Understanding Windows


Any opening in a building’s envelope – in its outer shell – is technically called fenestration, a term that includes windows, skylights and doors. Obviously, fenestration is important if you’re concerned about energy efficiency. A typical house can lose 30 percent of it’s energy dollars out the windows and doors, according to the California Energy Commission. To put that in perspective – the amount of energy lost through doors and windows in the U.S. every year is roughly equivalent to all the energy we get from the oil carried by the Alaska pipeline! Since windows outnumber doors in most buildings, they deserve the most attention. You can either fix them to make them as efficient as possible, or you can replace them with some of the new technology that has been introduced in the last several decades.

Today’s Windows – How They Work

Early windows were little more than holes cut in walls to let light and air into rooms. With the addition of glass in movable frames, a major improvement in building comfort was achieved, allowing closed windows to let in light and block the winter chill. Over the past 20 years, windows have become increasingly more sophisticated, using new materials with more energy-efficient properties. Single-pane glass has been1919439_129380720457286_4138650_n replaced by double, triple and even quadruple panes, with insulating materials separating the layers. Inert gasses have been pumped between the panes, adding to the window’s insulating properties. Even the glass itself has been coated to reflect heat. These innovations mean that windows can significantly contribute to a home’s comfort and energy efficiency. By letting in sunlight, they provide warmth in winter, which will save energy and lower monthly heating bills. Proper design and the use of exterior shading can also lower cooling costs in the summer.

How Energy Flows

To appreciate the improvement in today’s windows, it is helpful to understand three ways that energy can flow through them.
  • Air can carry heat in or out of a window. Intentional air flow is called ventilation. Unintentional air flow – leakage – is called infiltration.
  • Heat – or cold – can flow through the frame and the glass.
  • Solar radiation – sunlight – can pass through the glass and can heat whatever is inside the building.

Ventilation and Infiltration

Well-placed windows allow for natural ventilation, which can significantly reduce cooling costs in the summer. But infiltration – unplanned air leakage through a window’s joints, cracks, frames and sashes – can account for as much as 15 percent of a home’s heating and cooling losses. It can make a home much less comfortable and more costly to operate. Different types of windows can allow more infiltration. For example, a horizontal sliding window may not be as airtight as a window with a swinging sash. In addition, the overall quality of the window can affect infiltration – some windows are built better than others.

Modern Windows – Think of Them as Thermos® Bottles

When there is a difference between inside and outside temperatures, heat transfers through a window. It’s lost to the outside during the heating season and is gained from the outside during the cooling season. A window’s thermal performance – which can be measured at the center of glass, the edge of glass and the frame – is rated with a U-Value, its overall ability to resist heat flow. Think or drinking hot coffee from a drinking glass. You know that glass is a very good heat conductor. The outside of the container can quickly become too hot to hold. Using two layers of glass with an air space between – the idea behind the Thermos bottle, dramatically cuts the heat flow. Single-pane windows can act like that drinking glass, easily conducting heat. Dual-pane windows, with a 1/2 inch to 3/4 inch air space between sheets of glass, work like a Thermos bottle to cut down the flow of heat. If you replace the air between the panes with an inert gas like carbon dioxide, argon or krypton, the window will transfer even less heat and be even more efficient.

The Frames

Before dual-pane windows, the material used for window frames was not of great concern, since a single pane of glass conducts heat about the same as a solid aluminum frame. With more efficient glazing, however, new materials and new designs were used to make window frames themselves less conductive. Today, wood, fiberglass, vinyl or vinyl-clad wood window frames will generally perform better and provide more comfort than metal frame windows that do not have a thermal break – a buffer of non-conductive material sandwiched between the metal parts to cut down the flow of heat.


Even in dual-pane windows with less conductive frames, the type of glass and type of spacers used to create the air space can improve energy performance. Low Emissivity (or “Low-E”) coatings can help to increase U-values. A low-E coating is a microscopically thin layer of metal or metal oxide deposited on window glass. The coating reflects warmth back into the home in the winter and prevents unwanted heat from entering the home in the summer. When shopping for windows, look for the overall U-value rating. The lower the U-value, the better the window’s energy performance will be.

Windows that Help Block the Sun

Sunlight passing through a window adds heat to a room – desirable in winter, but generally unwanted in the summer. Solar heat gain through windows may account for 30 percent or more of a home’s summer cooling requirements. The amount of heat from direct sunlight through a window is measured with a shading coefficient. The lower the coefficient, the less solar heat gain. Window tints such as bronze and green keep out more of the sun’s heat than clear glass does. Low-E coatings used to lower window U-values may also reduce a window’s shading coefficient. Keep in mind, however, that by tinting your windows to keep out heat, you may make it harder to see through them. Consider a window’s orientation to the sun before selecting window tints. South facing windows gain the most beneficial heat in winter, so their shading coefficients should be high. If these windows are properly shaded – say, by deciduous trees – they will allow little heat gain in summer. Shading coefficients for north windows can also be high, since they get little direct sun and do not contribute much heat any time of the year. East and west facing windows should have low SCs, since they get direct morning and evening sun and are responsible for most of the solar heat gain during summer. Often the best strategy to reduce heat gain in the summer is to provide exterior shading through overhangs, awnings, shade screens and trees. The National Fenestration Research Council (www.nfrc.org) has developed a rating system used by most window manufacturers. When making your window purchase, look for the NFRC label to determine how each product performs. The rating system uses five categories. A. U-Factor measures how well a product prevents heat from escaping a home or building. U-Factor ratings generally fall between 0.20 and 1.20. The lower the U-Factor, the better a product is at keeping heat in. U-Factor is particularly important during the winter heating season. B. Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat from the sun. SHGC is expressed as a number between 0 and 1. The lower the SHGC, the better a product is at blocking unwanted heat gain. Blocking solar heat gain is particularly important during the summer cooling season. C. Visible Transmittance (VT) measures how much light comes through a product. VT is expressed as a number between 0 and 1. The higher the VT, the higher the potential for day lighting. D. Air Leakage (AL) measures how much outside air comes into a home or building through a product. AL rates typically fall in a range between 0.1 and 0.3. The lower the AL, the better a product is at keeping air out. AL is an optional rating, and manufacturers can choose not to include it on their labels. E. Condensation Resistance (CR) measures how well a product resists the formation of condensation. CR is expressed as a number between 1 and 100. The higher the number, the better a product is able to resist condensation. CR is an optional rating, and manufacturers can choose not to include it on their NFRC labels. Go to https://nfrccommunity.org/page/CertifiedLogoLabel for help reading the NFRC label. While the NFRC label is the best source for window energy performance information, you should also look to purchase ENERGY STAR-certified windows. Energy Star requires a U-Factor that is less than or equal to 0.35. Energy Star windows also have low-e coatings and other energy efficient features.