer air molecules per sample and lighter) and will “rise.” Cooler air is denser (more air molecules per sample and heavier) and will “fall.” This stratification of air will always happen (inside a wall cavity, inside a room, etc.). The exact amount of air movement will vary but it increases exponentially with increased temperature differences between the inside of the house and the outside as well as the height of the building. The questions that remain are why do we care and what do we do about it.

Why should I care about stack effect?

In the winter months the warm air will rise and find its way out of your house, typically, through the attic. For every cubic foot of warm air that leaves the house, a cubic foot of outside air is going to come in somewhere lower in the house – wherever it can. Remember, air is lazy and will take the least restrictive path inside – through the crawlspace below your house, the walls or even the vents of combustion appliances themselves. This air, while possibly bringing undesirable contaminants (are there any that are desirable?), is also not conditioned, meaning your heating or cooling system has to work to bring it to the temperature you prefer.

So, what do I do?

First of all, you can’t stop this from happening, BUT you can reduce the effect. All penetrations found at the low points and high points of the house should be sealed. These include wire penetrations, plumbing penetrations (I bet you have a pretty large hole underneath your bathtub), light fixtures – everything. To get a better handle on the amount of air that is moving in and out of your house you should find a BPI GoldStar Contractor near you to perform a blower door test. Reducing air leakage by controlling stack effect and employing proper air sealing measures has the fastest return on investment of all measures; both in terms of comfort and cold hard cash. Who doesn’t like that? 

Matt Anderson

Matt joined BPI in 2001 and now holds the position of Director of Operations. Matt’s background is in residential, commercial, and industrial construction. He has provided home performance analyses on numerous residential projects.