Building Science
Air Sealing
Also known as airtightness
Closing unintended openings in the building enclosure so air moves where it is designed to and not elsewhere. A separate function from insulation, which slows conducted heat but does little to stop air from moving through an assembly.
In more detail
The leaks that matter are usually few, large, and hidden. Top plates open to an attic, rim joists, plumbing and flue chases, dropped soffits, attic hatches, recessed lights, and the joint between foundation and framing account for most measured leakage; the window-and-door weatherstripping that homeowners think of first accounts for comparatively little. Leakage is also driven from top and bottom rather than evenly, because warm air rises and creates a stack effect that pushes out high and draws in low.
Air leakage carries more than heat. It carries water vapor, and vapor moving through an assembly by air transport moves orders of magnitude more moisture than vapor moving by diffusion — which is why the air barrier does more for moisture durability than the vapor control layer does. It also carries whatever is in the space the air comes from, so a house that draws replacement air through a garage or a damp crawlspace is ventilating itself from the wrong source.
Sealing changes what the house then requires. A tight enclosure no longer supplies accidental outdoor air, so mechanical ventilation becomes necessary rather than optional, and appliances that vent by natural draft become vulnerable to depressurization. Tightening without addressing those two consequences is the failure mode.
Why it matters
This is the cheapest work in the entire build and the most expensive to retrofit. Sealing a top plate is a tube of sealant and access to open framing; reaching the same joint after drywall, insulation and finishes means demolition. The window between framing and insulation is when a blower door can find leaks while they can still be reached, and it closes permanently. Nothing else in a house has that ratio between the cost of doing it now and the cost of doing it later.
The common misunderstanding
That a tight house is an unhealthy house. Tightness determines whether air movement is controlled, not whether there is enough of it. A tight house with designed ventilation has better and more predictable air than a leaky one, because the outdoor air arrives filtered, in known quantity, and from outdoors rather than from a crawlspace. The problem is tightening without ventilating, which is a sequencing failure rather than an argument for leaving the holes.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.