Windows and Energy Efficiency
How energy loss through your windows works, and how to get better performance and save money on your heating and cooling costs
The replacement window market is diverse and broad. Window frames are made of various materials -- vinyl, wood and fiberglass. Glass types and insulating properties vary tremendously. In climates where temperatures are moderate year round (San Diego, perhaps?), the insulating properties of your windows may not have a dramatic impact on your energy costs. In the Northeast, however, temperatures can vary 90 to 100 degrees Fahrenheit over the course of the calendar year, and the efficiency of your windows has a significant impact on your energy bill. Moreover, while many consumers are informed about the R-value of the insulation in the walls of their home, they never consider the importance of their windows. Other than in extreme cases such as an old house with uninsulated walls, the R-value of your walls is going to be much higher than the R-value of your windows. So it makes sense to assess your current windows and to understand the payback you can realize from the proper selection of a replacement model.
Temperature Gains and Losses
A variety of factors determine how your windows control heat gain and loss. In colder months, you want a window that gains heat and keeps it inside. When it is hot outside and you are running air conditioning units at full throttle, you benefit from a window that retains the inside cool air and prevents outside hot air from entering your home or building. A well designed and manufactured window does both. If you make an informed choice when selecting replacement windows, you can often get a great payback on your investment with reduced energy bills. Generally, more expensive windows are more energy efficient than cheap windows, but price is definitely NOT the only factor. If you are focused on energy efficiency, this primer is intended to help you become an informed consumer.
R-value is a measure of the resistance of a window to heat flow. The higher the R-value, the better. You get greater resistance to heat flow and a higher insulating value. The benefits apply anytime there is a significant difference between indoor and outdoor tempartures, in both hot and cold weather.
The glass options in a window are the first factor to consider. Windows can be found with single, double, and triple panes of glass. Some windows also include a suspended film to improve R-value. Single pane windows are light, low cost to purchase, and have the lowest R-value. Many older windows found in New Jersey homes are single pane. If you are replacing single pane windows, and you select an alternative replacement window wisely, you can achieve the maximum energy savings advantage. Dual pane windows are common today, and they are definitely an improvement on a single pane product. But the highest R-value will be found in triple pane windows, or windows with suspended film glazing.
How Windows Lose Energy
The best replacement windows are designed and built to minimize the four main processes in heat or cold transfer found in most windows: radiation, conduction, convection, and air leakage.
Radiation – The amount of cold or hot air passing through the panes of window, known as "radiation," is responsible for the majority of the energy lost through the window. Older windows have single or maybe double panes, but are ineffective at reducing radiation. Heat transfers easily from the colder side of the glass to the hotter side, i.e. from the inside to the outside during the winter, and in reverse in the summer. The best replacement windows have special low-E coatings that control the heat transfer through the glass. Triple panes or suspended films are the best at reducing radiation.
Conduction – Temperature variations between inside and outside also pass through the edges and framing system of a window. The material of a window’s framing system is what determines how well a window controls conduction. Some materials are better conductors than others. Just as copper and aluminum pots and pans are great for cooking because they are extremely good conductors, your best bet in a replacement window are the ones of the least conductive materials such as vinyl, fiberglass or wood.
Convection – Another factor to consider is convection, which is experienced as a "drafty" feeling in the winter. This results from a convection process in which warm interior air hits the colder glass pane. The air is cooled, and then it drops toward the ground as cold air does, and circulates out toward the interior of the home. This movement of colder air is what causes that feeling of a draft in the winter. The same thing happens in the summer, although you do not experience it from inside. You should look for window products that have been engineered with optimal spacing between the panes of glass, reducing temperature fluctuations due to convection or conduction. The best engineered replacement windows have an R-value for the full frame that creates warm interior glass temperatures and eliminates that cold drafty feeling.
Air leakage – The last factor to consider in energy efficiency is air movement between the moveable parts of the window’s frame. Air leakage is most common between the sash and frame, or along the side rails of a sliding sash in operable windows. Air leakage performance is shown on a window’s NFRC label. Look for the rating called “air infiltration rate.” The lower the number, the better the performance. An air infiltration rating is not required; some manufacturers don’t supply it (usually the ones whose products don’t have a great rating). Don’t hesitate to ask for this information, and be skeptical of any company that cannot provide it.
