Humidity Control

Indoor air humidity levels need to be controlled in order to maintain thermal comfort, minimize air quality problems, and prevent structural damages.

Thermal comfort

Air humidity affects occupant thermal comfort. Typically in a house, 35%-50% relative humidity is considered the comfortable range. In summer, with a higher humidity, you will feel hotter since your sweat cannot evaporate from your body. Conversely, in winter, with lower humidity, you will feel cooler and dry since water is readily evaporating from your skin.¹³

Air quality problems

In hot environment, high humidity can lead to mold and mildew growth, causing indoor air quality problems and significant health concerns.⁵

Structural damages

High indoor air humidity can likely result in condensation problems on windows, walls, and other cold surfaces in your home, which in turn may cause structural damage. High humidity can also attract bugs, evaporate volatile compounds in paint, and can also damage appliance and electrical wiring. Conversely, a low humidity can lead to shrinkage of wood products, cracking of paint, and unwanted static electrical discharge. To avoid these issues, a relative humidity between than 25-60% is desirable.⁵

Humidity Measurement

Air humidity is typically measured as relative humidity or humidity ratio. Relative humidity is the percentage of moisture in the air compared to the maximum amount of moisture that the air could hold at a specific temperature. The humidity ratio indicates a mass ratio of water to dry air (lb H₂O/lb dry air). A 100% relative humidity is the most moisture the air can hold before water vapor condenses, and a 0% relative humidity means dry air and there is no moisture in the air. The warmer the air is, the more moisture the air can hold.

Measurement Equipment

Instruments that are able to measure humidity are known as hygrometers. They use a variety of measurement methods, though most sensors used in residential applications are electronic capacitive sensors. Electronic capacitive sensors are fairly cheap, with a cost ranging from $10-$100.⁹

When selecting a hygrometer for your home, you will want to consider:

Accuracy – how accurate do you need the sensor to be?
Price – how much are you willing to spend?
Application intent – what action do you want the sensor to take if the humidity is outside of your desired range, i.e. do you want an alarm?
Design – size, placement, etc.

A discussion of some hygrometers used in home’s can be found in “8 Best Digital Hygrometers Suitable for Your Home” by nwclimate.org. The review provides valuable selection criteria information, which you can use for comparing all hygrometers (not just the 8 discussed in the article).

Standards

There are different standards for human thermal comfort zones in terms of humidity and temperature set points. The most commonly adopted standards are provided by ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers). Please refer to ASHRAE Standards 55—Thermal Environmental Conditions for Human Occupancy or chapter 9 of ASHRAE Fundamentals for specific air temperature and humidity set points in different seasons.¹³

Dehumidification Methods

Dehumidification is the process in which you remove moisture from the air. Dehumidification tends to be the most energy consuming process in maintaining desirable humidity levels.

The energy efficiency of dehumidification equipment is measured by "energy factor”, which is defined as the liters of water removed per kWh energy consumed by the equipment. A higher energy factor implies a greater efficiency. According to ENERGY STAR®, performance of dehumidifiers typically ranges from 1.2 L/kWh (worst) to 3.6 L/kWh (best).¹¹

Below you'll find some common techniques and equipment you may use to dehumidify your home.

Electrical Dehumidification Systems

Air conditioners and electric heat pumps dehumidify the air by cooling it to a temperature at which the water condenses onto a cooling coil.²

Desiccant Dehumidification

A desiccant is a chemical (liquid or solid) that absorbs moisture out of the air. In the desiccant dehumidification systems, air passes through a porous wheel containing solid desiccant, or through a shower of liquid desiccant, and moisture in the air is removed by the desiccant. This dehumidification technique is less common, but is a more energy efficient. Rather than using electricity, this process uses heat. Once the desiccant is completely saturated, it requires dehydration/heat to remove the absorbed water. Water evaporated in the desiccant wheel is typical exhausted to the outside air.²

Humidification Methods

Humidification is normally needed in winter time. Adding moisture to the air tends to be much easier than removing the moisture. It requires significantly less energy because it does not require cooling and heating as dehumidification does. Below you'll find some commonly used methods for humidification in your home.⁴

Water Evaporation

One simple way to add moisture to the air is to boil water, hang wet towels near a heater duct, or place water on the radiator in your home.

Steam Vaporizer

A humidifier that boils water and releases the warm steam into a room. These are the least expensive humidifiers.

Impeller humidifier

This type of humidifiers use a rotating disc that flings water at a diffuser. The diffuser breaks the water into droplets that evaporate into the air. These droplets can be seen as a cool fog.

Ultrasonic humidifier

A common humidifier, the ultrasonic humidifier uses a metal diaphragm that vibrates an ultrasonic frequency (hence the name), to create water droplets. This process is quiet and produces a cool fog.

Wick/Evaporative System

A humidifier that uses a wicking cloth, paper, or foam to draw water from a reservoir. A fan then blows over this wicking surface, and the water evaporates into the air. As the relative humidity of the room increases, the water will evaporate slower.