The attic is where several apparently unrelated problems originate — ice dams in Spokane, condensation and mold in Seattle, and heating costs everywhere.
What you are looking for
Insulation depth and coverage. Is it adequate, is it even, and are there gaps? Insulation pushed aside during previous work and never replaced is extremely common, particularly around anything an electrician or plumber has worked on.
Compressed insulation. Performs far below its rating. Anything that has been walked on, stored on, or squashed under boarding is doing less than you think.
Blocked soffit vents. The most common single fault. Insulation pushed into the eaves blocks the airflow the ventilation system depends on. Baffles exist to prevent this and are frequently absent.
Exhaust terminating in the attic. Bathroom and kitchen fans should discharge outside the building. A duct that stops in the attic, or one that has come loose from its outlet, pumps warm humid air into the space continuously.
Staining and condensation. Dark staining on the underside of the roof deck, frost on nails in winter, or damp insulation all indicate a moisture problem.
Daylight. Visible daylight where there should not be any means a hole, and where there is a hole there is water.
Why ventilation matters
Attic ventilation exists to keep the roof deck close to outdoor temperature and to carry away moisture that gets in.
It works as a system: cold air enters low at the soffits, warms slightly, rises and exhausts at the ridge or upper roof vents. Blocking the intake breaks the whole thing, which is why blocked soffits matter more than most homeowners realise.
What it prevents east of the Cascades
Ice dams. A properly ventilated attic keeps the roof deck cold, and a cold deck does not melt the snow sitting on it. Combined with air sealing and adequate insulation, this is the actual fix for a problem most people try to solve with roofing.
What it prevents west of the Cascades
Condensation and mold. Warm moist air from the house reaching a cold roof deck condenses there, and in a marine climate that moisture does not readily dry.
The result is staining on the underside of the deck, degraded insulation and eventually damage to the structure — developing quietly over years.
Air sealing comes first
The most important point and the most frequently skipped. Adding insulation on top of a leaky ceiling is treating the symptom.
Warm air gets into the attic through gaps around light fittings, plumbing penetrations, the attic hatch, chimney chases and top plates. Sealing those is more effective than adding depth, and doing it first means the insulation you add actually performs.
Recessed lights and the hatch
Two specific offenders. Older recessed fittings are effectively holes in the ceiling, and some cannot safely be covered with insulation — which needs the right fitting rather than a workaround.
The attic hatch is frequently an uninsulated, unsealed panel in an otherwise insulated ceiling. It is cheap to address and disproportionately effective.
Doing it safely
Walk only on joists or proper boarding. Wear a mask, since old insulation is unpleasant and older homes may contain materials you should not disturb.
If your home is old enough that the insulation might contain something hazardous, do not disturb it — get it assessed. That is a specific concern in Washington’s pre-war housing stock.