What Are the Most Energy Efficient Windows?

The Role Energy Efficient Windows Play in the Sustainable Home 

According to the U.S. Department of Energy, old and inefficient window designs can be responsible for up to 25-30 percent of all heat loss and heat gain in your home. Old, single-pane windows that you found for a bargain are most likely causing your energy bills to go through the roof. 

In insulation terms, single pane glass that is only 1/4th of an inch thick will have an expected R-value of around .90. A double-pane window of similar thickness would have an expected R-value of at least 1.7 This means that you nearly double the insulation value of your windows through this upgrade. Some of the top-quality triple pane windows on the market today have an R-Value of 5, which would increase the energy performance of your windows five times over when upgrading from older, single-pane options. Even homes that undergo a complete low carbon retrofit would have a hard time achieving such drastic energy efficiency improvements. Replacing old windows, or at the very least installing window films over less energy-efficient options, is one of the best and most cost-effective strategies to improve the overall energy efficiency and thermal performance of your home. 

Energy-efficient windows can also significantly reduce the amount of energy we collectively use to light our home. Unless you live in Alaska where the sun hardly moves above the horizon during the winter months, almost all areas in North America receive enormous amounts of natural light throughout the year. According to the U.S. Energy Information Administration (EIA), “the U.S. residential sector and the commercial sector used about 232 billion kilowatt-hours (kWh) of electricity for lighting. It was about 8% of the total electricity consumed by both of these sectors and about 6% of total U.S. electricity consumption.” An average U.S. home with 45 light bulbs required to use many of these lights during the daytime hours due to a lack of natural lighting in the house, adds up to an enormous amount of energy usage over a year. Proper window placement can effectively eliminate the need for turning the lights on during the daytime, and thus save you money on your electric bill.

What to Look for in the Best Energy Efficient and High-Performance Windows

When searching for the best energy-efficient and high-performance windows, there are several different factors you need to focus on: 

• U-Factor: The U-Factor is the primary insulation rating for windows. It measures the rate of heat transfer, and generally speaking, the lower the U-Factor, the higher the insulation capability of the window. (Another way to think of it: the U-Factor is the inverse of the R-Value used primarily for insulation in walls and roofs. It takes into consideration both conduction and radiation for the entire window assembly.)
• Solar Heat Gain Coefficient (SHGC) Rating: This rating measures the amount of solar radiation emitted through a window, typically more prominent in southern, hotter climates. You can find information about how to choose windows with the maximum SHGC rating for your home here.
• The Glazing Package (number of panes): In much of Europe, triple pane windows are the norm for home construction and can lower the window's U-value by up to 50 percent. However, in North America, cost considerations often lead many homeowners to choose double pane windows, which have become the standard. While triple-pane windows might be the best option, double-pane options that rely on low emissivity (Low-E) glass are also a great option. It includes an invisible coating of metal oxide on one of the interior panes. 
• The gas between the Glass: You should consider finding window options that fill the spaces with gasses such as argon, xenon, or krypton. It can reduce condensation and also increase the thermal performance of the window. 
• Pane Spacers and Frame Materials: The pane spacers and frame materials are just as important as the glass itself when it comes to actual window performance. Pane spacers that contain little or no metal are known as warm edge spacers and can be much more energy-efficient. The frame materials can also affect the energy performance of the window. Although wood frames are the most natural and energy-efficient, they require maintenance and can lead to leaks. Composite frames that combine an inner timber frame covered with aluminum or plastic rely on wood while also reducing the need for maintenance. 

The Top 3 Most Energy Efficient Types of Windows on the Market Today (and What They Cost)

The best window options for your particular home will largely depend on the climate where your home is located and other contextual considerations. These three types of windows perform well in many types of environments.

Triple Glazed Vinyl Windows

Triple glazed vinyl windows offer perhaps the highest value in window insulation, as they incorporate insulation between all three panes. (If you are aiming for the Passive House standard, triple pane windows are a requirement.) Many manufacturers, such as the Milgard Essence Series Wood Windows, offer two separate insulating air spaces with Low E glass and dual spacers. It can interrupt wasteful heat flow through the window and thus save on heating and cooling costs. These types of windows will generally be the most expensive on the market, with an average price of between $550 and $850 USD. However, the expected energy savings, especially for homeowners upgrading from single-pane windows, can lead to relatively quick payback periods. 

Fiberglass Windows

Wood has been considered to be the most natural and energy-efficient material for window construction. Although, the tendency for wood to expand and contract with the seasons can lead to air leakage that compromises the energy efficiency of your home. Wood also requires some sealant or finish, so any condensation does not lead to rot. Fiberglass windows are often tougher and more durable than their wood counterparts, and once installed, these framed windows will require minimum maintenance. Marvin Windows and Doors offers manufactured window frames made from Ultrex®, which they claim to be eight times stronger than vinyl and three times stronger than vinyl/wood composites. These windows can resist fading, chalking, peeling and cracking, offers unparalleled strength and durability, and also meet or exceed federal ENERGY STAR® guidelines.

Gas-Filled Windows

Whether your budget allows you to choose between double or triple-paned windows, you should undoubtedly make it a priority to opt for gas-filled windows. As mentioned above, gas-filled windows offer increased insulation while simultaneously reducing issues with condensation. JELD-WEN® Windows offers a high-performance LoE3-366 glass with insulating argon gas to lower energy costs by assisting in keeping homes cooler in the summer and warmer in the winter. This technology also deflects direct sunlight to help reduce the fading of interior furnishings and prevent condensation. Ply Gem HP (high performance) Glass windows combine Low-E glass with argon gas fill and Warm Edge spacer options for super energy-efficient window options. This company also offers R-5 Windows, which provides some of the highest insulation capacity for windows on the market today. 

So take a look at your windows today—they might be the most important element in your house.