Thailand's Humidity Problem: What Every Building Owner Should Know

Thailand's average outdoor relative humidity sits between 74–85% year-round, and coastal areas like Koh Tao and Phuket can reach 90% or higher during the rainy season. Even with air conditioning, indoor humidity often remains higher than ideal.

The Ideal Range

The WHO and ASHRAE recommend keeping indoor relative humidity between 40–60%. Below 40%, respiratory irritation and static electricity become issues. Above 60%, biological contaminants begin to grow.

What Happens Above 60% Humidity

Between 60–70% humidity, mold begins growing on surfaces within 24–48 hours if temperature is also warm. Between 70–80%, dust mites reproduce rapidly and become a major allergen source. Above 80%, building materials begin to degrade, paint peels, wood warps, and electronic equipment is at risk.

How Air Conditioning Helps and Its Limits

When your AC cools air, it also dehumidifies — moisture condenses on the evaporator coil and drains away. The problem is that many Thai buildings run their ACs set too cold at 18–20°C. The AC reaches target temperature quickly and switches off, but does not run long enough to remove sufficient humidity. The result is a room that feels cold and damp. Setting your AC to 24–26°C means the unit runs longer cycles, removes more humidity, and uses less energy overall.

For Coastal Properties

Salt-laden humid air corrodes metals faster and infiltrates buildings more aggressively. Koh Tao and coastal Surat Thani properties need more frequent AC coil inspections, marine-grade equipment where possible, and ERV or HRV ventilation to manage fresh air intake without bringing in uncontrolled humidity.

Signs of a Humidity Problem

Watch for a musty smell even with the AC running, visible mold on walls or ceilings, condensation on windows, warping wooden furniture or floors, and frequent respiratory issues for occupants.