Volume 7 Issue 3 March 2006
Super Windows to the Rescue
by Joshua Apte, Dariush Arasteh and Christian Kohler
For many homeowners in the northern United States, the word winter conjures up not just the serene beauty of snowy fields, but also the chill of drafty rooms and the sting of high heating bills. Our home’s windows might do a nice job of framing the beautiful scene outdoors—but they also tend to be the weakest link in our house’s defenses against the cold. Over the past several years, energy-efficient windows-such as Energy Star® certified windows—have cut billions of dollars from the heating bills of millions of satisfied homeowners. However, the outer limit of window performance lies far beyond Energy Star’s minimum requirements. A high-tech breed of windows known as “super windows” offers energy performance superior to even a normally insulated wall. These windows have a variety of substantial benefits beyond their significant energy savings, as this article will show.
How Windows Keep the Cold Out
While super windows offer unparalleled energy performance and comfort benefits, the basic physical principles under which they operate are straight-forward. Engineers generally look at two physical properties—insulating value and solar heat gain—when they seek to characterize the performance of windows.
Engineers describe heat loss through a window with a value known as U factor, a measure of the rate of heat flow through a window. Thus, windows with lower U factors have lower rates of heat flow, and are better insulators. Today’s high-performance windows use a variety of technologies to achieve very low U factors:
• Low-E coatings are thin, invisible metal coatings applied to glass on the inside surfaces of double- or triple-pane windows. These coatings improve the insulating value of windows by acting like a mirror for radiant heat flow from one pane to the next. Low-E coatings are available in high, moderate and low solar heat gain versions (more on this later);
• Gas fills such as krypton and argon are used instead of air in the cavities of double- and triple-pane windows to improve their insulating performance. Krypton is the better insulator of the two, while argon is currently the more common gas fill; and
• Improved frames and glass spacers prevent heat from taking a short-cut through the solid materials of the window frame, which often have inferior insulating properties to that of the central glass area.
Solar heat gain is the second major component of window energy performance, and it refers to the amount of solar energy (heat from the sun) which can pass through a window into a home. Solar heat gain is commonly measured by a value known as solar heat gain coefficient (SHGC). The SHGC varies between a value of 0 and 1, with 0 meaning no solar heat (or light) passing through a window and 1 equivalent to all solar heat passing through a window1. Practically speaking, commercially available windows are sold with a range of SHGCs between about 0.25 and 0.75.
In many climates, solar heat gain is a double-edged sword. In the winter, a window with a high SHGC allows sunlight to help heat the home, lowering heating bills. On the other hand, in the summer, a window with a high SHGC will warm up the house as well, which may lead to more money spent on air conditioning. This means windows must be chosen carefully with respect to SHGC. In practice, however, a higher SHGC is desirable in many colder and northern climates in the United States.
With the price of oil and natural gas setting record highs, the energy-efficient properties of high-performance windows can lead to substantial savings in a homeowner’s energy bills. Figure 1 shows how Super Windows can offer energy savings of $150 to $200 annually over a standard double-paned window. This is more than twice the savings possible from windows just meeting the minimum Energy Star standard.
Even in an adequately heated room, the choice of windows makes a surprisingly large impact on how comfortable a room is to its occupants. Anyone who has sat with their back to a window on a cold winter night will understand this intuitively-the air in the room might be warm, but their back will feel the chill of the outdoors. This is because their body heat radiates towards the interior surface of the window, which can easily be 30 degrees cooler than room temperature.
Many rooms also feel drafty in the winter, reducing the comfort of the room. If the windows in that room are cold to the touch, then they are a likely culprit of the room’s draftiness. This is because cold windows cool air near the room’s outside walls, causing it to fall to the floor. Warmer air drops then down from the ceiling to replace this air, setting an air current in motion throughout the room and making it feel drafty. This problem is worst when the temperature of a window’s inside surface is below 50°F. As Figure 2 shows, high-performance windows can increase the inside surface temperature of a window by more than 20 degrees compared to standard double-pane windows, improving comfort dramatically.
The improved comfort offered by super windows can translate into cost savings. With super windows, homeowners can often turn down the thermostat and still enjoy a more comfortable room than before. Furthermore, in situations where a home’s current heating system is unable to adequately reach all parts of the house, high-performance windows can improve comfort in trouble spots without a costly overhaul of the entire heating system.
On a cold winter day, it’s not at all uncommon to see water condensation or even frost on the inside surface of many standard windows. Like draftiness, this phenomenon is related to the surface temperature of a window. When a window gets sufficiently cold, it causes water in the air to condense onto the surface of the glass, often making it hard to see outside. Prolonged accumulation of moisture on a window sill as a result of condensation can lead to wood rot and other material failures as well as mold growth. The problem of condensation is easily solved with more energy-efficient windows, which have much higher interior surface temperatures.
Many of today’s super windows do not cost appreciably more than windows just meeting the Energy Star standard, especially when one considers the other costs associated with replacing windows, such as installation costs, painting and labor. These high-performance products offer significant improvement in comfort and aesthetics, and their additional energy savings can pay for themselves in just a few years.
Figure 1 shows the potential annual energy savings that could be achieved by replacing double-pane windows in an existing home with a more energy-efficient product. Savings are estimated for a typical existing 2000-square foot home with 300 square feet of windows.
Please refer to Table 1 for performance properties of the windows presented in this chart. Savings estimates are based on energy simulations and fuel prices available at http://www.efficientwindows.org.
Joshua Apte, Dariush Arasteh and Christian Kohler serve as research associate, staff scientist and engineer respectively for Lawrence Berkeley National Laboratory.
© Copyright 2006 Key Communications Inc. All rights reserved. No reproduction of any type without expressed written permission.