All about Energy Efficient Windows

A comparison of different window materials

Glossary of Window Terms

A Window is a Window is a Window……. right?

Buying windows can be very complicated.  There are dozens of manufacturers that make dozens of brands.  Then there are wood windows, vinyl windows, aluminum windows, and fiberglass windows.  Then there is hard LowE, soft LowE, LowE/2  (energy efficiency of glass), different gas packages, and different Solar Heat Gain Coefficients, U-Values and DP ratings.  Are the windows Energy Star rated?  Do they meet NFRC requirements for your climate?  Where does it all end?

BuyWindowsDirect.com eliminates the guesswork.  All of the windows we sell are Energy Star compliant and meet NFRC ratings for your climate.  To keep things simple we sell only vinyl windows as they are the only window systems that are truly maintenance free and give you the best value for your money.  Also, our windows provide a limited lifetime manufacturer’s warranty.  See our Products page for more information.

If you want more knowledge about the complexity of windows and Energy Star ratings or the NFRC certification, keep reading.

What’s all the fuss over Energy Efficient Windows?

When you’re choosing new or replacement windows, it’s always good to look for the Energy Star rating. 

The U.S. Environmental Protection Agency (EPA) launched the Energy Star program in 1992 to protect the environment through energy efficient products and procedures. In order to receive an Energy Star rating, products falling under 50 home and office categories must meet strict energy conservation guidelines set by the U.S. Department of Energy, which partnered with the EPA on Energy Star in 1996.

The first Energy Star labeled products were computers and monitors, but this line quickly expanded to include printers, fax machines, residential appliances and fixtures, windows and even new homes and buildings. According to Energy Star, Americans saved enough on Energy Star products in 2006 alone to save $14 billion on utility bills and avoid greenhouse gas emissions equivalent to that released by 25 million cars.

In addition to providing a rating system, Energy Star helps organizations and consumers practice informed decisions, delivering technical information and tools through more than 9,000 partnerships in the public and private sector. Energy Star’s efforts have led to more widespread use of LED traffic lights, efficient fluorescent lighting, energy management systems for office equipment, and low standby energy use.

Consumers can look for the Energy Star label when shopping for a new home or searching for new home products, such as refrigerators, dishwaters, and home heating systems. The label will show the product’s typical yearly energy usage. Examples of Energy Star labeled products include full-sized dishwashers that conserve 493 gallons of water per year and washing machines that save over 12,000 gallons of water per year. Appliances with the Energy Star rating average 10 to 20 percent higher energy efficiency than their non-rated counterparts. Items labeled “Energy Efficient” may have low energy emissions without meeting the standards to be rated as an Energy Star product.

Independently Tested and Certified Energy Performance

The energy performance of all ENERGY STAR qualified windows, doors, and skylights must be independently tested and certified according to test procedures established by the National Fenestration Rating Council (NFRC).

NFRC is a third party, non-profit organization that sponsors certified rating and labeling programs to help consumers compare the energy and performance features of windows, doors, and skylights.

NFRC's programs do not distinguish between “good” and “bad” windows, set minimum performance standards, or mandate performance levels. This is where ENERGY STAR comes in. ENERGY STAR enables consumers to easily identify NFRC-certified products with superior energy performance.

Read the NFRC label and see for yourself why a window with this label earns the ENERGY STAR in all Climate Zones.

Performance Ratings

The NFRC label, which can be found on all ENERGY STAR qualified windows, provides performance ratings in a number of categories:

U-Factor measures the rate of heat transfer and tells you how well the window insulates. The lower the U-Factor, the better the window insulates. U-Factor values generally range from 0.25 to 1.25. Find the ENERGY STAR U-Factor for your region.

Solar Heat Gain Coefficient (SHGC) measures the fraction of solar energy admitted and tells you how well the product blocks heat caused by sunlight. The lower the SHGC, the less solar heat the window transmits. SHGC is measured on a scale of 0 to 1; values typically range from 0.25 to 0.80. Find the ENERGY STAR SHGC for your region.

Visible Transmittance (VT) measures the amount of light the window lets through. The higher the VT, the more light you see. VT is measured on a scale of 0 to 1; values generally range from 0.20 to 0.80.

Air Leakage (AL) measures the rate at which air passes through cracks in the window. AL is measured in cubic feet of air passing through one square foot of window area per minute. The lower the AL value, the less air leakage. Most industry standards and building codes require an AL of 0.3 cfm/ft².

Condensation Resistance measures how well the window resists water build-up. The higher the condensation resistance factor, the less build-up the window allows. Condensation Resistance is scored on a scale from 0 to 100. The Condensation Resistance rating is useful when comparing different products. ENERGY STAR qualification is based on U-Factor and SHGC ratings only.

Air Chambers - Small honeycomb spaces within the sash and frame, which help to insulate and strengthen the window.

Air Infiltration - The 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.

Argon Gas - An odorless, colorless, tasteless, non-toxic gas, which is six times more dense than air. It is used to replace air between the glass panes to reduce temperature transfer.

Awning - A top-hinged window that swings outward for ventilation.

Bay Window - An angled combination of three windows that project out from the wall of the home. The windows are commonly joined at 30- or 45-degree angles.

Beveled Exterior - An angled extension from the frame that adds an aesthetically-pleasing dimension to the exterior of the window.

Bow Window - An angled combination of windows in 3-, 4- or 5-lite configurations. The windows are attached at 10-degree angles to project a more circular, arced appearance.

Cam Lock and Keeper - The mechanisms which pull the sash together when placed in the locked position.

Casement  - A window with a side-hinged sash that opens outward for ventilation.

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 efficient the window and glazing system.

Constant Coil Spring Balance System - Device for holding vertically sliding sash in any desired position through the use of a spring or weight to counterbalance the weight of the sash.

Dead-air Space - The space between the panes of glass of an I.G. unit.

Double Hung  - A window that has two operable sash which slide vertically.

Double-strength Glass - Glass with a thickness of approximately 1/8".

Egress Code - The code that requires a minimum opening of a window for persons to exit or firefighters to enter a building.

Extruded Screen Frame - Different from a Rollformed frame, this frame is pressed through a form or die.

Fusion-welded - The process of joining materials by melting them together with extreme heat (over 500ºF), resulting in the materials uniting into a one-piece unit.

Geometric - Specially designed windows classified as either Straight line Geometrics such as rectangles, triangles, trapezoid, octagons, pentagons, etc., or Radius Geometrics which include Half-rounds, Quarter-rounds, Circles, Ellipses, Eyebrows, etc.

Glass - An inorganic transparent material composed of sand (silica), soda (sodium bicarbonate), and lime (calcium carbonate) with small quantities of alumina, boric or magnesia oxides. Available Styles: clear, bronze tinted and grey tinted.

Glazing - The process of sealing the glass to the sash.

Grids - Decorative horizontal or vertical bars installed between the glass panes to create the appearance of the sash being dividing into smaller lites of glass.

Head - The horizontal top portion of the main frame.

Hopper - A window with a bottom-hinged sash that opens inward for ventilation.

I.G. Unit (Insulating Glass Unit) - Two or more lites of glass separated by a spacer and hermetically sealed at the glass edges.

Intercept^® Spacer System - Spacer system using a U-channel design to reduce the number of conduction paths.

J-channel - Integral extension on the outside of a new construction window that eases installation on siding applications.

Jamb - Vertical sections of the main frame.

Krypton Gas - An inert, odorless, colorless, tasteless, non-toxic gas which is about 12 times denser than air. It is used to replace air between the glass panes to reduce temperature transfer and deter convection.

Laminated Glass - Specially designed glass where two panes of glass are bonded to a durable interlayer, providing increased safety, UV protection and noise reduction. If the window or door gets broken the glass will adhere to the to the plastic interlayer-preventing glass fallout in the home.

Lift Handle - A handhold for raising and lowering the sash. Handle implies that the handhold is not continuous across the sash.

Lift Rail - A handhold for raising and lowering the sash. Rail implies that the handhold is continuous across the sash.

Lite - A unit of glass in a window.

Lock Rail - The horizontal section of the sash where the cam lock is attached.

Lock Stile - The vertical section of the sash where the cam lock is attached.

Low E (Emissivity) Glass - Glass with a transparent 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-value.

Main Frame - The head, sill and jambs sections of a window.

Meeting Rail - The horizontal sections of a pair of sash that meet when the sash are closed.

Meeting Stile - The vertical section of a pair of sash that meet when the sash are closed.

Mesh - Fabric made of either fiberglass or aluminum, used in the making of screens.

Mullion - A vertical or horizontal connecting unit between two or more windows.

Nailing Fin - An extrusion attached to the main frame of a window used to secure the unit to the rough opening.

Obscure Glass - Glass that has been made translucent instead of transparent.

Oriel - A window with the meeting rail located off center of the frame. Most oriels have a 60/40 configuration.

Overlapping and Interlocking Meeting Rail - A meeting rail which overlaps and interlocks both sash.

Patio Door - A glass door that slides open and close on adjustable tandem rollers. Available in 2- or 3-lite configurations with the operable panel available in any position.

Picture Window- A window that has no moveable sash.

R-value - Resistance a material has to heat flow. The higher the R-value, the greater the resistance.

Rollformed Screen Frame - A method of fabrication in which a flat (usually metal) material is placed on a machine where the material is formed into shape using differently shaped rollers and pressure.

Sash - The part of the window which contains the glass.

Shading Coefficient - The ratio of solar heat that is transferred through a glazing material relative to the solar heat transferred through 1/8" clear glass. The lower the number the more efficient the window is at reducing solar heat gain.

Sill - The horizontal, bottom section of the main frame.

Single Hung - A window in which one sash slides vertically and the other sash is fixed.

Single-strength Glass - Glass with a thickness of approximately 3/32".

Slider Window - A window in which the sash move horizontally. Sliders are available in a 2- or 3-lite configuration, with the 3-lite having operable end vents.

Sloped sill - The sill of the window that has a downward slope to the outside. This sill has sufficient degree of slope to aid in water runoff.

Solar Heat Gain Coefficient - The percentage of heat gained from both direct sunlight and absorbed heat. The smaller the number, the greater the ability to reduce solar heat gain.

Spacer - Material placed between two or more pieces of glass in order to maintain a uniform width between the glass, and prevent sealant distortion.

Tempered Glass - Glass with a surface compression of not less than 10,000 psi, or an edge compression of not less than 9,700 psi. When broken, the glass breaks into pebbles instead of shards.

Tilt Latch - Mechanism that unlocks the sash and allows it to tilt in from the main frame.

Tilt-in/lift-out Sash - A sash that can be tilted to the interior and removed for cleaning and is manufactured by welding.

U-value - Amount of heat transferred through a material. The lower the U-value, the slower the rate of heat flow and the better the insulating quality.

UV Block - The percent of ultraviolet rays blocked from being transmitted through the glass. The higher the number the lower the percentage of ultraviolet rays transmitted through the window.

Visible Light Transmittance - The percentage of light that is transmitted through glass in the visible light spectrum (380 to 720 nanometers). The higher the number the higher the percentage of visible light transmitted through the window.

Weep Slots - Slots or holes in the sill (bottom) member of the sash frame that allows water to escape. Weep flaps add a vinyl flap to keep insects out.