9/25/2014

Absolute vs. Relative Humidity – What’s the Difference?

Absolute humidity is the measure of water vapor (moisture) in the air, regardless of temperature. It is expressed as grams of moisture per cubic meter of air (g/m3).

The maximum absolute humidity of warm air at 30°C/86°F is approximately 30g of water vapor – 30g/m3. The maximum absolute humidity of cold air at 0°C/32°F is approximately 5g of water vapor – 5g/m3.

Relative humidity also measures water vapor but RELATIVE to the temperature of the air. It is expressed as the amount of water vapor in the air as a percentage of the total amount that could be held at its current temperature.

Warm air can hold far more moisture than cold air meaning that the relative humidity of cold air would be far higher than warm air if their absolute humidity levels were equal.

Relative humidity is cited in weather forecasts as it affects how we “feel” temperature.

As an example, consider two containers:

Container one has a maximum volume of 30g of water and is half full- it contains 50% of its capacity.

Container two has a maximum volume of 5g of water and is three-quarters full- it contains 75% of its capacity.

Container one contains four times as much water as container two, yet actually contains a lower percentage.

If we now call container one “summer” and container two “winter”we can start to differentiate between “absolute” and “relative” humidity.

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Human body temperature is dependent on the air as it absorbs and removes moisture from our skin to cool us down.  If the relative humidity is high, the amount of water evaporating from our skin is limited so we feel warm and stifled.

What are the potential consequences of extreme humidity levels?

High and low levels of relative humidity within the home can have adverse effects for the occupants as well as the dwelling itself. Hygiene, health, retention of value, aesthetics, and comfort are all areas which can be affected by a failure to maintain an optimal level of relative humidity.

Health and comfort

This graph shows that by maintaining an optimal indoor RH of 40-60% the potential adverse effects for occupants, and the dwelling itself, are at their lowest levels.

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A pleasant indoor climate is essential for a sense of well-being in the home. Room humidity can have a major impact on the quality of the living environment. A relative humidity (RH) of 40-60% is generally considered to be optimal for a comfortable and healthy home. Too much moisture can lead to mold and overheating. Too little causes dry eyes, chapped lips and an environment in which bacteria and viruses can thrive.

Fluctuations in indoor RH are caused by the following factors:

  • Everyday domestic activities
  • Use of showers and baths
  • Natural transpiration of people, animals

How does a Zehnder Enthalpy Exchanger help?

The Zehnder Enthalpy (Energy) Exchanger recovers both thermal and humidity energy from the stale air extracted from wet rooms around the home. This additional humidity energy, which would otherwise have been lost, is transferred into the incoming fresh air stream before being supplied to habitable rooms. Choosing the Enthalpy Exchanger, rather than the standard heat exchanger, means that the unit becomes an Energy Recovery Ventilation (ERV) system, as opposed to solely a Heat Recovery Ventilation (HRV) system. The Zehnder Enthalpy Exchanger is designed to help maintain comfortable moisture levels within the home.