The Stack Effect

Stopping attic air leakage minimizes energy loss

The Stack Effect is a core principle of Building Science, which focuses on how heat, air and moisture flow in buildings. Understanding the Stack Effect will also help you understand how air-sealing the attic can make a big difference in your home's energy performance.

In most houses, more exfiltration (interior air leaking outside) occurs through the attic than anywhere else. This isn't surprising when you consider how many leakage points there are into the attic (see below) and how warm air always wants to rise. The warmest air in the house will always be at the top of the living space. When this warm interior air leaks out through cracks, gaps and openings near the top of the house, a similar volume of outside air needs to leak in (infiltration) to avoid causing a vacuum. Exfiltration through the attic combined with infiltration through lower parts of the house is called the Stack Effect. It's easy to understand how the Stack Effect contributes to poor energy performance. Interior air that you've just paid to heat or cool leaks out of the house before you get its full benefit, and a similar volume of unconditioned air leaks into the house, making your furnace or air conditioner work harder to maintain comfortable interior temperatures.

When an energy technician air-seals an attic, the stack effect is minimized. But this sealing work is extensive and challenging. Every hole cut in the ceiling for a recessed light provides leakage points that are tricky to seal. Holes drilled through the top plates of walls for electric wires and plumbing pipes also require sealing. There are plenty of other gaps as well, and it takes an experienced energy technician to find and seal these leaks.

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