Cool but Clammy: Air Conditioners, Oversizing and Indoor Humidity

• Air conditioning removes humidity, if it runs long enough
• Keep the unit maintained
• Portable units: skip single-hose
• Target: below 50%

Most of this site is about winter, but summer brings its own damp problem: a house that feels cold yet clammy, with a musty basement. Natural Resources Canada’s Keeping the Heat In (Section 9) explains one common cause.

§Air conditioning removes humidity, if it runs long enough

NRCan notes that air conditioning lowers both air temperature and, “very importantly,” humidity. Oversizing is “clearly detrimental”: an oversized system lowers the temperature too quickly without removing excess humidity. The result is a house that is cool and damp, which can promote mould growth and musty odours. NRCan adds that higher indoor temperatures, for example 26°C (79°F), with lower humidity are more comfortable and save energy.

§Keep the unit maintained

According to NRCan, air conditioners lose efficiency when the indoor coil is dirty, the outdoor condenser’s airways are blocked, or the refrigerant is low. Homeowners can clean or change the filter and keep plants and leaves away from the outdoor unit. A service contractor should handle periodic maintenance.

§Portable units: skip single-hose

NRCan advises avoiding single-hose portable air conditioners. They draw room air and exhaust it outdoors, creating negative pressure that pulls more hot, humid outdoor air into the home, so they run longer and use more power. A dual-hose unit draws its cooling air from outside, so it causes no negative pressure indoors and cools better.

§Target: below 50%

Health Canada’s 2025 guidance for indoor air quality professionals says to keep humidity below 50% in summer, using a dehumidifier or air conditioner if needed. Health Canada’s ventilation guide adds that when windows are closed for air conditioning, bringing in warm, humid outdoor air can push indoor RH above that level. It also says an ERV can transfer some of that outdoor humidity to the outgoing air, so the air conditioner works less. In non-air-conditioned houses, increasing ventilation in summer will usually raise humidity unless something is dehumidifying.

A hygrometer is the easiest check. If the house is cool but reads above 50%, the cooling isn’t drying the air enough. That’s the situation NRCan describes with oversized systems.

Sources: Natural Resources Canada, Keeping The Heat In – Section 9; Health Canada, Guidance for indoor air quality professionals (2025); Health Canada, Ventilation and the indoor environment (2018). All read in full.

Originally published at https://dry-glass-notes.surge.sh/articles/air-conditioner-oversizing-indoor-humidity.html (AI-assisted, sources cited). If this helped, zaps go to hollisvarrow3753@coinos.io.