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Windows
Components | Frames | Materials | Operation | Glass | Gas Fill | Glazing | Choosing Windows for Optimal Performance | Energy Efficiency Ratings for Windows | Heat Gain and Loss | Sunlight Transmittance | Summary of Window Ratings | Energy Star Window Prescriptive Requirements by Climate | Federal Tax Rebates for WindowsQuick Facts
- It is most cost effective to tighten a home’s building envelope by air sealing and insulating before replacing windows
- Properly selected windows will insulate a home as well as allow the appropriate amount of solar light and heat to enter the home
- 25% of a home’s heat loss can be due to windows
- Upgrading from single pane or simple double pane windows to high performance windows can significantly lower heating and cooling costs
- Different windows are best for different applications; standardized efficiency ratings help determine which windows are appropriate for a specific application
Components
Windows have varying qualities and components which aid (or inhibit) the window’s capacity to insulate a home from outside temperatures and to filter, reflect, or transmit light. Different window configurations are suitable for different climates and home orientations. A window is generally selected for a particular situation based on four different efficiency rating values.
Frames
In addition to affecting aesthetic value, window frame materials and operation style affect insulation and air leakage of windows.
Materials
Different frame materials provide varying levels of insulation and durability.
- Aluminum or Metal: sturdy and durable and range from cheap to architectural. Unfortunately metal is a very poor insulator and makes windows susceptible to heat loss. Some of the heat loss can be mitigated by internal “thermal breaks” (plastic spacers).
- Composite: made of wood products and binders, such as particle board and laminated strand lumber. Composite frames insulate at least as well as conventional wood and are more durable
- Fiberglass: a stable material with superior insulation qualities compared to wood or vinyl; they offer insulation comparable to that of insulated vinyl and durability and ease of maintenance to that of aluminum
- Vinyl: usually made of PVC plastic, vinyl frame have great insulating value but can warp in hot temperatures and crack in cold temperatures a; vinyl tends to fade with extended exposure to sunlight
- Insulated Vinyl: unlike standard vinyl, these frames have cavities filled with insulation, making them better insulators than standard vinyl or wood
- Wood: a good insulator but very susceptible to weather conditions which make them expand and contract and make wood frames need the most maintenance
Operation
Frames seal more or less well depending on their operation style and design/construction quality. It is important that frames themselves be thoroughly sealed when installed to prevent air leakage.
- Swinging Windows: create a tighter seal when closed because the window presses down on the frame
- awning: swing upwards
- casement: swing sideways
- hopper: swing downwards
- Fixed Windows: cannot be opened and do not create air leakage, but do not allow for any ventilation
Sliding windows: generally allow more air leakage than swinging windows
- single or double hung: slide upwards
- single or double sliding: slide sideways
Glass
Many technological improvements have been made over the single pane and even double pane windows of several years ago.
Gas Fill
In contrast to single pane windows, double pane (and triple pane) windows have more than one pane of glass with a space between the panes. The space can contain different invisible gases with varying insulating properties.
- Air: the least expensive double pane windows simply contain air which provides the least insulation
- Argon: an inert gas which is inexpensive, nontoxic, nonreactive, clear, and odorless; it the most commonly used alternative to air and provides better insulation than air
- Krypton: also an inert gas with the same basic properties as argon; it is more expensive but has a better thermal performance (it provides superior insulation compared to argon)
- Xenon: similar to krypton, yet more expensive and better performing. Examples include Serious Windows 1125 Series.
Glazing
The amount of heat or light which passes through glass can be altered in different ways by tinting the glass itself or by adding glazing or film to the panes.
- Insulated Window Glazing: simply refers to windows with two or more panes of glass (known as double pane and triple pane); the glass panes are spaced apart and hermetically sealed to form a single-glazed unit with an air space between each pane of glass, improving the insulation value of the window. Multi pane window have lower U-factors
- Tinting: pigments can be added to glass to filter all or part of the visible light spectrum. The effect varies with the color of the pigment.
- Reflective glass: has a coating which reflects more light than heat and is not appropriate for most residential applications
Low-e: refers to film with low emissivity, which means that the surface reflects radiant energy instead of emitting it through the surface. Low-e windows have lower SHGC . Low-E coatings typically cost about 10%–15% more than regular windows, but they reduce energy loss by as much as 30%–50%.
- spectrally selective coating: filter out 40%–70% of the heat normally transmitted through insulated window glass or glazing, while allowing the full amount of light to be transmitted; create a window with a low U-factor and SHGC but a high VT
- Suspended film glass: an lighter weight, more efficient, and more versatile alternative to triple pane glass, where a layer of high performing film is sandwiched between two window panes and air pockets. Manufacturers include Serious Windows.
Choosing Windows for Optimal Performance
Windows are an expensive investment. For the best return on investment, it is important to compare costs over the long term, including durability and energy savings potential for the windows.
In addition to budget and style, windows should be chosen based on climate, shading, and orientation using standardized ratings. Energy Star windows qualify for the energy efficiency label by adhering to certain values for these ratings which vary by climate.
Manufacturers of High Performance Windows include:
Search for Energy Star qualified windows.
Energy Efficiency Ratings for Windows
The NFRC label, found on all Energy Star windows (the ratings also apply to doors and skylights), contains four independently tested ratings which are summarized below. They measure:
Heat Gain and Loss
Windows, doors, skylights can gain and lose heat in the following ways:
- Direct conduction through the glass or glazing, frame, and/or door
- The radiation of heat into a house (typically from the sun) and out of a house from room-temperature objects, such as people, furniture, and interior walls
- Air leakage through and around them.
These properties can be measured and rated according to the following energy performance characteristics:
- U-factor: measures direct conduction of heat through the glass and frame. Directly related to the R-value used to rate insulation, U factor is the inverse of R-value (U=1/R); U values below 0.50 are in the same range as R-values above R-2, so the lower the better for U-factor values. NFRC ratings refer to whole window performance (including glass, glazing, frame and spacers).
- SHGC: a number between 0 and 1 which measures the fraction of solar radiation admitted through a surface. The lower the SHGC, the less solar heat it transmits and the greater its shading ability. Appropriate SHGC varies with exterior shading, orientation (the direction the windows is facing), and climate.
- lower SHGC: generally appropriate for warm climates where it is most important to avoid heat gain from the sun during the summer
- higher SHGC: generally appropriate for colder climates where it is more important to collect solar heat during the winter, which passively warms the home
- Air leakage: measures the rate of infiltration around the window at a specific pressure. Lower air leakage means higher efficiency
Sunlight Transmittance
A window’s ability to transmit sunlight into a home can be measured and rated according to the following energy performance characteristics:
- VT (Visible Transmittance): a number between 0 and 1 which measures the fraction of visible light transmitted through the window glazing. Values closer 1 indicate more natural light (tinted windows have low VT values); appropriate VT values for windows may vary throughout the home based on the need for natural daylighting versus the need to reduce glare.
- LSG: this is simply the ratio of SHGC and VT (LSG=SHGC/VT) and is not always on the NFRC label. The higher the number, the more light transmitted without adding excessive amounts of heat.
Summary of Window Ratings
| Rating | U-factor | SHGC | VT | Air Leakage |
| Typical range | 0.25 to 1.25 | 0.25 to 0.80 | 0.20 to 0.80 | mostly ≤ 0.3 |
| Units | Btu/h·ft 2 ·°F | N/A | N/A | cf·m/ft 2 . |
| Energy Star Req. | varies by climate | varies by climate | N/A | N/A |
| higher value means | more heat loss | more solar heat absorbed | more visible light | poorer insulation and more heat loss |
| factors which increase the value | single pane, metal frames, air between panes | plain glass | spectrally selective low-e film and tinting | sliding frames |
| lower value means | better insulation | less solar heat absorbed | less visible light | better insulation and less heat loss |
| factors which decrease the value | composite, fiberglass or insulated glazing (> 1 pane), inert gas between panes | low-e film | bronze or gray tinting | casement or awning frames |
Energy Star Window Prescriptive Requirements by Climate *
Homes in colder climates benefit from passive solar heat gain (higher SHGC) and need better protection from heat loss (lower U-factor). Homes in hotter climates have higher cooling costs and benefit from restricted solar heat gain (lower SHGC); while insulation is still important in warmer climates, it is less critical so U-factor values may be higher.
| Climate | U-factor | SHGC |
| Northern | ≤ 0.35 | Any |
| North/Central | ≤ 0.40 | ≤ 0.55 |
| South/Central | ≤ 0.40 | ≤ 0.40 |
| Southern | ≤ 0.65 | ≤ 0.40 |
| clear single pane** | 1.1 | 0.87 |
| air filled double pane** | 0.50 | 0.76 |
** Note: not Energy Star qualified; provided for reference.
Federal Tax Rebates for Windows
Federal tax rebates for high performance windows, as for other home energy efficiency measures, amount to 30% of the cost of materials up to a maximum of $1500. To qualify for rebates windows must meet the following requirements:
- U value greater than 0.3
- SHGC less than 0.3