How to buy an ENERGY STAR skylightWhen considering an eco-friendly, ENERGY STAR skylight or solar tube purchase, first consider what style you'll choose. A traditional skylight is usually rectangular in shape and is essentially a roof-mounted window. You can actually see the sky through these types of skylights.
But a new innovation in skylights is the tubular skylight, also known as a sun tube. These illuminators are reflective-lined tubes that channel daylight from the rooftop into the interior of a building. They're easier to install and less expensive than traditional skylights, although they don't allow an unobstructed view like their clear-glass counterparts.
After you've determined the style of skylight you'll install, think over the following feature possibilities:
- Interior blinds: To cut back on potential heat gain through your skylight, you may wish to have blinds pre-installed between the panes of glass.
- Venting options: Some skylights are sealed while others open like windows that can be louvered manually with a chain or crank or automatically by remote control. Some even include rain sensors that close the window when moisture is sensed. Since hot air rises, skylights can provide much-needed ventilation on summer days. Like a chimney, your vented skylight lets hot air escape providing natural cooling.
- Plastic or glass?: If you're in the market for a dome-shaped model, you may have to opt for a plastic skylight. These have the benefit of keeping debris and snow from accumulating, but plastic is more prone to scratches, discoloration, and stains, so does not last as long as glass skylights.
- Glass tinting: Bronze, gray, or high-performance (spectrally selective) tinted glass can reduce solar gain, which is especially helpful during hot summer months. Because these tints have no effect on the U-factor, they aren't recommended for colder-climate areas.
- Gas fills: Take advantage of the two panes of glass on your new skylight by filling them with gas, which will further boost their energy efficiency. Argon and krypton are the two most common options, either of which can be used on their own or in combination. Argon is less expensive than krypton, but since krypton has better thermal performance, it is preferred in colder regions.
- Low-e films or coatings: If you're concerned about heat gain, be sure to look into low emissivity (low-e) options to further cut back on energy use. Low-e glass has a thin, invisible layer of metal or metal oxide deposited on the glass, which reflects warmth into a building during cold months and keeps heat from entering a building during warm months. To see how it works, check out The Efficient Windows Collaborative Low-E Coatings illustrations. But if you've got an existing skylight in need of a facelift, consider window films, which can be applied to existing windows to accomplish the same purpose.
- Seal it up!: New or old, you'll want to ensure that your skylight is properly sealed so that you're not allowing heat energy to freely pass through your skylight.
Proper positioning of your skylight
Your skylight should add illumination without unwanted solar heat gain. To avoid excessive fluctuations in temperature, pay attention to your skylight's slope and position on your roof:
- Roof position: Those in warmer climes should choose to mount the skylight on the roof's north face where little passive heating will occur; south-facing is best for cooler climates where heat gain is desirable. An east-facing roof will provide lots of light and morning warmth, while a west-mounted skylight will bring afternoon sun as well as some passive solar heating.
- Slope: Generally, the less your roof slopes, the greater the heat gain during the summer and the lower in the winter (not exactly what you want). That's because the sun is higher on the horizon during the summer and lower during the winter. Your skylight should therefore be tilted to miss the high-noon sun during the summer but low enough to get the rays in the winter. A tried-and-true rule of thumb: tilt your skylight so that it equals your geographical latitude plus five to 15 degrees. So, if you're in San Francisco which is 37 degrees north, you'll want to position your skylight between 42 and 52 degrees. Many manufacturers produce pre-tilted skylights to help you achieve the perfect position.
Find it! ENERGY STAR skylightsHere's just a peek at what's out there in the land of skylights. Each one will add beauty and the free light of the sun, and if installed properly, should save you money on heating, cooling, and lighting. Just be sure to choose an ENERGY STAR model to ensure energy savings upwards of 40 percent compared to conventional models.
Before you buySelect a skylight with U-factors suitable to your area: Be sure to get a model with the correct U-factor, which is indicated on a new unit's label. In general, a 0.35 is best for colder locales; you can get away with a U-factor of 0.65 in warmer climes. Check out ENERGY STAR's Climatic Zones map or Efficient Windows Collaborative Window Selection Tool to find out which windows are best for your specific region.
Buying an ENERGY STAR skylight or solar tube helps you go green because
- They provide natural, energy-free lighting.
- They can help warm a space in the winter and cool it in the summer, thus reducing both heating and cooling costs.
Lighting energy savingsNatural sunlight provides somewhere between 7,000 and 10,000 foot-candles of light, which is far more than the 50 foot-candles needed by the average person for day-to-day activities. Even cloudy days result in 5,000 to 6,000 foot-candles of illumination hitting Earth's surface. Although sunlight provides as much heat as it does illumination, if harnessed efficiently through smart daylighting design, it can significantly decrease lighting, cooling, and even heating energy costs.
Lighting energy requirements decrease between 30 percent to 80 percent when daylighting features are employed. And because the light-to-heat ratio is significantly better with daylighting than conventional electric lighting, it also cuts down on cooling costs by as much as 30 percent.
Heating and cooling savingsSkylights, windows, and doors contribute up to 30 percent of heat loss and gain in a home, by direct conduction through the glass and via air leakage through the skylight assembly. Skylights can contribute to heat gain and loss in a home by as much as 35 to 45 percent more than traditional windows. Through both convection and radiation, skylights can allow heat to escape during cold weather and enter a home during warm weather, creating uncomfortable drafts and causing air conditioners and heating devices to use more energy.
The US Department of Energy (DOE) estimates that heat loss and gain through today's skylights and windows account for approximately 4 percent of total domestic energy consumption. Efficiency strategies vary widely based on regional climate considerations: residential glass can account for 25 percent of a typical home's heating energy load in cooler climates, and as much as 50 percent of the cooling load for homes in warmer climates. ENERGY STAR skylights cut energy consumption by up to 40 percent compared to conventional skylights.
Double-pane skylights employ two glass layers with an air space (sometimes gas-filled) between to cut heat flow. The biggest benefit is a lower U-factor, but double-pane models can also cut a skylight's solar heat gain coefficient (SHGC). Low-e coatings can further cut a skylight's SHGC by upwards of 30 percent. Compared to coat-less skylights and windows, though, low-e coated models provide up to 75 percent more protection from ultraviolet light (UV) rays that would otherwise damage (fade or discolor) artwork, furniture, carpeting, or photographs with repeated exposure.
Tax breaks and subsidiesIn the US, upgrading your home's skylights and windows may qualify you for tax incentives at the federal, state, or local levels. For detailed information, see these resources:
- American Council for an Energy-Efficiency Economy: Updates on potential energy legislation.
- Tax Incentives Assistance Project: Explains federal tax credits for energy efficiency.
- Alliance to Save Energy: Offers an index of energy efficiency programs by state.
- Database of State Incentives for Renewables & Efficiency: Provides information on state and federal incentives.
- Contact your utility provider for information on local offers.
- convection: The process by which heat is transferred by moving air.
- double-hung windows: The two sashes on these windows both slide vertically. On single-hung windows, only the bottom sash slides upward. Both single- and double-hung windows tend to experience more air leakage compared to projecting or hinged windows.
- foot-candle: A gauge of light intensity measured in lumens per square foot, or the amount of light actually falling on a given surface.
- low-emissivity (low-e): A microscopically thin metal or metal oxide layer deposited directly on one or more panes of glass (window, skylight) to reduce heat transfer and subsequently the U-factor. This layer is transparent to the visible solar spectrum (short-wave infrared radiation) but reflects long-wave infrared radiation.
- radiation: Energy transfer that takes place through open space via electromagnetic waves, including light.
- solar heat gain coefficient (SHGC): Solar heat can be admitted through a door or window either by direct transmission or through absorption. SHGC measures the fraction of heat caused by sunlight on a scale between zero and one, smaller numbers indicating lower levels of heat transfer.
- U-factor: The inverse of the R-factor (a measure of a material's insulating ability; higher numbers indicate better performance), U-factor also measures the rate of heat gain and loss. However, the higher the U-factor number (values range from 0.20 and 1.20), the less the product is able to resist heat flow (i.e., the worse its insulating ability).
- ultraviolet light (UV): The invisible rays of the light spectrum found in sunlight that are not visible to humans. UV can cause fading of carpets, fabric, and paint finishes.
- Consumer Energy Center - Windows of the Future
- Efficient Windows Collaborative - Window Technologies: Glazing Types
- ENERGY STAR - Anatomy of an Energy-Efficient Window
- Rocky Mountain Institute - Home Energy Briefs: Building Envelope
- US Department of Energy - Windows, Doors, & Skylights
- Daylighting Cooperative - What is daylighting? Myth: Daylit buildings need clear glass windows
- GreenBiz.com - Harnessing Daylight for Energy Savings
- ENERGY STAR - Daylighting Web Conference: Energy Savings with Daylighting Page 9
- Daylighting Cooperative - What is daylighting? Myth: Daylighting lets in too much heat
- US Department of Energy Abstract: - Zero Energy Windows: Conclusion page 14
- ENERGY STAR - High Performance Windows
- Efficient Windows Collaborative - Window Technologies: Low-E Coatings
- ENERGY STAR - More Than Just Dollar Savings: Safeguarding valuable interiors