Why air sealing matters

When people think about insulation they usually think about R-value. But there's another part of the building envelope that gets overlooked constantly, air sealing. Insulation slows down heat flow. Air sealing stops unwanted air movement through the building. Those are two different jobs, and a house needs both of them. Air can move through gaps around plumbing, electrical wiring, top plates, windows, doors, ductwork, and other penetrations. In an attic, there can be dozens of little openings between the conditioned space and the attic. Individually they might not look like much. Collectively, they can make a pretty big difference.

The reason air sealing is so freaking important is that air doesn't care about averages. A small and I mean tiny leak can let a lot of air through. Case in point, crack a window open just a little bit in the middle of the winter. That's why we recommend air sealing before adding insulation, especially in attics. There's not much point in burying a bunch of holes under a foot of insulation and pretending they don't exist. A properly air-sealed house keeps conditioned air where you paid to put it and helps the insulation perform as intended. If you've ever seen condensation around a bottom plate, that's because it hasn't been air sealed

Insulation slows heat. Air sealing stops the leaks. You need both.