Draughts at a window are noticeable, which is why they get attention. The leaks that actually move the most air are usually invisible and out of reach.
The stack effect
Warm air rises. In a house, that means warm air pushes upward and escapes through gaps in the ceiling plane, which draws cold air in low down to replace it — through crawl space gaps, around rim joists and at the base of walls.
The house is effectively acting as a chimney. This is why the ceiling and floor planes matter more than the walls, and why a house can feel draughty at floor level while the actual leaks are above your head.
Where the leaks are
Recessed light fittings. Older ones are effectively holes in the ceiling. A significant leak, repeated across a house.
The attic hatch. Frequently an uninsulated, unsealed panel in an otherwise sealed ceiling. Cheap to address and disproportionately effective.
Plumbing and wiring penetrations. Every pipe and cable passing through the ceiling plane has a gap around it.
Top plates. Where interior walls meet the ceiling, there is often a gap running the length of the wall into the attic.
Chimney chases. Frequently large openings, and they need appropriate fire-rated materials rather than ordinary sealant.
Rim joists. Where the floor framing meets the foundation. A major leak in the floor plane and accessible from the crawl space.
Ductwork. Leaking ducts in an unconditioned crawl space lose conditioned air directly into a space you are not trying to heat.
Why it matters more in Washington
Beyond energy, there is a moisture dimension specific to our climate.
Air leaking from the house into the attic carries moisture with it. When that warm humid air meets a cold roof deck, it condenses — producing staining, damp insulation and eventually damage to the structure.
Air sealing is therefore a moisture control measure as well as an efficiency one, and in a marine climate that matters.
The ice dam connection
East of the Cascades, air leakage into the attic is a direct cause of ice dams. That escaping warm air heats the roof deck, melts the snow above it, and the meltwater refreezes at the cold eave.
Homeowners who try to solve ice dams with roofing are treating a symptom. Air sealing is a substantial part of the actual fix.
Can you go too far?
A legitimate question. A very tight house needs deliberate ventilation, because moisture and indoor pollutants have to go somewhere.
In practice, most existing Washington homes are nowhere near that threshold and sealing improves them. But if you are doing comprehensive work, ventilation should be part of the conversation rather than an afterthought — that is why mechanical ventilation is standard in new construction.
Doing it
Sealing around penetrations and the attic hatch is within reach of many homeowners. Fire-rated detailing around chimneys and flues, and anything involving recessed lights that cannot safely be covered, is worth doing properly.
A professional assessment, sometimes using a blower door test to measure actual leakage, tells you where the air is going rather than where you guess it is. That targeting is worth the cost on a larger house.