Moisture Metric Tool Guide 


What this tool does – [Tool]

Sling psychrometer with wet-bulb (top) and dry-bulb thermometers

You can calculate up to seven moisture metrics — saturation vapor pressure, relative humidity, dew point temperature, water vapor density, vapor pressure, wet-bulb temperature, and humidity ratio — merely by entering dry-bulb temperature and one of the previous values. It’s like having all the benefits of a psychrometric chart without the chart complexities.

This unique RED Calc tool uses our “solve-all” technique, but in a different way than most of our other solve-all tools. Instead of selecting one of six values to solve, you select one of six to enter; the other five are solved by the tool. This tool works most similarly to our Air Leakage Metrics tool.

Calculated values

  • Saturation vapor pressure – at a given temperature, the pressure at which water vapor and liquid water can exist in equilibrium. This is also known as the equilibrium vapor pressure.
  • Relative humidity – the ratio of the density of the water vapor to the density at saturation, at the same dry-bulb temperature. Equivalently, this is the ratio of the current vapor pressure to the saturation vapor pressure.
  • Dew point temperature – the temperature at which water vapor reaches its saturation point or 100 percent relative humidity. Below this temperature condensation begins to occur.
  • Water vapor density – the density of water vapor.
  • Vapor pressure – the pressure exerted by water vapor. If mixed with air, this is the partial pressure of the water vapor (fraction of air pressure, by the number of molecules).
  • Wet-bulb temperature – the temperature indicated by a thermometer with a sensing element covered by a water-saturated wick over which air is caused to flow at a rate of at least 500 feet per minute (2.54 m/s). This thermometer and wick combination is called a wet-bulb thermometer. The heat for vaporization from the wick is supplied by the surrounding air.Your body is sensing the wet-bulb temperature as you get out of the shower, but before you dry off. Your skin is supplying the thermal energy for the evaporation of the liquid water. Once you dry off, the energy for evaporation is no longer needed, so you feel warmer.
  • Humidity ratio – the ratio of the mass of the water vapor to the mass of dry air for a given volume.


  • Clicking the label for any input or result will cause a popup help box to appear. This help box includes the allowed and normal values (for inputs).
  • For an enhanced conceptual understanding of moisture metrics, use a psychrometric chart in combination with this Moisture Metrics tool. There is a psychrometric chart below in the Background section.
  • The relationship between the three temperatures involved is
    Dew point ≤ Wet-bulb ≤ Dry-bulb.
    At saturation (100 percent relative humidity) all three temperatures are the same.
  • All temperatures in this tool (dry-bulb, wet-bulb, and dew point) must fall between -15 °C (5 °F) and 374 °C (705 °F) because the calculations depend upon laboratory measurements that are only valid in this range. This imposes restrictions (directly or indirectly) on the allowed ranges of all the inputs.
  • The allowed ranges of the inputs in the bottom section of the tool depend on the given Dry bulb temperature.
  • The Humidity ratio (the last result) of this RED Moisture Metrics tool is a ratio without mathematical units. If you are accustomed to working with this value in units of grains of water vapor per pound of dry air, merely multiply the Humidity ratio by 7000 (there are 7000 grains in one pound). For example, to find grains of water vapor per pound of dry air for a Humidity ratio of 0.008: 7000 grains/pound X 0.008 = 56 grains of water vapor per pound of dry air.
  • Mold growth is supported when the relative humidity is above 60 percent.

Practical Applications

  • As a sample or mass of air cools, its relative humidity increases. If it continues to cool, its relative humidity will eventually reach 100 percent, the dew point temperature. If it cools below the dew point (saturation) temperature, some of the water vapor in the air will change phase to liquid water. This is what might occur when moisture-laden air moves through a wall.
  • Condensation of water vapor often takes place on basement walls with an interior surface temperature below the dew point temperature of the basement air. This can lead to hazardous mold and mildew in a basement. It is common for homeowners to open basement windows if they discover such condensation. In many cases, this only makes the condition worse because the outdoor air is the major source of moisture in the basement. The basement windows should only be opened when the dew point temperature of the outdoor air is lower than the interior temperature of the basement wall.

    In order to estimate the interior temperature of an uninsulated basement wall below grade, subtract the latitude of the house from 90 degrees. For example, 90 degrees minus 40 degrees north latitude = 50 degrees Fahrenheit.

  • Water vapor diffusion through a material is driven by a difference of water vapor pressure across the material. You can use this tool to calculate the vapor pressure on both sides of the material and use that to determine the direction of water vapor diffusion. The vapor will move from higher vapor pressure to lower vapor pressure. By multiplying the difference in vapor pressure by the permeability of the material, you can calculate the rate of diffusion.


This Moisture Metrics tool solves most of the values on a typical psychrometrics chart (see below), including relative humidity, dew point temperature, wet-bulb temperature, and humidity ratio.

Courtesy of Building Science Corp. Used with permission.

As you can see from the inputs and results of the Moisture Metrics tool, it has to do with the properties of air/water-vapor mixtures. Heating, ventilation, and air conditioning engineers use psychrometrics to determine values such as the energy required to remove water vapor from the air, or to determine at what temperature water vapor will condense on a cool surface. The folks who predict weather use psychrometrics to determine when it is going to rain or snow.

For building science professionals, psychrometics has many uses. These include the prediction of condensation on cool surfaces; explaining why running ventilation during cold weather has a drying effect in the indoor air; understanding why objects made of wood dry out if they are near a hot appliance, such as a wood stove; and comprehending the need for air and vapor barriers in dwelling enclosures.

Best Practices

During cold weather when a dwelling is heated, it is recommended the relative humidity be kept within a range of 30 to 50 percent (a humidity ratio of 0.0046 to 0.0078 at 70°F (21°C)).

A sling psychrometer is probably the best tool for measuring the moisture characteristics of a sample of air. This instrument includes a dry-bulb and wet-bulb thermometer mounted on a hinged device that allows the operator to sling it through the sample of air for at least 90 seconds at a rate of at least 500 feet per minute (2.54 meters per second). Please see the photograph at the beginning of this User Guide.