
Insulate and air seal.
Your AC can provide either optimal efficiency or optimal dehumidification, but it can’t usually provide both. Air conditioners, designed for energy-efficiency, may not run cold enough to condense sufficient water vapor out of the indoor air. The air conditioner may fail to dehumidify a building for these reasons.
- The air conditioner is oversized and doesn’t run long enough.
- Thermostats control air conditioners by temperature, not Rh so if the temperature is satisfied, no dehumidification.
- Fish tanks, plants, cooking, and showering create too much humidity for the AC to remove.
- Energy-efficient AC evaporator runs too warm, or the airflow is too high, or both.
- Humid air leaks through the building enclosure, introducing too much water vapor for the AC to condense.
- Mild outdoor temperature during spring, fall, and at night doesn’t activate the AC thermostat.
- Air sealing, insulation, exemplary windows, and shading combine to make indoor temperature and sensible-cooling load too low to activate the AC during humid weather.
Try these potential solutions.

Install a Dry Well
- Do whatever possible to improve drainage and fix water leaks.
- Pay for air-leakage testing, air sealing, and insulation when possible.
- Ask your HVAC professional to adjust air-conditioner controls to run longer with a colder evaporator coil.
- Consider a dehumidifier if you live in a humid climate and your indoor relative humidity reaches 60% or more.
For more information on low-cost cooling see Cool Buildings for Hot Weather