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How to pick replacement windows: a guide to glass

Iris Price | Improvement Center Columnist | May 4, 2015

Have you been thinking about installing new home windows? How hard could it be to make a decision about what to buy, especially if you have a set budget and know the style you want? Actually, it might be a little more complicated than you think.

Energy-efficiency factors, for example, might be unfamiliar to you. It's a good idea to know a little about U-values, low-e coatings, and SHGC (solar heat gain coefficient) before you start shopping for replacement windows. Once you tackle the energy-efficiency options, the other home window choices like styles and hardware will probably seem pretty easy.

How a basic guide to buying home windows can help

Many homeowners make window buying-decisions during an in-home window sales presentation. These sales representatives give you a great deal of information in a short amount of time about the features and benefits of the products they offer. To make an informed decision, however, it helps to learn a bit about what other features may be available before you schedule a meeting with the salesperson.

The features that matter most when buying new home windows are those that relate to energy-efficiency, functionality, and appearance. The following guide breaks down and highlights the window components that contribute to energy-efficiency, specifically regarding window glass.

How to choose energy-efficient replacement windows: glass options

Ratings labels from the independent, non-profit National Fenestration Rating Council (NFRC) provide useful information relating to the energy-efficiency of new home windows. Look at the following two ratings in particular, and compare them to the latest Energy Star requirements for your climate zone:

  • U-factor or U-value. The lower the number of the U-factor, the better your replacement windows are at preventing heat from escaping your house. As of 2015, Energy Star requires U-values below 0.30 for cold, northern climates -- and for north-facing windows -- to keep your home more comfortable and energy-efficient in winter.
  • SHGC or solar heat gain co-efficient. The lower the number of the SHGC, the less solar heat transfers through your windows from the outside to make your room warmer. If you live in a Southern climate zone or have windows that get strong afternoon rays, a SHGC of 0.25 or less offers better comfort and energy efficiency.

When it comes to glass (or glazing), the quality of the following options contribute to lower U-factors and SHGCs:

  • Double and triple glazing (also known as insulated glass). An older home's existing windows are typically single pane, and in the winter you may install storm windows over single-glazed windows for an additional layer of glass to improve energy efficiency. Replacement windows, however, come double-glazed to give you the extra layer of glass already built in. For even better heat retention in colder climates, they also come with triple-glazing. Pros and cons: Insulated glass lowers both the U-factor and SHGC, which may not be optimal for all climate zones. Triple-pane windows can also be very heavy. They cost more, as well -- not always the most cost-effective option for your particular climate.
  • Gas fill. If you want better energy-efficiency from your double- or triple-glazed windows than just air between the panes can provide, they also come with an inert gas, typically argon or krypton, between the panes. Pros and cons: Because of the density of the gas, these windows offer more resistance to heat transfer than what air-filled compartments provide. According to the International Association of Certified Home Inspectors (NACHI), gas fill can leak as much as 1 percent annually, but even at that rate they would remain extremely energy-efficient for at least 20 years. Krypton gas-filled windows are more expensive to manufacture, making argon a better value. However, in very rare instances, argon-filled windows have imploded without warning.

Various types of glass coatings can also improve energy efficiency ratings:

  • Low-e (low emissivity) coatings. Invisible, microscopic low-e coatings are made from metallic oxide and applied directly to the glazing during manufacture. Pros and cons: According to energy.gov, low-e coatings can improve U-values and boost energy efficiency between 30 and 50 percent while at the same time offering SHGC ratings that accommodate the needs of your particular climate zone. On the flip side, they can raise the cost of your windows by 10 to 15 percent, as well as interfere with visible transmittance (VT) -- the amount of light that shines through the window -- making your interior space darker. Spectrally selective coatings used in combination with low-e have eliminated VT issues.
  • Spectrally selective coatings. These coatings reflect certain wavelengths and remain transparent for others to improve VT. Pros: These coatings can filter between 40 and 70 percent of the heat normally transmitted through the windows. Used in conjunction with low-e, solar heat gain and visibility can be customized for increased energy efficiency and the amount of visibility desired. According to energy.gov, homeowners in southern climates can save up to 40 percent of their electric cooling costs with the right combination of these coatings.
  • Reflective coatings. Applied to the glazing to reflect light away from it, these mirror-like, thin metallic coatings come in colors such as bronze, gold, and silver. Pros and cons: Used more often in Southern climates, they reduce excessive glare. However, VT is compromised, which can require additional indoor lighting to improve visibility. SHGC is also reduced, which can make them less energy-efficient.
  • Tinted glass. Tints come in various colors such as green, blue, and black. Blues and greens allow better visibility through the glass. Pros and cons: Tints can reduce SHGC. Gray and black tints absorb too much heat and light, cutting visibility way down: indoor plants may not get enough light to thrive.

A new glazing technology that can improve the insulation properties of windows to almost as much as that of walls uses two layers of film between the panes of glass, creating three interior air-filled chambers -- one chamber more than triple-paned windows. In addition to taking windows to an unprecedented level of energy-efficiency, such technology can reduce both the weight and cost of replacement windows.

Window technology has been improving steadily during the past couple of decades, and it continues to do so. You owe it to yourself to find out as much as you can about the best available energy-efficient features to ensure your comfort during the years to come that you are in your home.

About the Author

Iris Price is a single Baby Boomer whose antidote to a lack of retirement funds was to launch a long-delayed career as a writer. While others her age concoct bucket lists and travel the world, she bought a new-construction home and obsessively creates lists of must-have home improvements and personal realization goals. She specializes in writing about home services and self-motivation.