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Infrared R-Value

What this tool does

Side-by-side plain and infrared photo of a house side. The patio and roof fascia are significantly hotter than the shaded area under the eave.
Infrared photometry; temperature scale shown is in Celsius. (Courtesy of: Florida Solar Energy Center)

With the use of this tool and an infrared thermometer or camera, you can determine the R-value of an enclosed cavity. If the R-value is known you can also use this tool to determine the interior surface temperature given the exterior surface temperature, which can be useful for dew point considerations.

Calculated values

  • Interior surface temperature.
  • R-value of enclosed cavity.
  • Interior air-film R-value.

Tips

  • 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).
  • This tool is NOT appropriate for calculating the U-factor of a window. Only opaque surfaces should be considered.
  • The “Interior air-film R-value” is based on the selections you make for “Surface orientation” (Vertical, Horizontal, or 45° Slope) and “Direction of heat flow” (this drop-down menu is only visible if you select “Horizontal” or “45° Slope” on the previous drop-down menu).
  • The exterior and interior surface temperatures should be measured at the same spot on opposite sides of the building assembly. Choose an exterior surface NOT exposed to the sun. Avoid taking the temperature at framing members.
  • Use the same infrared device for all temperature measurements. Do NOT use the thermostat to obtain the indoor air temperature.
  • When using your infrared measuring tool, avoid the effects of infrared radiation from heating appliances, such as wood stoves or portable electric heaters. Avoid measuring the temperature of a surface above heating distribution devices such as furnace registers, hot water baseboard distribution, electric baseboard heat, or other such devices.
  • The greater the indoor/outdoor temperature difference, the greater the accuracy of the results. Do not attempt the calculation with less than a 10°F (6°C) temperature difference.
  • For the greatest accuracy, set the emissivity value of your infrared device for the surface being measured. Refer to the Procedures section below for guidance.

Procedures

The results of the Infrared R-value tool are only as accurate as your inputs. The accuracy of your temperature measurements is dependent on the proper emissivity setting. It is best to test the emissivity of a surface rather than looking it up on a table.

The most common way of determining the emissivity on site is to place a material of know emissivity in contact with the surfaces you want to measure. For example, place standard black electricians’ tape (emissivity of 0.97) on the wall close to the spot you wish to measure the temperature. Give the applied tape time to stabilize to the temperature of the surface underneath. Set your infrared device to an emissivity of 0.97 and measure the temperature of the black tape. This will give you an accurate temperature for the tape.

Now with your infrared device, “shoot” the surface next to the tape. If the temperature is not the same as the surface of the tape, adjust the emissivity on your device until the temperature is the same. This procedure will give you accurate temperature readings.

Interior air temperature can be measured by various methods. Because you have an infrared measurement device, it is best to measure the temperature of objects in the room and assume the temperature of these objects is the same as the room air temperature. One method is to measure the temperature of several interior surfaces that can be “seen” from the interior surface being measured; then take a simple average. The disadvantage of this method is that the emissivity of these interior surfaces is likely to vary, affecting the accuracy of your readings.

Another method is to place a piece of cardboard about one meter (three feet) from the interior wall surface to be measured. Allow the temperature of the cardboard to stabilize and then measure its temperature with your infrared device. Use this temperature as the indoor air temperature. An advantage of this method is that you can determine the emissivity of this cardboard and carry it from project to project.

Best Practices

Set the emissivity of your infrared measurement device for the surfaces to be measured. Refer to the Procedures section, above.

When measuring wall surfaces with the infrared device, the surfaces’ temperatures should be stable. The outdoor surface measured should not be affected by solar or wind loading. It is best to measure outdoor surfaces on cloudy days or at night.