“It’s not the heat, it’s the humidity.”
“Sure it’s hot, but it’s a dry heat.”
These and other bits of common knowledge tell us that the amount of moisture in the air does matter – and it matters a great deal when it comes to comfort. Humidity is an important part of the air conditioning picture, although most do not understand it.
What is it really?
In the simplest form, humidity is a measure of the amount of water vapor in the air. As a percentage, this is called the absolute humidity.1 Water vapor in air is a gas, so the absolute humidity is treated the same as any other component of the air. While the percentages vary, a normal atmospheric mix2 has 21% oxygen, 78% nitrogen, and 1% other gases, with water vapor adding a variable 1 to 4%.
The percentage is variable because temperature and pressure change and water moves in and out of the gas mix. When it moves out, we see clouds, rain, dew or snow. In the interior of our houses, water also moves in and out of the gaseous state. We rarely notice this because water is easily absorbed into the carpets, wood and fabrics throughout our home. We may see it as condensation on cold surfaces, or notice wood swelling or shrinking in the home as moisture is absorbed and released. Air, both inside and outside, is only capable of holding a certain amount of water vapor before condensation occurs.
Relative humidity is a percentage figure that relates how much moisture is in the air against how much it would take to saturate air at that temperature3. In a sauna, where water vapor is visible, the air has absorbed as much water as possible. The humidity is 100%. Hot air is capable of holding more water than cool air. As air is cooled, the same amount of dissolved water will result in a higher relative humidity.
How does it affect our comfort level?
Our bodies have a natural cooling system: sweat. As mammals, we sweat most of the time, usually in amounts that are not noticed. We also breathe out moist air. Both of these mechanisms put about 25 milliliters per hour4 of water into the air (more if we exercise).
When humidity is high, above about 45% relative humidity5, we begin to feel hotter than what the temperature registers. This happens because our sweat is not evaporating as quickly. Most people will not notice this until the relative humidity exceeds 60%, and heating and cooling engineers try to keep the relative humidity in a building between 25 and 60 percent6. (A relative humidity beneath 25% feels uncomfortably dry.)
Do I need a dehumidifier?
An air conditioner will remove excessive humidity from the air, but they are not designed primarily with this function in mind. Air conditioners are set to a specific temperature and most users will simply turn the temperature setting down if they feel too warm7.
There is a balance here. Since a lower relative humidity will feel cooler, it is sometimes wise to reduce moisture in the air, rather than cool the air directly. Ideally, this will allow a higher setting on the expensive air conditioner – but it comes at a cost.
A dehumidifier is very similar to an air conditioner, except it doesn’t exchange air with the outside. It removes moisture from the air by chilling whatever air moves through the unit, collecting this in a reservoir. Some have drain hookups, but others need to be emptied periodically.
The balance is between how much less the AC has to work (higher temperature setting) and how much it costs to operate the dehumidifier. Calculating the correct size is based on how large an area the unit serves and how moist the air usually is8. Capacity is measured in pints per 24 hours.
Signs you may need a dehumidifier9:
• Unable to sleep because of damp sheets
• Mold growth
• Condensation on water pipes that drips or puddles (some condensation is normal)
• Musty smells
• Water spots on walls
• Clothes that do not dry
Others may need a dehumidifier to combat allergies or for a medical condition. Higher humidity air has less oxygen and can make it harder for those with lung damage to breathe10. Asthmatics also benefit from keeping humidity under control. (You might want to check out acboy.org’s air purifier pages here if you have air quality issues or suffer from allergies.)
Other ways to reduce excessive humidity
Besides a dehumidifier, you can reduce the moisture content of indoor air with a few simple tricks11.
• Vent clothes driers outside
• Use exhaust vents in the bathroom and kitchen
• Cover cooking pots
• Use a moisture barrier beneath house and ground in crawl spaces
• Close off basements
• Close off kitchen and bathrooms from living areas
• Look for plumbing leaks and repair them
• Run kitchen exhaust vents for dishwashing (machines and manual)
• Reduce number of indoor plants
You can purchase a hygrometer (the instrument that measures humidity) where home supplies are sold, or anywhere you might find an indoor thermometer. They range in price from $8 to $60 and can be calibrated at home12.