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 a common problem with single pane windows, almost any modern insulated window will have excellent air-leakage ratings.
Annealed glass. 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, Argon is an inert, nontoxic gas used to fill the space between the two glass panes of an insulating glass unit in place of air. While Argon has minimal benefit in combating solar heat gain, it improves the insulation of a window against cold winter chill. (See U-Factor)
Awning Window. Window similar to a casement where the sash is hinged at the top and swings out along the bottom edge. (See casement and hopper windows.)
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.
Casement. A window sash that swings open on a side hinge, usually to the outside. (See casement and hopper windows. Also see Tilt-Turn Window.)
Caulk. 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. Custom Windows exclusively uses a modified urethane that contains no solvents, has no odor and will not shrink over time.
Condensation. The deposit of water vapor from the air on any surface where one side is significantly colder than the other. 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 adding a Custom Windows’ storm window to a single pane window to significantly reduce 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. during a Houston summer heat transfers 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, in insulated windows or doors, can occur between the panes of glass. Convection currents occur between the panes when the temperatures of the inside and outside panes of glass differ significantly. This causes the air or gas to circulate between the panes (hot air rises and cold air sinks) and causes ‘energy’ to transfer from the warmer pane to the cooler pane. The greater the difference between the exterior and interior temperature, the more critical convection can be. An inert gas such as Argon reduces convection in windows climates temperatures can vary substantially between inside and out.
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’s seal 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, virtually eliminating air-infiltration.
Double Pane Glass (Insulated Glass or IG). 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 or replaced by an inert gas, 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.
Double-strength glass. Float glass roughly 1/8” thick. While it is permitted to use thinner single-strength glass in producing windows, Custom Windows uses double-strength glass at minimum in all windows that we produce.
Egress. In relation to windows, one that meets minimum clear opening requirements for escape in case of an emergency. U.S. Building Codes specify minimum specifications for acceptable egress windows for first and second floors. See Egress.pdf for requirements.
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.
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. A properly sized and manufactured window will not require that a foam sealant is used. If foam sealant must be used, use only non-expanding types and use it sparingly!
Fogging. A deposit of moisture on the inside surface of a sealed insulating glass unit due to extremes of temperatures or a failed seal. Fogging can occur when external temperatures change rapidly or where the inside and outside temperatures differ substantially, but such fogging is temporary. If fogging is consistent regardless of the temperatures, the dual pane seal has probably failed and the IG must be replaced.
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. (See Argon)
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.
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 heated to just below melting point and then rapidly cooled. Heat-strengthening glass increases its strength beyond that of typical annealed glass. The most common heat-strengthened glass is tempered (or Safety) glass.
Hopper. Window similar to a casement except the sash is hinged at the bottom and always swings out along the top edge. See awning and casement windows.
Horizontal slider. A window with a movable panel that slides horizontally.
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 felt as heat and is the key component in controlling energy costs during summer. (See Low-E)
Insulated Glass or IG. See Double Pane Glass.
Krypton. See Argon or Gas Fills. Krypton is substantially higher in cost than Argon but is the more effective insulating gas for extreme winter conditions. Used in an EnergyCore™ triple pane, krypton can improve the insulating capability to R-7.
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 control. (See Noise Reduction
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 and is based upon dividing the Visible Light Transmission by the Solar Heat Gain Coefficient. A higher LSG results in a window that allows more visible light while also blocking infrared heat. A low LSG that blocks a similar amount of solar infrared is darker and allows less natural light.
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 as well as substantially reducing harmful UV radiation.
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 muntins (often referred to as grids) are placed between the panes of insulated glass to create the appearance of divided lites.
Nailing fin. An extension of a window frame which generally laps over the conventional stud construction and through which screws or nails are used to secure the frame in place. Used for new construction windows but not generally used for remodeling and replacement.
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. Sound vibrations are transmitted through every building material, but single pane windows especially transmit a significant amount of noise into a home. Simply adding a storm window can decrease noise levels in a cost efficient manner while replacing single pane windows with modern insulated windows combines superior energy efficiency with noise reduction. And for difficult problems, Custom Windows can improve things further by using special STC glass or laminated glass solutions that provide even greater noise reduction.
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. (See Single and Double Hung)
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. 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.
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). Single pane windows commonly are R-1, while modern Low-E windows are R-3 or better. Custom Windows currently manufactures windows up to R-6.
Radiation. The transfer of energy in the form of electromagnetic waves from one separate surface to another. Energy from the sun reaches the earth by radiation in the form of heat (infrared), fading (UV) and visible light.
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 frames are generally ½ inch smaller than the RO.
Safety glass. A strengthened or reinforced glass that is less subject to breakage or splintering. While tempered glass is the most common, laminated glass is also classed as a safety glass.