Window film is a popular way to improve the energy efficiency, comfort, and appearance of your home. But with the technology of window film for the home new to a lot of homeowners, its hard to know what to be looking at when making a purchasing decision.
One performance specification that is tested in the industry is a film’s emissivity. But what is it and how do we interpret the measurements when it comes to window film? Let’s explore just that.
What is emissivity?
Emissivity is a property that describes how well a material emits infrared radiation. IR radiation is the type of heat that is emitted by the sun and by warm objects.
According to Science Direct, it is a measurement that describes how well a material emits infrared (IR) radiation in comparison to a “blackbody” which is a theoretical object that absorbs all electromagnetic radiation and would then theoretically re-radiate all energy.
How is it measured?
Emissivity is typically represented as a value between 0 and 1, where 0 indicates perfect reflectivity (no radiation emission), and 1 indicates perfect emissivity (full radiation emission). To calculate emissivity, you can use various methods, including experimental measurements and theoretical calculations. One common method is using a device called an emissometer.
So, the lower the emissivity number, the less the object will emit IR radiation or heat. And the higher the number, the more heat or IR radiation the material emits.
Below is a list of common objects and their emissivity from thermal-engineering.org:
As you would expect, shiny metallic objects have a much lower emissivity than other objects because they reflect most infrared radiation and don’t absorb it.
The emissivity of glass
Before we think about emissivity with respect to window film, it’s important to set a baseline. All window film performance specifications are tested and rated by the NFRC with films being tested on 1/8” clear glass. The emissivity of 1/8” clear glass is: .86, which means it emits 86% of the IR radiation.
Solar control window film is typically concerned with blocking heat from entering a home – so it is best suited to situations where heat entering the home needs to be reduced. Because most solar control films work by reflecting a portion of solar energy that hits it, it will reduce the emissivity of your windows some. And as you can see from the chart below, the more reflective the film, the lower the measurement:
But, the amount of reduction is not significant because the other way solar control film manages the heat entering your home is by absorbing more of it than clear glass would, and that heat that is absorbed will re-radiate to a small extent.
Overall, if you are considering a solar control window film like ComforTech™ to reduce the heat coming into your home, emissivity is not a very important performance specification to consider. The more important specifications to consider for solar control film are Total Solar Energy Rejected and Solar Heat Gain Coefficient.