Window Glazing: What Do U-Factor and Low-E Have to Do With Selecting Your Windows?

Last Updated on March 7, 2022 by Kravelv

The glass or glazing is a very crucial part of a window, affecting how much light and heat is let through. Advancements in window manufacturing technology have greatly expanded and evolved glazing options over the years, making it possible for windows to also contribute to improving energy efficiency in the home.

When looking for energy-efficient windows, U-factor and low-E are just some of the terms that you’ll encounter. This post is here to give you the lowdown on the two, what they are and why they matter in window selection.

U-Factor: The Important Energy Efficiency Factor

U-factor is among the primary measurements used in determining the energy performance of fenestration products like windows, doors, and skylights. This is the measurement guideline used by the National Fenestration Rating Council (NFRC) to rate how well can a window prevent heat from escaping the home or building. Generally, the lower the U-factor, the greater a window’s ability to resist heat flow and the better its insulating properties are.

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Some, however, may confuse U-factor with R-value. The biggest difference between these two energy efficiency measurements is that U-factor measures the rate of heat transfer or loss, while R-value measures heat loss resistance or conductance. Basically, a product with high conductance conducts heat easily and quickly. While this is also important in determining how much heat can be absorbed by a product, energy efficiency is more than just that. U-factor aims to take into account not only conductance but also airflow and loss around the window and glass emissivity or radiation of heat.

To use U-factor ratings to your advantage, you need to know what the values mean. The ratings generally fall between 0.15 and 1.20. As mentioned earlier, the lower the value, the better a window insulates and keeps the heat inside the home. Generally, high-performance double-pane windows have U-factor values of 0.30 or lower.

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Not many people pay attention to what the labels in window products mean and what they can do for them. From a technical standpoint, U-factor isn’t merely a material property value. It is the result of a calculation of values that encompass not only the glazing but the framing, gas fill, spacers, weatherstripping, and sealants as well. U-factor is directly related to energy efficiency as it tells you how the window can help reduce heat transfer. Furthermore, the combination of outstanding values for U-factor with the other equally important measurements, solar heat gain coefficient (SHGC), air leakage (AL), and visible transmittance (VT) ensure optimal window performance.

The Lowdown on Low-E

To understand what low-E glazing is, it is important to first define what the “E” stands for. So what is it? “E” is for emissivity, the ability of a material to radiate or emit energy. The energy from the sun that goes through window glass can come in three general types: ultraviolet (UV) light, visible light, and infrared (IR) light. Low-E glass helps control which and how much of these energy types pass through the glass surface.

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Low-E glass features a microscopically thin and virtually invisible layer of metal or metallic oxide. Commonly used on multi-pane windows, this coating is what makes heat and energy control possible. Depending on heating and cooling needs, low-E coating may be applied on either the inner or outer surface of the window glass, or even on both sides.

When they were first invented in the 1980s, low-E windows came in just one variety and were designed to serve just one purpose: keep infrared light, and the heat it brings, indoors. Today, newer low-E glass products not only keep the heat in but can also keep the heat out, preventing unnecessary passive heating from being a problem.

Low-E glass addresses two main concerns homeowners have: keeping household energy consumption low and keeping the home comfortable. Low-E coatings have been proven to efficiently control heat transfer, especially on windows with insulated or multiple glazing and gas fills. Because of this, they assist in lowering U-factor and SHGC values in windows. And even though low-E glass generally cost about 10% to 15% higher than regular windows, they help reduce energy loss by as much as 30% to 50%, keeping energy bills down in the long run.

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Standard clear glass has an emittance rating as high as 0.84, meaning it can emit up to 84% of energy possible at its temperature and reflect back 16% of infrared light. Low-E glass, on the other hand, has an emittance rating as low as 0.04, meaning it emits only 4% of energy and reflects back 96% of infrared light.

Other benefits of low-E glass include:

  • Reduced UV damage on belongings such as furniture and fabrics
  • Decreased condensation in double glazed windows

By choosing windows that have a low U-factor ratings and fitted with low-E glazing, you can save on energy costs while keeping your home’s indoor environment comfortable throughout the year. To make the most of investing in energy-efficient windows, make sure you only work with a trusted window professional in your area.


Author Bio:

Alex Esler is the marketing manager of Renewal by Andersen of Denver and Colorado Springs. Always eager to assist homeowners in finding the best window replacement solutions for them, she is driven and draws inspiration from her work and the causes she cares about. For updates from Alex, check out the company blog!


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