What Is The Stack Effect?

If you have been reading about home energy efficiency, you have probably come across the term stack effect.  That term has been widely used in articles on various aspects of energy efficiency, often without a basic explanation of what the stack effect is.

The stack effect is one of the three major drivers of airflow inside your home.  The other two are wind and mechanical systems.

An easy way to understand the stack effect is to start with the fact that, anytime air is heated, it rises.  Inside your home, when air is warmed it expands, becomes buoyant, and rises to the top.  At the top it builds up pressure.  The pressure at the top forces air to leak out of the house and into the attic.  The air is forced through holes such as recessed lighting, whole-house fans, attic access, top plates and many more.

As air is forced out the top of our home, more air rises and creates a vacuum at the bottom.  The vacuum at the bottom pulls outside air into the house through holes such as the rim joist.  The air pulled in at the bottom of the house may be hot, cold, humid and/or dirty.  In winter, the cold air pulled inside has to be heated.  As the cold air is heated, it rises and is then forced out at the top of the house, into the attic.  This whole process is continuous and can waste large amounts of energy and money.

The stack effect is a type of convective loop.  Air is constantly being pulled in at the bottom of the house and forced out at the top.  A certain amount of this airflow is good.  We call it ventilation.  Too much of this airflow can be bad.  Excess airflow of this type is a waste of energy.

The stack effect is working in your home.  Is the stack effect so great that it is causing you spend too much money on heating and cooling your home?  How can you find out?

The answer to these questions is:  have an energy audit.  An energy auditor will use a diagnostic tool called a blower door to find out.  If the stack effect is causing you to waste money and energy, the auditor will prescribe air sealing at the top and bottom of your home to reduce the stack effect.  Once air sealing is complete, everything will stack up.