Stopping mold coming back.

Buy a cheap hygrometer, put it in the room that had the problem, and aim to keep relative humidity under about 50 percent. That single number is what decides whether it returns, because the spores and the food are already in the building permanently and moisture is the only one of the three requirements you control.

Everything else on this page is a way of moving that number.

Why humidity rather than cleaning

Mold needs three things at once: moisture, something organic to live on, and time. Your house supplies the second one generously and always will, in drywall paper, wood, dust, and the paper backing on a cabinet. Spores are in the air of every building on earth.

So the only lever is water, and the reason cleaning alone fails is that it removes the growth without changing the conditions that produced it.

Where the water is coming from

Work through these before buying equipment, because a dehumidifier fighting a leak is an expensive way to lose.

A leak. Fix it. Any leak, however slow. If you cannot find it, work from when it appears.

Ground water. A wet basement wall or a slab that stays damp is a drainage problem outside, and the fixes are downspouts, gutters, and grading rather than anything indoors. The outside work that actually matters.

Everyday living. Showering, cooking, drying laundry indoors, and simply breathing put a surprising amount of water into the air of a closed house. This is the usual cause in bathrooms and bedrooms and it needs ventilation, not a repair.

Cold surfaces. Condensation forms where warm moist air meets a cold surface, which is why growth appears in the corner of an exterior wall, behind a dresser pushed against one, and on a window reveal. Insulation and air movement fix that; cleaning does not.

Unvented appliances. A clothes dryer that is not vented outside, or a bathroom fan discharging into a roof space rather than through the wall.

The things that move the number

Ventilation, and running it for long enough. Exhaust fans in bathrooms and kitchens, run during and for twenty to thirty minutes after, with the door shut. Bathrooms in particular.

A dehumidifier, in a basement or a persistently damp room, with a hygrometer next to it so you know it is achieving something. Empty it or plumb it to a drain.

Air movement. Pull furniture a couple of inches off exterior walls. Leave closet doors ajar in a room that gets it. Still air against a cold surface is the exact condition to avoid.

Heat, evenly. A room kept much colder than the rest of the house is the room that gets condensation. Unheated spare rooms with the door shut are a classic.

Do not dry laundry indoors without ventilation, and vent the dryer outside.

Cover pans when cooking, and run the kitchen exhaust fan.

Insulate cold spots where you can. It is the permanent version of the fix.

Keep gutters and downspouts working, so the outside of the wall is not soaking.

Aim for a range, not a floor

Under about 50 percent is the target in most guidance, and 30 to 50 is a reasonable band. Going much below that is not a bonus: very dry indoor air is uncomfortable, and in cold weather it is difficult and expensive to achieve.

In summer, in a humid climate, air conditioning does most of the dehumidification for you. In winter the risk shifts from humidity in the air to condensation on cold surfaces, which is a temperature problem more than a moisture one.

What does not work

• Mold resistant paint over a colony. It works on a clean, dry surface and it is a lid on anything else.
• Sprays that claim to prevent regrowth while the moisture stays.
• Ozone generators. Not effective for this, and ozone is a respiratory irritant.
• Sealing a room up. Less ventilation is the opposite of the fix.
• Cleaning on a schedule without changing the conditions. If you are cleaning the same patch every spring, the patch is not the problem.

Worth doing once, then forgetting

• Put a hygrometer in the worst room and one in a normal room, and compare.
• Test each exhaust fan with a square of paper.
• Check where the dryer vents to.
• Check the attic for damp patches on the underside of the roof, which is where a misdirected fan shows up.
• Look behind the furniture against exterior walls, once a season.
• After any water event, take the drying seriously rather than assuming. The window is 24 to 48 hours.

The short version

Keep it under about 50 percent relative humidity and everything else follows. Fix leaks, run exhaust fans for twenty minutes after, vent the dryer outside, pull furniture off cold walls, and heat rooms evenly. A hygrometer is worth more than any product sold for this.