Cleavon Abu-Warda

We’ll also tell you how these factors translate into the final glazing product, the so called glazing package, and and show you how to incorporate glazing performance into your window buying decision.

First, let’s consider the two most basic functions of the glazing: creating a physical barrier and letting light pass from the exterior to the interior. Everything that matters about window glazing performance relates to these very fundamental properties.

As a physical barrier, the glazing contributes both to the thermal performance of a window and its impact resistance.

The better the glazing insulates the interior from the exterior, the less energy is required to maintain a constant temperature difference between the inside and the outside. This aspect of energy efficiency is referred to as thermal performance. The thermal performance is represented by the U-value, which gives the thermal transmittance per hour and per square foot. The lower the U-value, the less thermal energy is transmitted through the glazing. This means a lower U-value equates to better thermal efficiency. (Experts: don’t worry, we’ll cover solar heat gain in a little bit).

The impact resistance of glazing does not come with a standard measure, but there are a few things to keep in mind:

Thermal efficiency and impact resistance don’t always set the same priorities for window glazing if the maximum thickness is limited. For example, a massive, one inch thick single pane of glass breaks less easily than a set of two quarter inch glass panes with half an inch of gas in between. (That combination of glazing panes and gas is called a glazing package; we’ll provide more details about that in a moment.) However, a glazing package will certainly perform better from a thermal efficiency standpoint.

Besides its properties as a physical barrier, the glazing’s light transmittance represents the other fundamental aspect of its performance.

The glazing’s light transmittance matters in two ways. First, it affects how much heat is generated by the sun on the interior of a home or building, an effect known as solar heat gain. Second, it affects how much visible light is able to pass through the glazing.

Solar heat gain is often represented by a measure called the Solar Heat Gain Coefficient (SHGC). SHGC values for glazing range between 0 and 1, where the higher the value, the more solar energy passes through the glazing, and the more heat is generated by it.

Whether a higher or lower SHGC value is desirable depends on where you live. If you’re in a cold region of the United States, you would want to get as much of the solar heat gain as possible in order to reduce the amount of energy that you spend on heating. If, however, you’re in a hot part of the country, you would want to have a low SHGC so that your air conditioning doesn’t have to work so hard.

The amount of visible light that is able to pass through the glass is represented by the so called Visible Transmittance (VT). The higher the VT, the more transparent the glazing appears and the more daylighting is allowed inside the home or building.

Solar heat gain and visible transmittance are both forms of light transmittance. The only difference is that the solar heat gain refers to the light frequencies that cause a heat gain on the interior, while visible transmittance refers to the frequencies that the human eye can process.

This serves you well if you’re in a cold climate and want both a lot of daylight and a lot of solar heat gain on the interior. Your goals align as you simply want your glazing to block as little of the light as possible, no matter what frequency.