Window insulation can be approached in two ways: through the windows themselves and through window coverings. In this short guide, we’re going to offer an introduction to both of these methods and help you get a foundational knowledge of window insulation.
Insulated windows are a powerful asset for every home, being useful in both cold and warm climates. In a warm climate, high-performance windows are particularly useful as the temperature climbs towards 100°F. Good windows help in keeping the inside temperature to a more comfortable level.
The most effective insulating windows have a low U-factor, and a high R-value. The U-factor measures how well the window keeps the heat in the room (heat transfer). A lower U-factor means the window is more insulating. The R-value measures the resistance to heat transfer, so a high R-value is what you want. When purchasing windows, you will most often encounter the U-factor, while other insulation methods will be measured with the R-value.
Windows have different U-factors across their surface. The edges and frame have higher U-factors, while the center of the glass has the lowest U-factor. In some cases, manufacturers and dealers will only relay the U-factor for the center of the glass, making the window seem more effective than it actually is. Make sure that you find out the U-factor for the entire window before making a purchase. Ask the manufacturer or dealer for the NFRC (The National Fenestration Rating Council) ratings.
The NFRC U-factor rating always refers to the whole window. Most windows have an energy information sticker that gives NFRC ratings. The NFRC sticker means the window has been independently tested and certified to determine the effectiveness. This is a bit different from the Energy Star label. Energy Star labels tell you if the product is energy efficient while the NFRC ratings define the performance of the product. Always compare same size windows when determining which product to purchase as the ratio of glass to frame can affect the rating.
Window Panes and Frames
Window panes and the window frame also help to determine the effectiveness of windows. Double pane windows trap a layer of still air or gas between the panes which helps to insulate the house. Double pane windows tend to insulate twice as much as single pane windows.
Gas filled windows are filled with still air, argon, or krypton gas. Krypton gas is usually used in windows with multiple air spaces such as triple or quadruple paned windows. It is a more expensive gas and is usually found in upper scale designed windows. Argon is almost as effective as krypton and is less expensive. Argon works best in ½” spaces.
Low-Emittance coatings also help with window insulation. Low-E coatings are clear coatings of microscopic metal oxide that allow heat from the sun to come through the window but keep the radiant heat inside.
Window frames can account for up to 15% of energy loss through the window. Aluminum frames have the highest U-factor unless they also have a thermal break. A thermal break is a strip of urethane that interrupts heat transfer through the window frame. Wood and vinyl frames have similar U-factors and are both better than aluminum frames without a thermal break. Some vinyl frames are filled with foam insulation which can make them a bit more expensive. Fiberglass frames are only slightly better in performance but are the most durable.
Energy efficient window coverings such as drapes, shades, and films can be good options for window insulation. They can have U-factors of anywhere from 0.5 to 0.25 (or R-values of 2 to 4), which means that in essence, these window coverings double the insulating value of a standard double pane glass window with low-E coating.
So let’s start with honeycomb shades. These shades work similarly to polyurethane foam, in that they trap air within their structure, effectively creating a barrier of inert air between the window and the room. They can have R-values from 2 to 4, with some manufacturers claiming R-values of 7.8. The main drawback of these shades is that they block light from entering your home, but there are translucent options available. Honeycomb shades can cost anywhere from $55-$250 for a 3×5 window.
Plantation shutters are usually chosen for their aesthetic appeal, but they can have R-values of anywhere from 2.77 to 3.17. They can be either wood or vinyl, both being excellent insulators. The main advantage of plantation shutters is that they fit tightly in window frames, which means that they protect against air movement. Off-the-shelf plantation shutters go for $165 to $375 for a 3×5 window, with custom shutters costing double.
When you think of window insulation, you don’t usually think draperies. However, drapes that have lining and interlining can protect against both the summer heat (blackout lining) and against the cold (thermal lining). Surprisingly, thermally lined drapes can have an R-value of 3 to 5, depending on the fabric and the thickness of the lining and interlining. Drapes can cost anywhere from $100 to $140 per panel, with lined draperies having a price tag that’s 20% higher, and draperies that have both lining and interlining costing up to 25% more.
Finally, we have window films. Window films are sheets of self-adhering plastic that can be both aesthetic and insulating. Films are applied directly to the window and can be aesthetically pleasing to the eye by adding the appearance of stained or frosted glass. These films may qualify for energy-efficiency tax credits as well. They are effective at reducing solar heat by as much as 70% and they can block up to 95% of UV light. Furthermore, they can stop up to 50% of radiant heat transfer. Window insulation films act as a low-E coating to regular glass. However, windows that are already low-E coated do not benefit as much. Pricewise, they range from $25 to $150 per window, depending on factors such as scratch resistance, visibility, solar heat reflectivity, glare reduction and design.
Which Window Insulation Works Best For You?
Do you need a touch-up with window coverings or are you interested in a full insulating project, where insulating windows are installed? Contact us today for a free energy audit!
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