Humidity, airflow and mold in a tropical house

Mold needs three things: moisture, something to grow on and time. A house cannot do without wood, fabric, leather and dust, and an empty house has plenty of time. Moisture is the only one of the three that can be controlled, so that is where the work goes.

The numbers

The United States Environmental Protection Agency advises keeping indoor relative humidity below 60 percent, and ideally between 30 and 50. A study of closed-up seasonal homes by the Florida Solar Energy Center, in another hot and humid climate, treated 70 percent as the level above which mold becomes likely when it is sustained.

Those figures are worth holding against the air outside. In the green season on this coast the outdoor air is often more humid than that, day and night. A simple humidity meter in a bedroom and another on the terrace will show the difference, and they cost very little.

Why open windows are not enough

Ventilation swaps indoor air for outdoor air. When the outdoor air is itself above 70 percent humidity, no amount of it will bring a room down to 60. Moving air still helps, because still corners stay damp longest: behind wardrobes, inside closed cupboards, under beds. But a ceiling fan moves air. It does not take water out of it.

What works in a closed house

The Florida study tested the usual strategies in empty houses. Leaving a house to itself did not reliably keep humidity down. Setting the air conditioner to a high temperature did little, because the unit hardly ran. A dehumidifier on a timer, with a fan to circulate the air, controlled humidity in every house tested: a few hours a day in a small, well-sealed house, and far more in a large, leaky one.

Two practical points follow. An air conditioner only removes moisture while it is running, and a unit too large for its room cools the air quickly and switches off before it has dried it. And a dehumidifier in an empty house has to drain through a hose to a floor drain or outside, because a full bucket switches it off.

Small habits do the rest. Wardrobe and cupboard doors are left open in a closed house. Furniture stands a little off the walls. Linen is stored dry, never straight from a humid laundry room, and nothing is kept in cardboard.

Find the water first

Humidity in the air is one source of moisture. Leaks are the other, and no dehumidifier keeps up with one. The usual suspects are roof leaks, plumbing, bathrooms without an extractor and the condensate drains of air conditioners, which the EPA specifically says to keep clean and flowing. Its rule of thumb is worth memorising: materials dried within 24 to 48 hours of getting wet will, in most cases, not grow mold.

Cleaning it properly

The EPA's guidance for homes is that a patch smaller than about ten square feet, roughly one square metre, can usually be handled without a specialist. Hard surfaces are scrubbed with detergent and water and dried completely. Porous materials such as ceiling tiles and carpet may have to be thrown away, because mold grows into them.

Three more points from the same guidance. Bleach and other biocides are not recommended as a routine part of cleanup. Moldy surfaces should not be painted or sealed over: clean and dry them first. And whoever does the work should wear an N-95 respirator, gloves and goggles.

Bigger areas, or mold that returns after cleaning, mean there is a water problem that has not been found yet. That is the thing to fix.

Mold can trigger allergic reactions and asthma. Anyone with symptoms should speak to a doctor, and large or recurring growth should be assessed by a specialist.