Understanding Window Energy Efficiency Ratings

Aside from their aesthetic appeal, perhaps one of the most important factors to consider when buying windows are their energy efficiencies. However, there are many different measures currently being used to define the energy efficiency of certain types of windows. With the abundance of terminologies, it can become too overwhelming.

Don’t fret. We’ve prepared a brief overview of the different performance markers used to determine a window’s energy efficiency rating. This guide can help make things easier for you.

U-Value/U-factor

The U-value, interchangeably referred to as the U-factor, measures a product’s ability to resist heat loss. The numerical value assigned to this factor indicates the amount of heat that the window allows to escape. This means that a lower U-value allows less heat to escape and, therefore, provides better insulation. Windows with low U-values are recommended for the colder months to save on energy by not having to run the heating system too high or too often.

Visible Transmittance

The amount of visible light allowed to pass through the window is referred to as its visible transmittance. The lower the number, the lesser the amount of light entering the building. For instance, Low-E glass uses a special coating to reduce visible transmittance to the inside of the building.

Solar Heat Gain Coefficient

In conjunction with the visible transmittance, the amount of solar radiation transmitted through a window is also caused by visible light entering a home or building. This solar radiation is measured through the solar heat gain coefficient (SHGC) and will appear as a number between 0 and 1, with a lower number indicating that the window transmits less solar heat. Building on the previous example, Low-E glass uses a special coating to reduce solar heat gain. Higher quality Low-E glass is able to achieve this while still allowing ample natural light to enter.

Air Leakage

The amount of air that passes through cracks in a window is measured by the air leakage. A higher rate of air leakage indicates that the window is more susceptible to heat loss and heat gain, which reduces the overall energy efficiency inside your home. This can result from improper installation and having damage sustained from either calamities or human activities.

R-Value

The R-value is often mentioned in relation to the U-value since both measures the same process. One key difference, however, is that the R-value refers to the window’s ability to retain heat, as opposed to the rate at which a window expels heat, which is measured by the U-value. Ideally, an energy-efficient window should have a low U-value and a high R-value.

What is Energy Star?

Energy Star is a certification system used to identify energy-efficient products, such as windows and doors. Having the Energy Star logo on a product indicates that it has met or exceeded certain standards after being tested by an independent laboratory.

A single set of standards is essential in providing customers with more convenience in comparing different products and their efficiencies. Although these standards may have to change to keep up with evolving technology, there will still be indicators for the standards used. To be sure, always check if the figures shown are accurate and up-to-date.