Humidity Defined



by Brandon Burton

The restorer must use several different measures of humidity to fully understand how to increase evaporation.

These measures are in two categories: Those that describe the actual amount of water vapor in the air (vapor pressure, gpp, dew point, humidity ratio, absolute humidity) and those that relate the actual amount of water vapor to the potential amount (“relative humidity”).

Relative humidity

Relative humidity (RH) has traditionally been used to indicate the air’s moisture content. Currently the restoration industry has adopted the simplified definition of relative humidity, which is the amount of moisture contained in the air as compared to the maximum amount the air could hold at that temperature.

The reason RH is used in restoration is that hygroscopic materials have an equilibrium moisture content that is mainly determined by the RH. In simplest terms, when RH is low, materials will generally lose moisture. When RH is high, especially above 60 percent RH, materials will generally gain significant moisture. When conditions exceed 60 percent RH, the structure is more likely to suffer secondary damage.

RH does not define the water content of the air unless the temperature is given. Air can hold more water vapor when the temperature of the air increases. Therefore, changes in air temperature will change the relative humidity. Relative humidity will decrease when air temperature increases. Conversely, RH will increase as air temperature is decreased.

Specific measures of humidity

There are many ways to measure the actual amount of water vapor in the environment, regardless of temperature. In the restorative drying industry, the weight measurement used to measure water vapor in air is grains per pound of dry air, abbreviated as GPP. This is referred to as humidity ratio. GPP expresses the weight of water in a given weight of air (a pound of dry air). The metric equivalent of GPP is grams per kilogram of air, abbreviated as g/kg.

Specific measures of humidity give the restorer the opportunity to compare different environments on a drying project. GPP is most commonly used for this purpose.

GPP is directly related to dew point and vapor pressure, because all three are specific measures of humidity. Therefore GPP can be used as a constant measure for these metrics as well. When the GPP in an environment goes down, the dew point and vapor pressure decrease as well. For example, if the GPP is low inside a structure and the GPP is high outside, it can be predicted that moisture will move from the outside toward the inside. This is not because of the GPP differences but because the vapor pressure (associated with GPP measures) is greater on the outside of the structure.

When using humidity ratio to describe the amount of moisture in the air, restorers make use of two significant factors for proper decision making during a water loss:

14 cubic feet of air = 1 lb. air

7,000 grains = 1 lb.

With the understanding that one pound of air occupies approximately 14 cubic feet and 7,000 grains weigh one pound — a restorer can use humidity ratio to calculate amounts of water removed:

  • By dehumidification systems
  • By negative pressure systems (such as inter-air dryers)
  • In heat and air exchange drying systems
  • When flushing a structure
  • During combination drying systems.


Excerpted from Restoration Science Academy’s Complete Guide to Restorative Drying, a collection of all ASD classroom course materials, including water damage restoration, fire and smoke restoration, odor control, microbial remediation, trauma scene cleanup, upholstery and fabric cleaning and carpet cleaning. Authors: Brandon Burton, Kevin Fisher and Bob Lintzenich. Technical Review: Brendan Kimmel, David Oakes, Rick Ruch, Richard Driscoll, Darren Hudema, Bill Weigand, Gary Loiben, Cara Driscoll, Gary Funari, Lee Senter and Mitch Byrom.

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