BTU SizerHumidity and Comfort
Ask somebody to describe an uncomfortable room and they will mention temperature. Measure it and the temperature is usually fine. What is wrong is the moisture, and almost every sizing decision in the house affects it.
The numbers
Aim for 45 to 50% relative humidity in living space year round. The practical boundaries:
| Relative humidity | What happens |
|---|---|
| Below 30% | Dry skin and airways, static, timber and joinery shrink |
| 30–40% | Comfortable, common in winter with heating on |
| 45–50% | The target |
| 55–60% | Starts to feel close; dust mites become happy |
| Above 60% | Mould risk, musty smell, sleep quality drops |
At 78 °F and 45% a room is pleasant. At 78 °F and 65% the same room feels stuffy and the instinct is to turn the thermostat down — which treats the wrong variable and costs money.
Why an air conditioner is also a dehumidifier
Cooling and drying happen together. Warm humid air passes over a coil below its dew point, water condenses out onto the coil and drains away, and the air leaves both cooler and drier. That second half only happens while the compressor is running and the coil is wet.
Which is why an oversized air conditioner leaves a room damp: it reaches the setpoint in a few minutes, shuts off, and the coil dries before much water has run off it. Long, steady cycles dry the air; short ones do not. This is the single most common cause of a house that is cold and clammy, and it is a sizing problem rather than an equipment fault. It is covered in full in why an oversized AC is worse.
When you need a dehumidifier instead
An air conditioner only dehumidifies while it is cooling. That leaves three situations where a dehumidifier is the right machine:
- A basement. Cool and damp at the same time — the air conditioner is not running, because the temperature is fine.
- Spring and autumn. Humid without being hot, so cooling never calls for and the moisture stays.
- A room with a moisture source. Drying laundry, an aquarium, a bathroom without adequate extraction.
The winter problem is the opposite
Heating air does not remove water, but it drops the relative humidity because warm air can hold more. That is why houses get dry in winter: the same absolute moisture in warmer air reads 25% instead of 50%. Below 30% is where static, dry airways and shrinking timber start, and a humidifier sized to the space is the answer — not a higher thermostat setting, which makes it worse.
Ventilation is the other half
Moisture generated indoors — showers, cooking, laundry, people breathing — has to leave. A bathroom fan run for twenty minutes after a shower and a range hood that actually vents outside do more for a house’s humidity than any amount of dehumidifier capacity, because they remove the moisture at the source rather than collecting it afterwards.
A cheap hygrometer in the room that bothers you is the best five dollars in this whole subject. Without one, every conversation about comfort is guesswork.