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Windows and Glass Terminology

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The technology behind quality windows, sliding doors, and French doors is always evolving and improving and it can be difficult to stay up-to-date with the latest in windows and glass terminology, products, and standards.

We put together a glossary to help you understand some of the terms you’ll hear when you are considering replacement windows or doors.

Be sure to keep an eye out as we add to this list of windows and glass terminology!

Windows Terminology

  1. Arch-top: One of several terms used for a variety of window units with one or more curved frame members, often used over another window or door opening. Also referred to as circle-heads, circle-tops and round-tops.
  2. Awning: Window with sash swinging outward from bottom.
  3. Bay: A combination of window units that projects to the exterior. Usually features a large center unit with two flanking units at 30 or 45 degree angles to the wall.
  4. Bow: A combination window that projects to the exterior. Usually features four or more window units in a radial or bow formation.
  5. Box bay: A combination of window units that projects to the exterior. Usually features a large center unit with two flanking units at 90 degree angles to the wall.
  6. Casement: Window with sash cranking outward, to the right or left.
  7. Circle-top: One of several terms used for a variety of window units with one or more curved frame members, often used over another window or door opening. Also referred to as arch-tops, circle-heads, and round-tops.
  8. Cottage double-hung: A double-hung window in which the top sash is shorter than the bottom sash.
  9. Double-hung window: Window featuring two operable sash that move vertically in the frame.
  10. Egress window: Window designed to be large enough for a firefighter to climb in or a person to climb out of in an emergency. U.S. building codes require each bedroom of a home to have an emergency exit window, with minimum sizes specified.
  11. Fanlight: A half-circle window over a door or window with radiating bars.
  12. Glider: A window with a movable sash that slides horizontally. Also referred to as a horizontal sliding window.
  13. Hopper: Window with sash that swings inward from the top.
  14. Horizontal slider: A window with a movable sash that slides horizontally. Also referred to as a gliding window.
  15. Jalousie: Window made up of horizontally mounted glass louvers or slats that abut each other tightly when closed and rotate outward when cranked open.
  16. KD (Knocked down): Unassembled window or door.
  17. Mechanical window: A term for a product, usually vinyl, in which the corners are assembled using screws or other fastening mechanisms, as opposed to a welded corner construction. Also referred to as a mechanically fastened window.
  18. Oriel: Type of bay window which protrudes from building, but does not touch the ground.
  19. Palladian: A large, arch-top window flanked by smaller windows on each side.
  20. Picture window: Large, non-operating window. It is usually longer than it is wide to provide a panoramic view.
  21. Pivot window: A unit with a sash that swings open or shut by revolving on pivots at either side of the sash or at top and bottom.
  22. Pocket window: A unit designed for replacement applications that is installed into the existing window frame after removal of the sash, balance hardware and parting stops. Also called an insert window, these units allow existing interior and exterior trim to be maintained.
  23. Prime window: A primary window, as opposed to a storm or combination unit added on.
  24. Projected window: A window in which the sash opens on hinges or pivots. Refers to casements, awnings and hoppers.
  25. Roof window: An operable unit similar to a skylight placed in the sloping surface of a roof.
  26. Round-top: One of several terms used for a variety of window units with one or more curved frame members, often used over another window or door opening. Also referred to as arch-tops, circle-tops and circle-heads.
  27. Single-hung: A window resembling a double-hung, or vertically sliding window, with a fixed, non-operating top sash.
  28. Smart window: Generic term, sometimes used for windows offering high energy efficiency or windows featuring switchable glass to control solar gain.
  29. Splayed window: Window unit set at an angle in a wall.
  30. Super window: A generic term for a window with a very low U-value. Typically, it incorporates multiple glazings, low-E coatings, gas fills and an insulating spacers.
  31. Tilt window: A single- or double-hung window whose operable sash can be tilted into a room to allow cleaning of the exterior surface on the inside.
  32. Transom: Window used over the top of a door or window, primarily for additional light and aesthetic value.

Glazing Terminology

  1. Acrylic: Thermoplastic glazing material.
  2. Aerogel: A microporous, transparent silicate foam currently under development for potential use as a glazing cavity-fill material, offering very high thermal performance.
  3. Argon Gas – Argon is a colorless, odorless, non-toxic, noncorrosive and nonflammable naturally occurring and non-reacting gas. Argon is inert, or chemically inactive. Argon lowers the conductive heat transfer across the cavity of the insulated glass unit and improves the U-value. The addition of argon greatly increases the insulating performance of the Low E glass.
  4. Double glazing: Use of two panes of glass in a window to increase energy efficiency and provide other performance benefits. May or may not refer to an insulating glass unit.
  5. Electrochromic glazing: Glass or other glazing material that can be switched from clear to opaque electronically.
  6. Glazing: Glass (and other materials) in a window or door. Also, the act or process of
  7. fitting a unit with glass.
  8. Glazing stop: A component of the sash or door panel that holds the glass in place.
  9. Laminated glass: Two or more sheets of glass with an inner layer of transparent plastic to which the glass adheres if broken. Used for enhanced safety and security, as well as sound reduction.
  10. Low E – Low E glass stands for Low Emissivity Glass. This type of glass coating was developed to provide improved solar heat gain coefficient performance.
  11. Low E² – Low E² is an industry term to reflect the application of two coats of metal onto a window. Low E² was developed to provide better summer daytime performance with only a slight reduction in visible light transmittance. Problems with this coating include direct exposure to air causing the silver to tarnish and inherent gas leakage one experiences and any weaknesses that might have existed in the spacers. The quality of those should be double-checked if one decides to go with this solution.
  12. Low-E³ – Low-E³ is another industry term that simply reflects the application of three coats of metal onto a window. This style of window tends to be a great all climate window because it offers great performance in colder weather (U-Factor) and enhanced performance in summer (SHGC). The third coating layer provides great benefit for keeping unwanted heat out of a building but one can only see so much gain from applications for insulating qualities. This is as close to an all-purpose window as one is going to be able to get. In the past, solar gain prevention windows operated largely with deeper tints. This product permits great performance coupled with easily visibility through it for homeowners.
  13. Polycarbonate: A plastic material used for glazing.
  14. Polyvinyl Butyral (PVB): Plastic material used as the interlayer in the construction of some types of laminated glass.
  15. Pyrolytic Coating (Hard Coat) – As the name implies, these coatings are applied to hot glass. This type of coating gives a “Mid-E” performance ar from that of Low E². It provides rather poor optical quality (color) and consistency of coating. Another disadvantage of pyrolytic coatings is that they exhibit high haze levels compared to that of Low E². Most Low E glass available today on the market is made this way.
  16. Resin: A term commonly used within the industry that refers to the raw materials used by PVC extruders to produce vinyl window profiles. The word is also used to describe a liquid material that is used in the production of laminated glass.
  17. Safety glass: A strengthened or reinforced glass that is less subject to breakage or splintering and less likely to cause injury if broken. Law requires glass in doors to be some type of safety glazing product, such as tempered or laminated glass.
  18. Single glazing: Use of a single lite of glass in a window. Generally not as energy efficient as insulating glass or other forms of double glazing.
  19. SPD: Suspended particle device. A type of switchable glazing that typically uses laminated glass construction with the interlayer material featuring “suspended particles” that align when the glass unit is charged to provide a clear view and scatter when there is no charge, changing the glazing to translucent.
  20. Spectrally selective glass: A coated or tinted glazing with optical properties that are transparent to some wavelengths of energy and reflective to others. Typically, spectrally selective coatings are designed to allow high levels of visible light or daylight into a building and reflect shortwave and longwave infrared radiation.
  21. Sputtered Coatings (Soft Coat) – This is how Low E² is made. The advantage of this coating is that it provides overall qualities of high visible light transmission, neutral color, coating uniformity and low emissivity. This combination provides a nearly invisible coating with the highest level of performance possible.
  22. Tempered glass: Glass heat-treated to withstand greater than normal forces on its surface. When it breaks, it shatters into small pieces to reduce hazard.
  23. Triple glazing: Use of three panes of glass or plastic with two airspaces between. Generally refers to a sealed insulating unit.

Industry Standards & Ratings Terminology

  1. AAMA: American Architectural Manufacturers Association, a national trade association that establishes voluntary standards for the window, door and skylight industry.
  2. ANSI: American National Standards Institute, a clearinghouse organization for all types of standards and product specifications.
  3. ASHRAE: American Society of Heating, Refrigeration and Air Conditioning Engineers, a national association that establishes standards for building energy performance.
  4. ASTM International: Formerly, the American Society for Testing and Materials, an organization that establishes material standards (including glass) and test methods. It has also produced a window installation standard.
  5. BETEC: Building Environment and Thermal Envelope Council. Part of the National Institute of Building Sciences, an organization representing government and industry, BETEC is involved in communicating government policy and influencing standards development within the industry.
  6. Emissivity – A measure of a surface’s ability to emit long-wave infrared radiation or room temperature radiant heat energy. Emissivity varies from 0 (no emitted infrared) to 1 (100% emitted infrared). The lower the emissivity, the lower the resultant U-Value.
  7. BOCA: Building Officials and Code Administrators. One of the three model code groups in the U.S. that has now merged into the International Code Council.
  8. Energy Star: A program sponsored by the U.S. Environmental Protection Agency that establishes minimum performance standards for windows to be recognized as energy efficient. Four different sets of standards for U-value and solar heat gain have been established for four different climate zones in the U.S. See the Association Directory for more information on the Energy Star Windows program. More stringent requirements are planned for 2015.
  9. EuroWindoor: A consortium of European window, door and curtainwall industry associations involved in the development of common EU standards.
  10. Green building: A movement in architectural and building circles aimed at creating structures that are occupant and environmentally friendly. Criteria such as sustainability, energy efficiency and healthfulness are considered.
  11. Haze – Haze is the percentage of transmitted light which in passing through the specimen deviates from the incident beam by forward scattering. In essence, the higher the haze value, the poorer the product is for clarity. The haze % for Low E² is 0.14 as compared to anywhere from 0.74 up to 1.40 for pyrolytic Mid-E coatings.
  12. IBC: International Building Code. Published by the International Code Council, the IBC primarily covers nonresidential construction. See International Code Requirements for Windows and Doors page for more information on the International Codes.
  13. ICBO: International Council of Building Officials. One of the three model code groups in the U.S. that has merged to form the International Code Council.
  14. IECC: International Energy Conservation Code. Published by the International Code Council, the IECC sets forth compliance methods for energy-efficient construction of both residential and nonresidential construction. See International Code Requirements for Windows and Doors page for more information.
  15. IRC: International Residential Code. Published by the International Code Council, the IRC primarily covers low-rise residential construction. See International Code Requirements for Windows and Doors page for more information.
  16. LEED rating system: A “green building” rating system established by the U.S. Green Building Council. Currently applicable to new commercial construction and major renovations, the program is being expanded to include residential construction as well. LEED stands for Leadership in Energy and Environmental Design.
  17. Life-cycle analysis (LCA): An assessment of the environmental impact of a product that takes into account its entire lifespan. For a window or door, this would include energy and materials used to manufacture, its energy savings contribution during its useful life, and its disposal and/or capacity to be recycled.
  18. LRRP: The lead paint Renovation, Repair and Painting program established by the Environmental Protection Agency for pre-1978 homes to address health and safety issues associated with lead paint. Firms involved in such projects must be trained and certified in lead-safe work procedures. Also referred to as RRP. Information available on EPA Web site.
  19. MEC: Model Energy Code, established by Energy Policy Act of 1992 to serve as a baseline for state energy codes. Although referenced in some state codes, it has been succeeded by the International Energy Conservation Code (IECC).
  20. NFRC: National Fenestration Rating Council. A body that has established methods for rating and certifying the energy performance of windows, doors, skylights and other fenestration products.
  21. SDJA: National Sash and Door Jobbers Association. Trade organization of window, door and millwork distributors. It is now the Association of Millwork Distributors (AMD).
  22. NWWDA: National Wood Window and Door Association. Trade organization that has established many standards related to wood window and door products. It is now the Window and Door Manufacturers Association (WDMA).
  23. RESFEN: A computer program designed to calculate energy use based on window selection in residential buildings. Created under sponsorship of the U.S. Department of Energy by Lawrence Berkeley Laboratory.
  24. RRP: The Renovation, Repair and Painting program established by the Environmental Protection Agency for pre-1978 homes to address health and safety issues associated with lead paint. Firms involved in such projects must be trained and certified in lead-safe work procedures. Also referred to as LRRP. Information available on EPA Web site.
  25. SBCCI: Southern Building Code Congress International One of the three model code groups in the U.S. that merged to form the International Code Council.
  26. Shading coefficient (SC): A measure of a window’s ability to transmit solar heat, relative to that ability for 1/8-inch clear glass. The lower a unit’s shading coefficient, the less solar heat it transmits, and the greater its shading ability. It is being phased out in favor of the solar heat gain coefficient (SHGC).
  27. Six Sigma: A statistics-driven approach to quality control developed originally by Motorola.
  28. Solar Energy Reflectance – In the solar spectrum (300 to 2500 nanometers), the percentage of ultraviolet, visible and near infrared energy from the sun that is reflected from the glazing surface(s).
  29. Solar Energy Transmittance – In the solar spectrum, the percentage of ultraviolet, visible and near infrared energy from the sun that is transmitted through the glazing.
  30. Solar Heat Gain Coefficient (SHGC): A rating, which is now generally replacing shading coefficient, measuring a window’s ability to transmit solar heat. It measures both the solar radiation which is directly transmitted, as well as the solar radiation absorbed by the glass and subsequently transmitted. The lower a unit’s solar heat gain coefficient, the less solar heat it transmits, and the greater its shading ability. It is approximately equal to the shading coefficient divided by 1.15. It is expressed as a number without units between 0 and 1.
  31. Sound transmission class (STC): A rating measuring a window’s acoustic properties or its ability to reduce sound transmission. An STC rating is determined by measuring the sound transmission over a selected range of sound frequencies. The higher the number, the less sound transmitted.
  32. U-factor: Rate of heat flow-value through a building component, from room air to outside air. Also referred to as U-value. The lower the U-factor, the better the insulating value. U-factor, a rating more generally used in the window industry, is the reciprocal of R-value, a rating commonly used in the insulation industry.
  33. Visible Light Reflectance – In the visible spectrum, the percentage of light that is reflected from the glass surface(s) relative to the C.I.E. standard observer.
  34. WDMA: Window and Door Manufacturers Association. Formerly the National Wood Window and Door Association, this trade organization has established many standards related to wood window and door products.

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