In a typical residence, approximately 25 to 30% of heating and air conditioning is lost or gained through the windows¹. Energy dissipation occurs via conduction, radiation, convection currents, and air leakage. However, energy-efficient windows offer an effective strategy to counteract these processes and significantly reduce energy consumption.
The effectiveness of energy-efficient features varies depending on the climate, the window’s orientation, and how it integrates into the overall home design. Therefore, homeowners should carefully assess which window types best align with their specific needs. Organizations such as the National Fenestration Rating Council (NFRC), ENERGY STAR, and the U.S. Department of Energy (DOE) provide ratings for windows based on their efficiency and suitability for different regions and applications. Many of the guiding principles for selecting efficient windows also apply to skylights and glass doors.
Key Specifications
- U-factor quantifies the rate at which heat is conducted through a window. This rating can apply to the entire window fixture (including glass, frame, spacers, etc.) or solely to the glass itself. A low U-factor indicates that a window is well-insulated and, consequently, more energy-efficient. A rating of 0.30 or below is considered very good.
- Solar Heat Gain Coefficient (SHGC) represents the fraction of solar heat energy that permeates a window. SHGC can enhance indoor climate control through the installation of high-SHGC (greater than 0.55) windows in cold climates and low-SHGC (less than 0.40) windows in warm climates. Windows with a medium to high SHGC are most appropriate for homes in Pennsylvania. Allegheny County falls within ENERGY STAR’s Northern Climate Zone.
- Visible Transmittance (VT) measures the amount of visible light that can pass through a window. VT is scaled from 0 to 1, where 0 signifies no light transmission and 1 represents complete light transmission. A high VT can diminish the need for artificial indoor lighting. Coatings, glazes, and tints can reduce VT—unless they are spectrally selective (meaning they selectively screen out certain wavelengths of solar radiation, primarily infrared, while allowing most visible light to pass through).
- Light-to-Solar Gain (LSG) is the ratio of VT to SHGC. A high LSG rating is assigned to a window that permits a significant amount of light to pass through relative to the amount of heat.
- Low-emissivity (low-e) glazes are transparent metal-oxide insulators that effectively block longer wavelengths of energy while still allowing visible light (shorter wavelengths) to pass. Low-e windows are highly effective at retaining heat inside or reflecting it outside a building, depending on the prevailing climate. In fact, adding a low-e coating to a window provides as much additional insulation as incorporating an extra pane of glass! Glazes can also help prevent condensation on the interior surface of windows, as well as fading of fabric, paper, or wood furnishings. They are typically applied to one of the two inner pane surfaces, based on climate requirements and manufacturer specifications.
- Air leakage: Minimizing air leakage is always beneficial. Windows should ideally have a leakage rating of less than 0.30 cubic feet per minute per square foot of window area and must be installed correctly to prevent leaks around their edges. Windows that open outwards, such as awnings or casements, generally form a tighter seal than sliding windows.
- Gas fills: Some double-paned windows are manufactured with an inert gas, such as argon or krypton, in the space between the panes because these gases are superior insulators compared to air. Manufacturers may utilize a blend of gases, as argon is more economical, but krypton offers greater effectiveness.
- External factors: Window efficiency considerations are not solely limited to window specifications. Shading windows with overhangs or strategically placed plants can help keep a house cool in the summer when the sun is high. Curtains or shutters can also be used to block light or heat as desired. Whenever feasible, window orientation should be integrated into the home’s design. For instance, in colder climates, high-SHGC windows should primarily face south to maximize heat capture when the sun is low in the sky during winter. North-facing windows, conversely, tend to provide light rather than heat. Tubular daylighting devices, which channel sunlight through a reflective tube, can be an excellent way to introduce natural light into rooms where traditional windows or skylights are not practical.
Cost and Maintenance
Window prices are more dependent on type and size than any other factor, leading many manufacturers to now offer comparably priced efficient windows. With this in mind, energy-efficient windows represent an investment in a building. Even when they come at a higher initial cost, they typically pay for themselves within a few years through reduced heating and cooling expenses—and will continue to save money throughout their lifespan.
Strategically placed windows can decrease the need for artificial lighting by allowing ample natural light, which further contributes to energy cost savings. If efficient windows are incorporated into a new construction project, a smaller, more affordable heating and cooling system may suffice for the building.
For homeowners not looking to invest in new windows, several actions can significantly improve the efficiency of existing ones. Air leakage can be reduced by applying caulking or adding weather-stripping material around window edges, or by installing storm windows. Homeowners can also affix low-e coatings and window films to increase a window’s efficiency, although these might impart a slight tint or a mirror-like appearance to the glass.
Advantages and Disadvantages
| Feature | Advantage | Disadvantage |
| Long-term Savings | DOE-estimated annual savings for Pennsylvania are $401 for replacing single-pane windows and $166 for double-pane replacements, plus the benefits of a smaller carbon footprint. | |
| Home Comfort | Efficient windows lead to less temperature fluctuation indoors and fewer drafts. | |
| Upfront Costs | As an investment in a house, efficient windows may have a higher initial price tag. | |
| More Complex Planning | Different considerations are necessary for various windows within a house, but this advanced planning ultimately yields significant rewards. |