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Glossary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
A
Acrylic. An optically clear thermoplastic that has good weather resistance and is more shatter resistant than an equally sized pane of annealed glass. Custom Windows uses acrylics, other thermoplastics and laminated glass when resistance to breakage is a key concern.
Air-leakage or Air Infiltration. This is one of four key measurements on a window’s NFRC label. The lower a window's air-leakage rating, the more air-tight it is and the less transfer of heat or cold (as well as dust and pollen) through the window. While almost any modern insulated window will have excellent air-leakage ratings, this is one of the key failings of older single pane windows, especially those with Divided Lites. (NEW! See Custom Widnows' new IN'FLECTOR Window Insulators!)
Annealed glass. This is a sheet of float glass which has not been heat-strengthened or tempered. Most insulated glass units use two sheets of annealed glass as their basis.
Argon. Like Krypton or Xenon, Argon is an inert, nontoxic gas sometimes used to fill the space between the two glass panes of an insulating glass unit in place of air. Heavily marketed and very effective in cold winter climates, Argon and other gas fills actually do not contribute any meaningful benefit to energy savings in Houston’s hot and humid climate. The same investment in a higher efficiency Low-E glass will have far greater benefit in reducing energy consumption in Houston and along the entire gulf coast.
Awning Window. Window similar to a casement except the sash is hinged at the top and always swings out. (See casement and hopper windows) Top
B
Bay window. An arrangement of three or more individual window units, attached so as to project from the building. The most common arrangement uses two operating windows (flankers) on the side of one or more center windows that are set parallel to the building’s wall.
Bow window. A configuration of multiple windows that that projects from the wall in an arc shape, commonly consisting of five or six units. Top
C
Casement. A window sash that swings open on side hinges, usually with a locking mechanism and a crank handle for operation.
Caulking. A mastic compound for filling joints and sealing cracks to prevent leakage of water and air, commonly made of silicone, bituminous, acrylic, or rubber-based material. See also Foam Sealant.
Condensation. The deposit of water vapor from the air on any surface where one side is significantly colder than the other and the air contains significant humidity. Euphemistically called ‘sweating,’ single pane windows and some aluminum frames will ‘sweat’ in Houston’s hot and humid summers. Modern insulated glass panels virtually eliminate sweating, regardless of the frame material used. Other potential solutions include Custom Windows' new IN'FLECTOR Window Insulators or adding a storm window to a single pane window to significantly reduces condensation.
Conduction. Heat transfer through a solid material by contact of one molecule to the next. Heat flows from a higher-temperature area to a lower-temperature one, i.e. in Houston’s summers heat works to find its way from outside to inside where in a Minneapolis winter the problem is reversed. This is one reason that the right window material and glass is NOT the same for every climate.
Convection. A heat transfer process which, for our purposes, can occur between the panes of glass in an insulated window pane. While the use of dual panes with an insulating air or gas filled gap substantially reduces the problems of conduction, convection can occur between the panes due to temperature differences of each pane causing the air or gas to circulate between the panes (hot air rises and cold air sinks). This can cause ‘energy’ transfer from one pane to the other. The greater the difference between the exterior and interior temperature, the more critical convection can be.
This is one reason that gas fills such as Argon, which is denser than air and therefore sets up slower convection currents, are effective in the cold winter climates where the temperature differences between inside and outside can be 50-70 degrees. However in heat-dominated climates such as Houston, where the difference between inside and outside temperatures are usually between 15 to 25 degrees, the effects of a gas fill on solar heat gain is virtually impossible to measure.
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D
Desiccant. An extremely porous crystalline substance used to absorb moisture that is used in the spacers that separate insulated glass panes. It is this desiccant that helps maintain the proper humidity in an insulated unit and keeps it from fogging as long as the spacer is not breached.
Divided Lite (DL). A window with a number of smaller panes of glass, sometimes known as colonial lites. In older single pane windows these smaller panes are individual pieces of glass held in place by muntins, however these multiple small panes are often a key source of energy loss due to air-leakage. Modern insulated windows mimic this look by using muntin bars (or grids) between the panes of glass or SDL bars on the outside of the glass, but the actual glass panel is one solid unit, eliminating air-infiltration.
Double glazing (Insulated Glazing). In general, two thicknesses of glass separated by an air space to improve insulation against heat transfer and/or sound transmission. In factory-made double glazing units, the air between the glass sheets is thoroughly dried, the space is sealed airtight and a desiccant filled spacer used to eliminate possible condensation and providing superior insulating properties.
Double-hung window. A window in which both the upper and lower halves can be slid up and down. While common in more temperate climates where windows are left open much of the year, double-hung windows add unnecessary mechanisms, costs and potentials for air-leakage in heat dominated climates such as those along the gulf coast.
Double-strength glass. Float glass roughly 1/8” thick. While it is permitted to use single-strength glass (i.e. 1/16”) in producing windows, Custom Windows uses double-strength glass at minimum in all windows that we produce. Top
E
Emergency or Egress window. An escape window that is large enough for a person to climb out. In U.S. building codes, every bedroom must be provided with an egress window.
Extrusion. Produced by forcing heated material (vinyl, aluminum, steel) through an orifice in a die to create lengths of shaped material, extrusions of various types make up the materials that are precisely cut to create our window frames. The term Lineal is also common when referencing vinyl extrusions.
Eyebrow windows. Related to an arch window, an eyebrow shaped window is effectively an arch window with vertical side legs. Top
F
Fenestration. The placement of window openings in a building wall, one of the important elements in controlling the exterior appearance of a building. Also, a window, door, or skylight and its associated interior or exterior elements, such as shades or blinds.
Fixed window (or Picture Window). A window with no operating components.
Float glass. Glass formed by a process of floating the material on a bed of molten metal. It produces a high-optical-quality glass with parallel surfaces, without polishing and grinding.
Foam Sealant. Sometimes used to fill a gap between the window frame and the building wall, especially if the window is too small for the opening. (Better to have the window custom manufactured to fit right in the first place!) Be very careful as some installers mistakenly use expanding foam which can expand through crevasses into the home as it sets and even push sheetrock away from the framing. If foam sealant’s must be used, use only non-expanding types and use it sparingly!
Fogging. A deposit of contamination left on the inside surface of a sealed insulating glass unit due to extremes of temperatures or failed seals. Top
G
Gas fill. A gas other than air, usually argon or krypton, placed between window to reduce the U-factor, the winter efficiency measurement of a window. Heavily marketed because most company’s only resell windows made to be sold in other climates, Argon and other gas fills actually do not contribute any meaningful benefit to energy savings in Houston’s hot and humid climate. The same investment in a higher efficiency Low-E glass will have far greater benefit in reducing energy consumption in Houston and along the entire gulf coast.
Glazing. The glass or plastic panes in a window, door, or skylight.
Glazing bead. A molding or stop around the inside of a window frame to hold the glass in place. Top
H
Heat gain. The transfer of heat from outside to inside by means of conduction, convection, and radiation through all surfaces of a house.
Heat loss. The transfer of heat from inside to outside by means of conduction, convection, and radiation through all surfaces of a house.
Heat-strengthened glass. Glass that is reheated, after forming, to just below melting point, and then cooled, forming a compressed surface that increases its strength beyond that of typical annealed glass.
Hopper. Window similar to a casement except the sash is hinged at the bottom and always swings out. See awning and casement windows.
Horizontal slider. A window with a movable panel that slides horizontally. Top
I
IECC. International Energy Conservation Code published by the ICC. The successor to the Model Energy Code, which is cited in the 1992 U.S. Energy Policy Act (EPAct) as the baseline for residential Energy Codes in the United States.
Infiltration. See air leakage.
Infrared radiation. Invisible, electromagnetic radiation beyond red light on the spectrum, with wavelengths greater than 0.7 microns. More importantly, infrared light is what we feel as heat and is the key component to control to keep the summer heat out.
Insulating glass. In general, two thicknesses of glass separated by an air space within an opening to improve insulation against heat transfer and/or sound transmission. In factory-made double glazing units, the air between the glass sheets is thoroughly dried and the space is sealed airtight, eliminating possible condensation and providing superior insulating properties. Top
J
Jalousie. Window made up of horizontally-mounted louvered glass slats that abut each other tightly when closed and rotate outward when cranked open. Unfortuatately, a remarkably inefficient window due to extreme air-leakage!
K
Krypton. See Argon or Gas Fills Top
L
Laminated glass. Two or more sheets of glass with an inner layer of transparent vinyl or resin, heat bonded together. Used for both safety (the glass adheres to the inner layer if broken), infiltration control (even with broken glass the inner layer is difficult to break, keeping debris and rain out) and for noise reduction (see Noise Control).
Lite. See Divided Lite
Light-to-solar-gain ratio (or LSG). While we all want to reduce the energy costs caused by Houston’s intense summer sun, most of us still want to allow in as much natural light as possible. LSG is a derived measurement from two other NFRC window ratings: visible light transmission divided by the Solar Heat Gain Coefficient,. The higher the LSG, the more visible light is allowed vs. heat. A window with low LSG may block a lot of the sun’s heat but also blocks a lot of the natural light at the same time.
Low-emittance (Low-E) coating. Microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface that suppresses or reflects radiative heat flow. Low-E coatings are ‘spectrally selective’, i.e. they are transparent to visible light and reflective of infrared (heat) radiation. Top
M
Marketing Hype. What is often confused for facts when window resellers, who often do not know much about glass properties, push products or features that do not have a material benefit to the consumer, but it sounds good! See Argon, for example!
Metal-clad windows. Wood windows whose exterior surfaces are covered with extruded aluminum or other metal, with a factory-applied finish to deter the elements.
Mullion. A major structural vertical or horizontal member between window units or sliding glass doors.
Muntin. A secondary framing member (horizontal, vertical, or diagonal) to hold the window panes and divided lites in place in a single pane window. In a modern insulated window wood, plastic, or metal grids are placed between the panes of insulated glass to create the appearance of divided lites. Top
N
Nailing fin. An extension of a window frame which generally laps over the conventional stud construction and through which nails are driven to secure the frame in place. Used for new construction windows.
NFRC. National Fenestration Rating Council. The NFRC provides the measurement parameters for key window ratings such as U-Factor, SHGC, etc. and also sets the requirements for window labels to convey these ratings to consumers.
Noise Reduction. There are a number of ways to reduce that traffic noise that intrudes on your peace! Adding a storm window can decrease noise levels in a cost efficient manner. Replacing older single pane windows with modern insulated windows will also reduce the amount of noise while providing superior energy efficiency.
But if noise is a serious issue, the solution is laminated glass, either as a part of a storm or replacement window. While more expensive than standard glass, a single sheet of 1/4" laminated glass will reduce more noise transmission into the home than a standard insulated glass panel. When combined as a part of an insulated glass panel with Low-E, you can have replacment windows that are energy efficient, have a higher level of security against storms and break-ins while also significantly reducing the noise!
O
Obscure glass. Any textured glass (frosted, etched, fluted, ground, etc.) used for privacy, light diffusion, or decorative effects.
Operable window. Window that can be opened for ventilation. Top
P
Pane. One of the compartments of a door or window consisting of a single sheet of glass in a frame; also, a sheet of glass.
Panel. A major component of a sliding glass door, consisting of a light of glass in a frame installed within the main (or outer) frame of the door. A panel may be sliding or fixed.
Picture window. A large, fixed window framed to provide a panoramic view.
Plate glass. A rolled, ground, and polished product with true flat parallel plane surfaces affording excellent vision. It has been replaced by float glass.
Polyvinylchloride (PVC). An extruded or molded plastic material used for window framing and as a thermal barrier for aluminum windows. Top
R
R-value. A measure of the resistance of a glazing material or fenestration assembly to heat flow. It is the inverse of the U-factor (R = 1/U). While common when rating attic and wall insulation, U-factor is used for windows.
Radiation. The transfer of heat in the form of electromagnetic waves from one separate surface to another. Energy from the sun reaches the earth by radiation, and a person's body can lose heat to a cold window or skylight surface in a similar way.
Reflective glass. Window glass coated to reflect radiation striking the surface of the glass.
Refraction. The deflection of a light ray from a straight path when it passes at an oblique angle from one medium (such as air) to another (such as glass).
Rough opening (RO). The opening in a wall into which a door or window is to be installed. Used primarily for new construction windows with nailing fins. Actual window sizes are generally ½ inch smaller than the RO. Top
S
Safety glass or Tempered glass. A strengthened or reinforced glass that is less subject to breakage or splintering.
Shading coefficient (SC). A measure of the ability of a window or skylight to transmit solar heat, relative to that ability for 1/8-inch clear, double- strength glass. SC has been phased out in favor of Solar Heat Gain Coefficient (SHGC).
Simulated divided lights (SDL). A window that has the appearance of a number of smaller panes of glass separated by muntins, but actually is a larger glazing unit with the muntins placed on the surfaces of the glass.
Single glazing. Single thickness of glass in a window or door. For most residential purposes, single glaze windows can only be used to replace a broken single glaze window since energy codes now require insulated glass units in place of single glazing.
Single-strength glass. Glass with thickness of roughly 1/16”. Windows can be produced using single strength glass and are sometimes found at building supply warehouses, however Custom Windows will not produce windows with thin single strength glass.
Solar heat gain coefficient (SHGC). The fraction of solar radiation admitted through a window or skylight. Unlike Shading Coefficient, SHGC includes both directly transmitted energy as well as absorbed and reradiated energy. It is expressed as a number between 0 and 1. The lower a window's SHGC, the less solar heat it transmits. SHGC is a far more critical measurement of a window’s energy efficiency in a climate like Houston’s than U-Factor which is a winter rating.
Solar radiation. The total radiant energy from the sun, including ultraviolet and infrared wave lengths as well as visible light.
Solar screen. A sun shading screens that is installed to reduce solar radiation. It is not spectrally selective, however, so both visible light and heat are both reduced to some degree.
Sound Transmission Class (STC). The sound transmission loss rating of a material over a selected range of sound frequencies. The higher the number, the less sound transmitted. Improved STC in windows can come from using insulated glass with larger air gaps, insulated glass with different thicknesses for each pane and, most effectively, by using laminated glass in either a monolithic or insulated unit.
Spectrally selective. A coated glazing (typically Low-E) with optical properties that are transparent to some wavelengths of energy and reflective to others. Advances in Low-E coating technologies continues to improve the spectrally selective properties in insulated glass units, allowing most of the natural light while reducing significant amounts of the solar heat from entering the home.
Storm windows. A second set of windows installed on the outside or inside of the primary windows to provide additional insulation and wind protection. Glazed with a polycarbonate or laminated glass, storm windows can also significantly increase security and reduce noise. Top
T
Tempered glass. Treated glass that is strengthened by reheating it to just below the melting point and then suddenly cooling it. When shattered, it breaks into small pieces. Approximately five times stronger than standard annealed glass; is required as safety glazing in patio doors, entrance doors, side lights, and other hazardous locations. It cannot be recut after tempering.
Thermal break. An element of low conductance placed between elements of higher conductance to reduce the flow of heat. Used in aluminum windows such as Custom Windows’ T110 windows.
Tilt window. A single- or double-hung window whose operable sash can be tilted into the room. Well marketed but rarely used, a tilt sash can, if not constructed of heavier components, cause earlier seal failure, allowing more airflow through the interlock.
Tinted glass. Glass colored by incorporation of a mineral admixture. While tinting can reduce some heat gain into the home, tinting is not spectrally selective so it reduces visible light as well.
Transom. A horizontal window above a door or other window. Top
U
U-factor (U-value). A measure of the rate of non-solar heat loss or gain through a material or assembly. Importantly, for windows this is measured using extreme winter conditions of 0° F outdoor temperature, 70° F indoor temperature, 15 mph wind, and no sun . . . fairly rare conditions in Houston! The U-factor may be expressed for the glass alone or the entire window. U-factor ratings are clearly improved with the use of Argon or other gas fills as well as using vinyl or wood frames in place of aluminum. However virtually all dual pane Low-E windows, whether aluminum or vinyl meet or exceed the Energy Star prescriptive requirements for U-factor for the gulf coast.
Ultraviolet light (UV). The invisible rays of the spectrum that are outside of the visible spectrum at its short-wavelength violet end. Ultraviolet rays are found in everyday sunlight and can cause fading of paint finishes, carpets, and fabrics. Spectrally selective glass such as Low-E coated glass significantly reduce UV rays into the home and therefore help to reduce fading of furniture and carpeting.
V
Vinyl. Polyvinyl chloride material, which can be both rigid or flexible, used for window frames.
Vinyl-clad window. A window with wood parts covered with extruded vinyl.
Visible light. The portion of the electromagnetic spectrum that produces light that humans can see. Wavelengths range from 380 to 720 nanometers.
Visible transmittance (VT). The percentage or fraction of the visible spectrum (380 to 720 nanometers) weighted by the sensitivity of the eye, that is transmitted through the glazing. Top
W
Warm-edge technology. The use of low-conductance spacers to reduce heat transfer near the edge of insulated glazing.
Weep hole. A small opening in a wall or window sill member through which water may drain to the building exterior.
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