A leaky house exchanges air constantly without anyone arranging it. That wastes energy, but it does move moisture and stale air out. As homes get sealed and built tighter, that accidental ventilation disappears and has to be replaced deliberately.
Why it matters in Washington
Households produce a lot of moisture — showering, cooking, laundry, breathing. In a marine climate the outside air is frequently humid too, so there is limited natural drying capacity.
Moisture that cannot escape raises indoor humidity, condenses on cold surfaces and supports mold growth. This is why the tightening of homes and the mold conversation are connected, and why ventilation deserves attention alongside sealing.
The simplest layer: spot ventilation
Bathroom and kitchen extraction, removing moisture at source where it is generated. This is the foundation and it is frequently the weak point.
Three things to confirm on any bathroom fan: adequately sized for the room, ducted to terminate outside the building rather than in an attic or soffit, and a short direct run. A fan ducted into an attic causes two problems — it does not remove the moisture and it deposits it against cold roof structure.
Then use it. Running the fan during a shower and afterwards does more than any whole-house system will if the source ventilation is not working.
Whole-house options
Exhaust-only. A continuously or intermittently running fan removing air, with replacement air entering through leakage or dedicated inlets. Simple and inexpensive.
Supply ventilation. Bringing filtered outside air in deliberately, sometimes integrated with a forced-air system. Allows filtration of incoming air, which matters during smoke season.
Balanced with heat recovery. Systems that exhaust stale air and supply fresh air simultaneously while transferring heat between the two streams. More expensive and more efficient, and increasingly common in new Washington construction.
Which suits a given house depends on its construction, existing systems and how tight it actually is. This is a question for an HVAC contractor who does this work rather than a general rule.
The smoke season complication
A genuine Washington tension. Ventilation strategy assumes outside air is worth bringing in, and during wildfire smoke events it is not.
Systems that bring in outside air should have a way to be closed or bypassed during smoke events, and filtration on incoming air is worth specifying where possible. Know where your fresh air intake is and how to close it before smoke season rather than during it.
Signs your house needs more ventilation
Persistent condensation on windows. Musty smells. Mold appearing on cold surfaces — exterior wall corners, behind furniture against outside walls, in closets. Air that feels stuffy. Humidity that stays high regardless of heating.
In Western Washington, check the crawl space before concluding it is a ventilation problem. A damp crawl space raises humidity throughout the house above, and no amount of ventilation upstairs fixes a moisture source below.
Getting the sequence right
If you are air sealing comprehensively, discuss ventilation as part of the same project rather than afterwards. Sealing a house tighter without considering where moisture goes is how efficiency work produces a humidity problem.
For most existing Washington homes, working spot ventilation plus attention to crawl space moisture addresses the majority of the issue. Whole-house systems come into their own in genuinely tight construction.