Air Sealing vs. Insulation: What Comes First?
Insulation slows heat transfer. Air sealing stops air movement. They sound similar — but they solve different problems, and the order matters.
The sweater analogy
Insulation is your sweater. Air sealing is your windbreaker. A thick sweater on a windy day still leaves you cold. The same is true for your attic — fiberglass without air sealing lets warm air blow right through it.
Always seal first, then insulate
Air sealing the attic plane (around penetrations, top plates, recessed lights, the attic hatch) before adding insulation makes the new insulation work the way it's supposed to. Skipping this step is the #1 reason homeowners are disappointed with insulation results.
What about basements and walls?
Same rule. Rim joists should be air-sealed (often with closed-cell spray foam) before adding fiberglass batts. Wall cavities benefit most from dense-pack insulation, which provides air-sealing and insulation in one step.
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