Humidity Control
Why humidity matters as much as temperature, and how equipment adds or removes moisture.
Concept
Apprentice
#building science#iaq#comfort#humidityWhat it is
Comfort depends on both temperature and moisture. An air conditioner removes moisture as a side effect of cooling coil surfaces below the dew point. In heating season, tight buildings can hold too much moisture or, in cold dry climates, too little.
Why it matters
- High indoor humidity feels clammy, supports mould growth and dust mites, and makes occupants lower the thermostat.
- Low indoor humidity causes static, dry skin and shrinking woodwork.
- Oversized cooling equipment cools quickly and shuts off before it has removed enough moisture, which is the most common cause of a cool but clammy house.
- Moisture that condenses inside walls or on cold surfaces causes building damage well beyond comfort complaints.
Control strategies
| Strategy | How it works | Watch out for |
|---|---|---|
| Correct equipment sizing | Longer run times remove more moisture | Requires an honest load calculation, not a rule of thumb |
| Lower indoor fan speed in cooling | Colder coil removes more moisture | Too low risks coil freezing |
| Dedicated dehumidifier | Removes moisture independent of cooling demand | Needs drainage and adds heat to the space |
| Humidifier in heating season | Adds moisture to dry winter air | Over humidifying causes window condensation and hidden moisture damage |
| Ventilation with an ERV | Manages moisture exchange with outdoor air | Must be balanced and commissioned |
Measure before recommending
Log indoor temperature and relative humidity over several days before proposing equipment. A single spot reading rarely captures the pattern the occupant is describing.
Symptoms
Practice & Mastery
- Questions1
- Humidity Control Troubleshooting Scenarios2
Practice is part of HVAC Pro — $5/month or $50/year.
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