Read the NRFC Label
Any replacement window you purchase should come with a label from the National Fenestration Rating Council (NFRC). The NFRC is a program established by the U.S. Department of Energy to help consumers compare window products and options. Window manufacturers participating in the program are required to label every window to its specific thermal performance level. Customers are then ensured that the products they select meet the requirements for their application. Participation in the NFRC program is voluntary. Not all manufacturers participate because it requires outside third party inspection and extensive product testing.
The NFRC conducts testing and has established industry standards for rating the energy performance of replacement windows, including:
- Solar Heat Gain Coefficient
- Visible Transmittance
- Air Leakage*
- Condensation Resistance*
The most important measure of a window’s performance is its U-value. The U-value indicates the rate of heat flow through a window. The lower the U-value, the more energy efficient the window will be. U-value measures the entire window unit — glass, frame, sash, spacers — and is the only measurement accepted by the U.S. Department of Energy’s EnergyStar program.
U-value depends on:
- Quality of the materials used
- Type of materials used
- Engineering and design of the window
- Quality of assembly
R-value is the measure of the resistance of glass to the flow of heat. The higher the R-value, the better the glass will be at insulating your home. But R-value is NOT an accepted form of measurement by the NFRC or EnergyStar, as it does not measure the overall window unit.
Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits.
Visible Transmittance (VT) measures how much light comes through a product. The visible transmittance is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.
Air Leakage (AL) is indicated by an air leakage rating expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.
Condensation Resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation. While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation formation. CR is expressed as a number between 0 and 100.
*This rating is optional and manufacturers can choose not to include it.
Choosing a replacement window type and style is a big decision. A Plus Roofing has assembled a list of words and terms you may hear when replacing your home’s windows.
Air Infiltration. Amount of air that passes between a window sash and frame. In windows it is measured in terms of cubic feet or air per minute, per square foot of area. The lower the number, the less air the window lets pass through.
Aluminum-clad window. Window with wood construction covered with aluminum sheet. Has a factory-applied finish to protect from weather and solar degradation.
Apron. Horizontal trim board under a window stool.
Argon Gas. An odorless, colorless, tasteless, nontoxic gas which is six times denser than air. It is used to insulate better and reduce heat transfer.
Awning window. Hinged at the top and swinging out at the bottom, operated by a cranking mechanism. Usually rectangular, and wider than they are long.
Bay window. A series of usually three windows assembled in a polygon shape that projects outward from the side of a building, normally extending to the ground.
Bow window. Projects from the side of a building, normally with opaque elements extending to the ground, like a bay window, but in this case usually composed of a series of five window units assembled in an arc, rather than a polygon.
Casement window. A window unit hinged at the side and swinging outward, often operated by a cranking mechanism. Usually more energy-efficient because they have fewer moving parts and seal tighter than double-hung windows. In-swinging are French in origin while out-swinging are from England.
Center-hung sash. A sash that pivots on pins in the middle of the sash stiles and sides of the window frame to allow access for cleaning from the inside.
Check rail (also meeting and lock rail). Horizontal members of a double-hung window which come together.
Condensation Resistance Factor. A measure of the effectiveness of a window or glazing system to reduce the potential for condensation. The higher the condensation resistance factor, the more energy efficient the window and glazing system, generally.
Desiccant. Material used in insulating glass to absorb water vapor which causes fogging.
Diffusing glass. Glass with an irregular surface for scattering light; used for privacy or to reduce glare. Also called “obscure” glass.
Divided Lites. Division of glazing by the use of muntin bars.
Dormer window. Window in a wall that either projects from a sloping roof, or is recessed (inset dormer) into the roof, or a combination of both.
Double-hung window. Has two vertically moving sashes, each opening and closing a different part of the window. A single hung window has only one moving sash, usually the bottom half of the window.
Double-hung Sash Window. A design invented in 17th-century Holland comprising two panels that slide up and down in vertical grooves with the aid of cords or chains concealed in the window jamb.
Double-strength Glass. Glass with a thickness of approximately 1/8″.
Edge of Glass U- and R-values. The U- and R-values measured from the frame of the glass to 2-1/2″ from the frame.
Fixed light (also fixed sash). Window which is non-operative (doesn’t open).
Flat Casing. The exterior trim applied to the side jambs and header on wood and clad units.
Frame. The stationary portion of a window installed into the rough opening in a wall, enclosing the sash (operating and/or stationary).
French Window. A casement window that extends from the ceiling to the floor and features glass panes that run its entire height. Introduced at Versailles in the seventeenth century.
Gas-fill. An inert gas, usually argon, sealed between the panes of glass in a window instead of air. The gas is a far better insulator than just air, thus further increasing the thermal value of a window.
Head casing. Top or upper member of any element or structure. In windows, it refers to the top of the frame.
Header (also lintel; beam). Supporting member or beam above window opening which transfers building weight above to the supporting wall structure on each side of the window. The term header is generally in reference to a wood beam, whereas “Lintel” often refers to a steel or masonry beam.
Header. The horizontal top of the frame.
Horizontal sliding window (also horizontal slider). Windows which slide horizontally.
Insulated glass. Two or more panes of glass separated by insulation at the edges and air (or a more insulating gas) in the middle to provide greater thermal efficiency to a window.
Jalousie windows (also louvered windows). A window composed of overlapping narrow glass, metal, or wooden louvers, operated with a crank handle for adjusting the louver angles.
Jamb. A side jamb is the vertical molding of a window; the head jamb is the horizontal molding at the top. The bottom framing member is referred to as a sill.
Krypton Gas. An inert, odorless, colorless, tasteless, nontoxic gas that is about 12 times denser than air. It is used to replace air between the glass panes to reduce heat transfer and deter convection. Used when a higher performance is desired than that produced with Argon gas.
Laminated Glass. Two or more pieces of glass bonded together over a plastic interlayer. (See impact glass.)
Light or Lite. A window; a pane of glass within a window. Double-hung windows are designated by the number of lights in their upper and lower sashes, as in “6-over-6.”
Light-to-Solar-Gain Ratio (LSG). Ratio of visible transmittance to solar heat gain coefficient.
Lintel. Horizontal member (wood, steel, or stone) over a window opening to support the weight of the wall above. A header.
Low E (Emissivity) Glass. Glass with a transparent, usually multi layer, metallic oxide coating applied onto or into a glass surface. The coating allows short-wave energy to pass through but reflects long-wave infrared energy which improves the U-factor. Properly placed in a window, a “high-solar-gain” low-e coating can cut the loss of heat during the winter. A “low-solar-gain” low-e coating cuts the admission of solar radiant heat during the summer by reflecting this heat back to its source, thus providing year-round savings by lowering utility bills.
Mullions. Vertical members between window units. They are sometimes confused with muntins, which are secondary framing members that hold multiple panes of glass in the sash. Other parts of the sash include stiles (the outside vertical members) and rails (the top and bottom horizontal members).
Muntin (also sash bar; window bar; glazing bar). A secondary framing member (horizontal, vertical, slanted) to hold the window panes in the sash. Often confused with “mullion”.
NFRC label. NFRC stands for the National Fenestration Rating Council. This non-profit organization sets energy certification and labeling standards for windows in the United States.
Obscure Glass. Glass that has been made translucent instead of transparent, usually by its diffuse optical quality.
Palladian Window. A window with three openings, the central one arched and taller and wider than the others. Popularized by the Renaissance Italian architect Andrea Palladio.
Pivot window units. Window units in which the sash hardware is located near the midpoint of the stile or rail to permit sash rotation.
R-Value. A measure of insulation value. The higher the R-value, the better insulated are windows, walls, and ceilings. R-Value is the reciprocal of the U-factor.
Rough Opening. The hole in the wall where a window or door unit will be installed. Openings are larger than unit size to allow room for insulation and shimming the unit square.
Sash. The entire window, including the glass and the surrounding pieces that hold it together. The sash fits into a frame that is actually tied into the surrounding wall and holds the sash in place.
Sill. The horizontal bottom of the frame.
Single-strength Glass. Glass with a thickness of approximately 3/32″.
Slider Window. A window in which the sash moves horizontally. Sliders are available in a 2- or 3-lite configuration, with the 3-lite having operable end vents.
Solar Heat Gain Coefficient (SHGC). Ratio of the solar heat gained through a fenestration to the solar radiant heat incident on the fenestration. Solar gain per unit area of window divided by average solar irradiance incident on the window.
Tempered glass. Special heat-treated, high-strength safety glass which shatters into pebble-sized particles and not in slivers.
Total Unit U- and R-values. The U- and R-values of the window calculated from the average of the center of glass, edge of glass and frame U- and R-values.
U-factor. A measure of the rate of heat conduction through a surface—a wall, ceiling, or window. The lower the U-factor, the better job a window does in keeping out heat and cold by conduction.
Visible Light Transmittance or VT. The fraction of light that is transmitted through glass in the visible spectrum (380 to 720 nanometers). The higher the number the higher the fraction of visible light incident on the window that is transmitted.
Glass Package Options
When it comes to replacement windows; your options include the frame material, type of glass and how many layers of glass to have in the frame. Since 80% of a window is glass, most energy savings come from improved glass performance, not a high R-value of the frame.
Single, Double or Triple Pane Glass
The biggest choice in glass is the number of panes of glass: one, two or three. The decision often comes down to a choice of weighing your initial investment against expected long-term benefits.
A single pane window is the least expensive alternative. They are also the easiest to repair and weigh the lightest too. But a single pane window won’t do much to lower heating bills as they permit heat to escape easily.
Most homeowners now opt for insulated glass when replacing existing windows, with either double or triple window panes. A double pane window with inert gas between the panes is a big improvement over a single pane window. But a quality window with three panes, inert gas fill, and low-e glass will always have the lowest U-factor, an important measure of insulation value.
Dual pane windows are made up of two pieces of glass separated by an airspace. The panes are physically sealed together, and cannot be separated. Dual panes can be made up of countless combinations of glass and spacers to make the overall thickness anywhere from 7/16″ all the way up to 1 and 1/4″.
A triple pane glass package consists of three panes of glass separated by two sealed chambers of inert gas, usually argon or krypton. This creates a glass “sandwich” that provides substantially more insulating value than a single pane of glass, or even a double-glazed window. Insert gases like argon and krypton are used because their molecular composition make them particularly effective at blocking energy transfer.
In general the more airspace that there is in the unit the better the insulation the window provides. There are other factors that can improve the efficiency of your windows. Among the best options you can have on your windows is a low-e coating. This will allow your windows to reflect much of the heat and UV radiation away from the inside of your home.
Low-e glass is short for low-emissivity glass. It has a very thin coat of material on the glass to make it more efficient. It helps reflect standing heat away from the surface of the glass, keeping unwanted heat out in the summer and desired heat inside in the winter. Low-e glass is the most cost effective way to increase the energy efficiency of the windows.
Low-e coatings can also help reduce furniture and carpet fading by reducing the amount of ultraviolet (UV) radiation that enters the home. Harmful ultraviolet radiation can alter the chemical structure of dyes and other colorants in carpets and furniture causing fading.
Low Conductive or “Warm” Spacers
In order to provide proper insulation the layers of glass in a window must be appropriately separated by spacers. In the 1960s, window manufacturers began using aluminum spacers. They soon realized because aluminum is an excellent heat conductor when used as a spacer for most edge systems there was a major heat loss at the edge of the insulated glass unite, reducing the energy efficiency benefits. This heat loss also made the windows susceptible to condensation.
Modern windows use spacers made of thermoplastic or other advanced materials that will not conduct heat and cold like aluminum, helping to further reduce heat convection and energy transfer through the window.
Can New Windows Actually Save You Money?
Q: Do replacement windows really pay for themselves or is that just a sales line?
A: It’s true, if you select high-quality, energy-efficient windows. Savings will vary, but expertly engineered and well-built windows lower home energy consumption. With vinyl framed or fiberglass framed windows, maintenance is also virtually eliminated. No need to scrape and paint windows. These energy and maintenance savings will allow you to recoup your window investment over time.
Q: How can energy-efficient replacement windows save me money?
A: Replacement windows with insulated frames and energy-efficient glass packages can dramatically help prevent the transfer of heat and cold into and out of your home all year long. By controlling this natural conduction, you can better manage the comfort level in your home. You can save money and realize a significant return on investment by lowering the amount of energy it takes to heat or cool your home throughout the year.
A: Just like roofing and siding, windows need to be replaced every 20 years or so. Make it a part of your yearly home maintenance checklist to carefully evaluate your windows. Older windows can lose efficiency, causing heating and cooling bills to rise. When it’s time to replace the windows, look for upgraded window packages that are Energy Star® qualified. These can help lower your monthly energy bills.
Q: How do I evaluate my windows?
A: To determine if your current windows are energy-efficient, here are a few easy do-it-yourself steps:
- Look for condensation between the panes of glass on double- or triple-glazed windows. This could indicate seal failure. If this is the case, you might need to replace the glass or the entire window.
- Check to see if there are any faded areas on your carpets or furniture where the sun’s ultraviolet rays have caused damage. This is an indication that your windows need spectrally selective Low E glass to block the sun’s harmful rays.
- Research to determine if you have the appropriate glass package for where you live. For example, if you live in a cooler northern region you’ll find increased performance in the insulation properties of a window with low U-value. If you are located in a warmer region you’ll benefit from a lower solar heat gain coefficient to help keep the sun’s heat from entering your home. For more information visit Energy Star.
Q: What window frame type is the most energy efficient?
A: Vinyl or fiberglass window frames with multi-chambered construction and fusion-welded corners are one of the most energy-efficient available. Insulated frames are especially effective when combined with a Low E glass package.
Q: What kind of glass should I get in my replacement windows?
A: At a minimum, you’ll want to start with Low E glass in your window. Double-pane and triple-pane windows offer additional layers of insulation. Make sure your multi-pane windows are filled with an inert gas, like Argon or Krypton, between the panes of glass. This gas fill acts as a type of barrier and helps prevent the transfer of heat or cold into your home. Controlling the amount of convection between the panes of glass will help you use less energy to heat or cool your home.
Q: Which is more important in saving energy, the frame or the glass?
A: Seventy percent of an average window is glass; therefore, your biggest opportunity to save on energy comes from the glass. Remember, however, that the frame still makes up 30% of the window. Quite often, a replacement window will feel drafty around the edges. A frame that lacks insulating value generally causes this problem. That’s why every vinyl window we install includes a frame that is foam filled, giving it superior insulating ability.
Q: Why should I consider getting replacement windows?
A: Modern replacement windows are durable and help you lower monthly energy costs, increase the value of your home, reduce maintenance hassles and add beauty to the interior and exterior of your home. According to the U.S. Department of Energy (DOE), the typical home loses more than 30% of its heat through windows. The DOE reports that the best replacement windows have moderate to high R-values, a measure of both the construction of the window and the materials used versus heat flow. That translates into reduced heat loss and greater thermal efficiency that can help you save money and reduce energy costs. Another reason to consider replacement windows is that they can be custom made to fit your design and remodeling requirements.