Portland winter damp: what a cleaner can fix and what needs a specialist

From roughly October to April a Portland house is trying to hold moisture. Not because anything is broken, but because that is what a mild wet winter does to a building where people breathe, cook, shower and dry laundry indoors. The question is never whether there is moisture. It is where it lands and how long it stays.

The four places it shows up first

On a recurring visit we check all four without being asked, because they are early and they are cheap to fix while they are early.

• Window sashes, especially single glazing. Condensation runs down the glass and pools in the bottom corner of the frame. On old painted wood that corner is the first thing to go black. At week two it wipes off. At month six it is in the paint.
• The bathroom ceiling above the shower. Usually a diffuse grey bloom rather than spots. Almost always a fan problem rather than a cleaning problem.
• The wall behind a bed pushed against an exterior wall. No air movement, a cold surface and a person exhaling at it for eight hours a night. Pull the bed out four inches and most of it never starts.
• The inside of a north-facing closet. Particularly one on an exterior wall with the door kept shut and clothes packed tight. This is the one people find in March and are genuinely shocked by.

What we can actually do

Surface mould on a non-porous or painted surface, caught early, comes off with detergent, friction and drying. That is genuinely most of it. We do not use chlorine bleach as a first move: on a porous surface bleach's water content can feed what is below while the chlorine bleaches only what is visible, and it produces exactly the kind of indoor air nobody in a sensitised household wants.

We use a detergent, mechanical action, and then we make the surface dry. Drying is the active ingredient. Mould does not need a special product to be defeated, it needs to be unable to reproduce, and moisture is the only thing standing between those two states.

The single most useful thing we do in a Portland bathroom is clean and check the extractor fan cover. A fan choked with lint moves a fraction of its rated air. That is the difference between a bathroom that dries in twenty minutes and one that stays damp until lunchtime, every day, for six months.

Where it stops being a cleaning job

The commonly cited line, from EPA guidance for homeowners, is around ten square feet of contaminated area. Past that, it is remediation rather than cleaning, and it needs containment, negative air and somebody licensed to do it. We will tell you when you have crossed the line and we will give you names.

You are also past a cleaner rather than a remediator when any of these are true.

• It is inside the wall cavity, or the drywall is soft to press.
• It keeps coming back within weeks in the same spot, which means the moisture source is still running.
• There is a smell with no visible growth, which usually means it is behind or under something.
• It followed a leak, a flood or a burst pipe, where what you can see is a fraction of what is there.
• Somebody in the house is immunocompromised, in which case the threshold for calling a professional should be much lower.

The unglamorous prevention list

None of this needs a product. All of it works better than one.

• Run the bathroom fan during the shower and for twenty minutes afterwards. A timer switch is the best forty dollars in a Portland house.
• Do not dry laundry indoors without ventilation. A single load releases a remarkable amount of water into the air.
• Keep furniture a few inches off exterior walls so air can move behind it.
• Open the closet doors on the north side of the house occasionally over winter.
• Use the kitchen extractor when boiling anything, and check it vents outside rather than recirculating.

We put all of this in the notes after a winter visit rather than saving it for an upsell.What a visit covers, room by room, is here.