Choosing an Energy Efficient Window

Windows bring light, warmth and beauty into our homes while providing openness and space to living areas. Unfortunately, they can also be a major source of heat loss in the winter and heat gain in the summer. However, when properly selected and installed, windows can actually help to minimize a homes heating, cooling and lighting costs.

No one window is suitable for every application. Many types of windows and window films that serve different purposes are available. Moreover, you may discover that you need two types of windows for your home because of the directions that your windows face and your local climate. To make wise purchases, first examine your heating and cooling needs and prioritize desired features such as daylighting, solar heating, shading, ventilation and aesthetic value.

Eliminating air leakage
Air leakage through the centers, edges and frames of windows contributes to wasted energy. Eliminating or reducing these paths' air flow can greatly improve the energy efficiency of windows and, ultimately, of homes. Several options are available to reduce air leaks around windows.

Caulking
Proper application of caulking is an inexpensive solution to air leakage. By caulking the unwanted gaps around window frames and joints, air leakage can be significantly reduced, thus reducing the amount of energy wasted.

Before applying new caulk, old caulk or paint residue around a window should be removed using a putty knife, stiff brush or special solvent. After old caulk is removed, new caulk can then be applied to all joints in the window frame and the joint between the frame and the wall.

Caulk in warm temperatures so the caulk will set properly and adhere to the surface.

Weather stripping
Another cost-effective solution to wasted energy through air leakage is weather stripping. Weather stripping is a narrow piece of metal, vinyl, rubber, felt or foam that seals the contact area between the fixed and movable section of a window joint.

It should be applied between the sash and the frame, but should not interfere with the operation of the window.

What is an R-value?
Manufacturers usually represent the energy efficiency of windows in terms of their U-values (conductance of heat) or their R-values (resistance to heat flow).

If a windows R-value is high, it will lose less heat than one with a lower R-value.

Conversely, if a windows U-value is low, it will lose less heat than one with a high U-value.

Most window manufacturers use R-values in rating their windows performance. When comparing windows you should make sure
(1) that all U- or R-values are based on current standards set by the U.S. Department of Energys (DOE) ENERGYSTAR™ program or the National Fenestration Rating Council (NFRC)
(2) that values are calculated for the entire window, including the frame, and not just for the center of the glass
(3) that the values represent the same size and style of window

The following five factors affect the R-value of a window:

Low-emissivity (low-E) glass has a special surface coating to reduce heat transfer back through the window. These coatings reflect from 40% to 70% of the heat that is normally transmitted through clear glass, while still allowing the full amount of light to pass through.

Heat-absorbing glass contains special tints that allow it to absorb as much as 45% of the incoming solar energy, thus reducing heat gain. Some of the absorbed heat, however, passes through the window by conduction and reradiation.

Reflective glass has been coated with a reflective film and is useful in controlling solar heat gain during the summer. It also reduces the passage of light all year long, and, like heat-absorbing glass, it reduces solar transmittance.

Plastic glazing materials — acrylic, polycarbonate, polyester, polyvinyl chloride and polyethylene — are also widely available. Plastics can be stronger, lighter and easier to cut than glass. Some plastics also have higher solar transmittance than glass. However, plastics tend to be less durable and more susceptible to the effects of weather than is glass.

Layers of glass and air spaces
Standard single-pane glass has very little insulating value (approximately R-1). It provides only a thin barrier to the outside and can account for considerable heat loss and heat gain. Traditionally, the approach to improving a windows energy efficiency has been to increase the number of glass panes in the unit, because multiple layers of glass increase the windows ability to resist heat flow.

Storm windows can increase the efficiency of single-pane windows. For people with tight budgets, a less expensive option to replacement windows is to use storm windows. Even though storm windows add to the insulating performance of single-glazed windows that are in good condition, they can help to reduce air movement into and out of existing windows, and thus reduce heating and cooling bills. They are made for installation on the interior or exterior of the primary window.

They range from the inexpensive plastic sheets or films designed for one heating season, to triple-track glass units with low-e coatings that offer many years of use. No matter what type you choose, the storm window frame must be hung square with the primary window and sealed to the opening. Exterior-mounted storm windows must have "weep holes" at the bottom of the frame to allow any moisture that collects between the primary window and the storm window to drain out.

Double- and triple-pane windows have insulating air- or gas-filled spaces between each pane. Each layer of glass and the air spaces resist heat flow. The width of the air spaces between the panes is important, because air spaces that are too wide (more that 5/8 inch) or too narrow (less than 1/2 inch) have lower R-values (i.e., they allow too much heat transfer). Advanced, multi-pane windows are now manufactured with a less conductive, more viscous, or slow-moving gas (e.g., argon or krypton). By minimizing the convection currents within the space, conduction through the gas is reduced, and the overall transfer of heat between the inside and outside is reduced. The introduction of argon and krypton gas fills has lead to a considerable measure of improvement in thermal performance over traditional air-fills.

Though ideal for strength and customized window design, aluminum frames conduct heat and therefore lose heat faster and are prone to condensation. Through anodizing or coating, the deterioration of aluminum frames can be avoided. Additionally, the thermal resistance of aluminum frames can be significantly improved by placing continuous insulating plastic strips between the interior and exterior of the frame. These insulating strips will virtually halt temperature transfer.

Wood frames have higher R-values, are not as greatly affected by temperature extremes and do not generally promote condensation. Wood frames, however, do require considerable maintenance in the form of periodic scraping, painting and/or staining. If not properly protected, wood frames can swell, develop wood rot, warp and eventually become inoperable.

Fiberglass frames are relatively new and are not yet widely available. With some of the highest R-values, fiberglass frames are excellent for insulating and will not warp, shrink, swell, rot, or corrode. Some fiberglass frames are hollow while others are filled with insulation for greater heat gain protection.

Spacers are used to separate multiple panes of glass within the windows. Although metal (usually aluminum) spacers are commonly installed to separate the glass in multipane windows, they conduct heat. During cold weather, the thermal resistance around the edge of a window is lower than that in the center; thus, heat can escape, and condensation can occur along the edges. To alleviate these problems, many manufacturers have developed multipane windows using a butyl spacer along the perimeter and between both panes of glass. These spacers help to alleviate thermal transfer through the glass.