Blinding glare. Glare that is so intense that, for an appreciable length of time after it has been removed, no object can be seen.

Borate glass. A glass whose essential glass former is boron oxide rather than silica.

Brightness. The subjective perception of luminance.

Brise-soleil. An architectural device on a building (such as a projection, louvers, or a screen) that blocks unwanted sunlight.

Btu. An abbreviation for British thermal unit – a standard measure of the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit.

Building envelope. The outer elements of a building, both above and below ground, that divide the external and internal environments.

Casement. A window sash which swings open on side hinges.

Casement window. A window containing one or more side-hinged sashes hinged that project outward or inward from the plane of the window in the vertical plane. A conventional casement window in North America swings outward, while in Europe it swings inward.

Casting. A process of shaping glass by pouring hot glass into or onto molds or tables.

Caulk, caulking compound. 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.

Caulking. Filling joints, cracks, voids, or crevices with a sealant (caulk) in order to prevent the passage of air or water.

Ceramic glass enamel (also ceramic enamel or glass enamel). A vitreous inorganic coating bonded to glass by fusion at a temperature generally above 500 degrees Celsius.

Chromogenic glazing. A broad class of switchable glazings including active materials (e.g. electrochromic) and passive materials (photochromic and thermochromic).

Clear glass. Architectural clear glass is mostly of the soda-lime-silica type, and composition varies between manufacturers, but is generally 70 – 74 percent silica, 5 – 12 percent lime, and 12 – 16 percent soda, with small amounts of magnesium, aluminum, iron, and other elements.

Clerestory. That part of a building rising clear of the roof or other parts, whose walls contain windows for lighting the interior.

Clerestory window. A venting or fixed window positioned above other windows or doors on an upper outside wall of a room.

Coating. A thin layer applied to the surface of a glass in either a chemical deposition technology (i.e., vapor, liquid, etc.) or a vacuum sputtering process. After application it is converted to a solid protective, decorative, or functional adherent film.

Color of transmitted light. The human eye’s and brain’s subjective interpretation of the spectral distribution of transmitted visible radiation. Transmitted light is said to be colorless (white) if it matches the spectrum of the external incident light, while any imparted color is due to the subtraction of the complementary wavelengths by absorption or reflection of those wavelengths by the glazing system.

Commercial entrance system. Products used for ingress and egress in nonresidential buildings. Commercial entrance systems typically utilize panic hardware, automatic closers, and relatively large amounts of glass.

Composite frame. A frame consisting of two or more materials – for example, an interior wood element with an exterior fiberglass element.

Condensation. The deposit of water vapor from the air on any cold surface whose temperature is below the dew point, such as a cold window glass or frame that is exposed to humid indoor or outdoor air.

Conduction. Transfer of heat through a material via molecular contact; heat flows from a higher-temperature area to a lower-temperature one.

Conductivity, thermal. The time rate of steady state heat flow through a unit area of homogenous material induced by a unit temperature gradient in a direction perpendicular to that unit area.

Convection. A heat transfer process involving motion of a fluid (such as air) caused by either the difference in density of the fluid and the action of gravity (natural convection) or by mechanical forces such as blowers, fans, etc. (forced convection). Convection affects heat transfer from the surface to air, whether it is for enclosed spaces (like insulated glazing unit cavity) or open spaces (like indoor glass surface to room air).

Cool daylight glazing. Spectrally selective glazing that employs tinting and/or surface coatings to achieve a visible transmittance that exceeds the solar heat gain coefficient (total solar energy transmittance). See also Light-to-solar-gain ratio.

CR. Condensation Resistance index; an indication of a window’s ability to resist condensation developed by NFRC. The higher the CR, the less likely condensation is to occur.

CRF. Condensation Resistance Factor; an indication of a window’s ability to resist condensation developed by AAMA. The higher the CRF, the less likely condensation is to occur.

Crown glass. Large panes that first became available in the seventeenth century and were incorporated in wooden sash windows. The glass was hand-blown through a pipe (pontil) into a circular disc, leaving a bubble or bullion where the pipe was inserted. Also known as bottle glass or bull’s eye glass.

Curtain wall. An external non-load bearing wall, applied in front of a frame structure, thereby bypassing floor slabs. The cladding is intended to separate the internal and external environments, and is distinct from the building structure. There are now many curtain walls systems manufactured from a variety of materials; the systems typically include both windows and spandrel sections.

Cylinder glass. A glass that is blown in the shape of a cylinder and flattened into a sheet.

Egress window. A window providing egress, as defined in applicable building codes. Also referred to as emergency exit window, escape window, and fire-escape window.

Effective thermal conductivity. The combined effects of conduction, convection, and radiation in fluid-filled (gas-filled) enclosures and cavities, converted into an apparent or effective conductivity of a solid.

Electrochromic(s) (glazing). Glazing with optical properties that can be varied continuously from clear to dark with a low-voltage signal. Ions are reversibly injected or removed from an electrochromic material.

Electromagnetic spectrum. Radiant energy over a broad range of wavelengths.

Emissivity. The relative ability of a surface to reflect or emit heat by radiation. Emissivity factors range from 0 to 1; the lower the emissivity, the less heat is emitted through a window system. Emissivity is typically measured by U-factor (or its inverse, R-value).

Etch. To attack the surface of glass with hydrofluoric acid or other agents, generally for marking or decoration.

Evacuated glazing. An insulating glazing composed of two glass layers, hermetically sealed at the edges, with a hard vacuum between (< 10-3 Pascals) to eliminate convection and conduction. A spacer system (commonly referred to as “pillars”) throughout the surface of glass (rather than just at the edges) is needed to keep the panes from touching.

Exterior stop. The removable glazing bead that holds the glass or panel in place when it is on the exterior side of the light or panel, as contrasted to an interior stop located on the interior side of the glass.

Extrusion. The process of producing aluminum shapes by forcing heated material through an orifice in a die by means of a pressure ram. Also, any item made by this process. An example is the complex cross-section of an extruded aluminum or PVC window frame.

Fenestration. The placement of openings in a building wall, such as windows, doors, skylights, etc., designed to permit the passage of air, light, or people; one of the important aspects of a building’s exterior appearance. Also, associated interior or exterior elements, such as shades or blinds. From the Latin word, fenestra, meaning “window.”

Fiberglass. A composite material made by embedding glass fiber in a polymer matrix. It may be used as a diffusing material in sheet form, or as a standard sash and frame element.

Film conductance. The time rate of heat flow from a unit area of a surface to its surroundings, induced by a unit temperature difference between the surface and the environment.

Fire resistance. As applied to buildings, the property of a material or assembly to withstand fire or give protection from it, characterized by the ability to confine a fire or to continue serving a structural function, or both.

Fixed (window). A single sash fastened permanently in a frame so that it cannot be raised, lowered, or swung open; a non-venting or non-operable window unit.

Flashing. (1) Sheet metal or other material applied to seal and protect the joints formed by different materials or surfaces. (2) Applying a thin layer of opaque or colored glass to the surface of clear glass, or vice versa.

Flat glass. A general term covering sheet glass, plate glass, float glass, window glass, and various forms of rolled glass, and named according to the method used in its manufacture. See also Float glass, Plate glass, Rolled glass, Sheet glass.

Float glass. Glass formed by a process of floating the molten glass (at approximately 1000 degrees Celsius) on a shallow bed of molten tin. Thickness is controlled by the speed at which the solidifying glass ribbon is drawn off the tin bath. The surfaces of the glass do not come into contact with any rollers or mechanisms that could cause damage until the glass has solidified; therefore it produces a high-optical-quality glass with parallel surfaces without polishing and grinding.

Flush glazing. A method of glazing wherein the surfaces of the glass retaining members (stops or beads) are in the same plane normal to the glass as the side faces of the frame members; often achieved by providing pockets in these faces.

Foam spacer. Nonconductive, foam material (often closed-cell silicone foam) used to separate the double- and triple-pane insulating glass units; improves the thermal performance of the window.

Fogging. A deposit of contamination left on the inside surface of a sealed insulating glass unit due to extremes of temperatures or failed seals.

Fourcault process. The method of making sheet glass by drawing vertically upward from a slotted debiteuse block.

Frame. The fixed, enclosing structure of a window or other fenestration system which holds the sash, casement, door panels, etc., as well as hardware. Frames can be constructed from aluminum extrusions, steel, PVC extrusions, wood, composite materials, or a combination of these materials.

Freeze-thaw resistance. Resistance to cycles of freezing and thawing that could affect application, appearance, or performance

Frit. Ceramic frit opacification is one or more coats of durable colored ceramic material fire-fused onto compatible base glass. The firing also produces a heat-treated product. Since the basic purpose is generally to render the glass opaque, the frit is typically applied to the second surface of monolithic glass or the fourth surface of an insulating unit (counting from the outside surface in). The opacity can be improved with thicker or multiple coats, which are available in a wide range of colors.

Impact resistance. The ability to withstand mechanical blows or shock without damage seriously affecting the effectiveness of the material or system.

Inert gas. Refers to the use of chemically nonreactive gas(es) within the cavity of a sealed insulating glass unit for the purpose of reducing conductive/convective heat transfer. See Gas fill.

Infiltration (air). The movement of outdoor air into the interior of a building through cracks around windows, doors, and the building envelope in general.

Infiltration heat loss. The heat loss due to infiltration. The loss depends upon the indoor and outdoor temperatures, the crack perimeter, and the rate of air leakage per foot of crack.

Infrared Radiation (IR). Invisible electromagnetic radiation, beyond red light on the spectrum, with wavelengths greater than 0.7 microns. Short-wave infrared radiation is from 770 nm to 2500 nm (0.77 to 2.5 microns), while long-wave infrared is from 2.5 microns and beyond.

Insulating Glass (IG) Insulating Glass Unit (IGU). A combination of two or more panes of glass with a hermetically sealed air space between the panes of glass, separated by a spacer. This space may or may not be filled with an inert gas, such as argon.

Interior venetian blinds. A venetian blind installed between two panes of glass and remotely controlled.

Jal-awning window. A window consisting of multiple top-hinged ventilators arranged in a vertical series and operated by one or more control devices that swing the bottom edges of the ventilator outward. The window does not contain a cross shaft or torque bar, but does have an individually operated locking mechanism.

Jalousie (window). Window made up of horizontally mounted, louvered glass slats that tightly abut each other when closed and rotate outward when cranked open. See also Louvered window.

Jamb. The main vertical members forming the sides of a window or door frame.

Krypton. An inert, nontoxic gas used in insulating windows to reduce heat transfer.

kWh. Kilowatt-hour; a unit of energy equal to one thousand watt-hours.

Laminated glass. Two or more sheets of glass bonded together with one or more inner layers of transparent plastic (interlayer) to which the glass adheres if broken. The bonding is achieved by heating the glass/interlayer sandwich under pressure in an autoclave. The glass is used for overhead, safety glazing, and sound reduction.

LCD switchable. Form of chromogenic (switchable) glazing that employs a liquid crystal device to modulate transmittance of solar radiation.

Light. A window; or a pane of glass within a window. Double-hung windows are designated by the number of lights in the upper and lower sashes, as in six-over-six. Also, spelled informally, lite.

Lightscoops. Clerestory roof monitors oriented away from the sun, utilized when and where indirect light is desired or solar heat gains are undesirable.

Light pipe. A generic term for a system employing bulk optics (lenses, mirrors, reflective ducts, or other optical waveguide technology) designed to transport light (natural or artificial) to parts of a building remote from the envelope. Also known as tubular daylighting devices (TDD).

Light-redirection system. A glazing unit or panel, possibly retrofitted, which intercepts incident sunlight and sky light and specularly reflects it in another direction, usually toward the ceiling. See also Prismatic glazing and Holographic glazing.

Light shaft. An insulated shaft built to direct the light from a roof window or skylight through the attic to the room below.

Light shelf. A daylight-enhancement device; an internal and/or external overhang with a reflecting upper surface normally above head height. Designed to reduce glare near the window and improve illuminance uniformity along an axis normal to the window wall.

Light-to-solar-gain ratio (LSG). A measure of the ability of a glazing to provide light without excessive solar heat gain. It is the ratio between the visible transmittance of a glazing and its solar heat gain coefficient.

Lightwell. An open shaft in a building that provides air and light to windows opening onto the shaft.

Liquid crystal glazing. Glass in which the optical properties of a thin layer of liquid crystals are controlled by an electric current, changing from a clear state to a diffusing state.

Litrium (also Literium). An atrium designed to optimize daylighting in adjacent spaces. See also atrium.

Load. The amount of energy that must be added to or extracted from a space to thermal comfort. Sensible or latent cooling or heating loads are due to accumulated heat gains or losses through the building envelope, window, infiltration or ventilation, and occupancy.

Long-wave infrared radiation. Invisible radiation, beyond red light on the electromagnetic spectrum (above 2.5 micro meters or microns), emitted by warm surfaces such as a body at room temperature radiating to a cold window surface.

Louvered window. A window having louvers or slats that fill all or part of the opening.

Louvers. Slanted fins or slats in a window, ventilator, or venetian blind; the slats may be fixed or adjustable, and made of wood, metal, glass, or plastic.

Low-conductance spacers. An assembly of materials designed to reduce heat transfer at the edge of an insulating window. Spacers are placed between the panes of glass in a double- or triple-glazed window.

Low-E (low-emittance) coating. A microscopically thin (less than 100 nm) metal, metal oxide, or multilayer coating deposited on a glazing surface to reduce its thermal infrared emittance and radiative heat transfer. Near-infrared emittance may also be reduced depending on whether solar heat is to be rejected or admitted. Low-emissivity glass is used to increase a window’s insulating value, block heat flow, and reduce fading

Low-iron glass. Glass with a low concentration of ferrous compounds, which are absorbing in the near-infrared part of the solar spectrum. Particularly used for solar collector covers and equator-facing windows in cold-climate, passive-solar buildings where solar transmittance must be maximized.

Luminous efficacy (Ke). The ratio of the visible transmittance to the shading coefficient; it is a measure of the light-to-heat ratio of the transmitted energy. See also Light-to-solar-gain ratio.

Permeance. A measure of the transmission of water vapor through a material expressed in units of “perms.”

Photochromic glazing, photochromics. Glazing which changes its thermal, solar, and visible transmittance in response to outdoor illuminance or ultraviolet (UV) radiation. See also Switchable glazing.

Photopic response function. See V-lambda curve.

Photovoltaic. A device that produces electricity (voltage) directly from sunlight (photons).

Pivot (pivoted) window. A window with a sash that swings open or shut by revolving on pivots at either side of the sash, or at the top and bottom.

Plastic film. A thin, plastic substrate sometimes used as the inner layers in a triple- or quadruple-glazed window.

Plate glass. Flat glass with surfaces that are essentially plane and parallel; it is formed by a rolling process, ground, and polished on both sides. It is available in thicknesses varying from 1/8″ to 1-1/4″ (3.2 mm to 31.8 mm), but has been replaced by float glass.

Point fixings. In contrast to mullions or patch fittings, which project beyond the plane of the glazing, point fixings are interior. Typically, holes are drilled into the glass, and bolts or screws attach the glass in an interior frame structure.

Polished wire glass. Wire glass that is ground and polished on both sides.

Polyvinylchloride. See PVC.

Prismatic glazing. A daylighting device that consists of a light-redirecting glazing with a fine-structure, sawtooth cross-section, designed to refract incident sunlight and sky light toward the ceiling.

Projected window. A window fitted with one or more sashes opening on pivoted arms or hinges. The term refers to casements, awnings, and hoppers.

Protected opening. A window with a fire-resistance rating suitable for the wall in which it is located.

Psychrometric chart. A chart which shows various psychrometric quantities, like dry bulb and wet bulb temperatures, moisture content, partial pressure of water vapor, etc.

PVC. A polymer known as polyvinylchloride made by combining several chemicals, fillers, plasticizers, and pigments. It is often used as an extruded or molded plastic material for window framing or as a thermal barrier for aluminum windows.

Pyrolytic coating. A low-E, thin-film coating applied at high temperature. See also Hard coating.

Radiant temperature. The temperature describes the infrared radiant field at a certain position and is the weighted average of surface temperatures surrounding the location; the weighting is dependent on surface emissivity and the view factors to the measurement point.

Radiation. The transfer of heat, in the form of electromagnetic waves, from one surface to another. For example, 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.

Rail. A horizontal member of a window sash or door panel. Also known as head rail, top rail, bottom rail, meeting rail.

Rebate. Part of a surround, the cross-section of which forms an angle into which the edge of the glass is received.

Reflectance. The fraction of incident radiation upon a surface that is reflected from that surface.

Reflection. The process by which incident flux leaves a surface or medium form the incident side, without change in frequency.

Reflective glass. Window glass that is coated to reflect radiation striking the surface of the glass.

Reflectivity. The reflectance of a microscopically homogeneous sample with a clean, optically smooth surface and of thickness sufficient to be completely opaque.

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).

Relative humidity. The percentage of moisture in the air in relation to the amount of moisture the air could hold at that given temperature. At 100 percent relative humidity, moisture condenses and water droplets are formed.

Ribbon window (window band). A series of windows in a row across the face of a building.

Rolled glass. A flat glass with a patterned or irregular surface, produced by rolling, and having varying transparency. Types include flat wire glass, corrugated glass, patterned glass, obscured glass, cast glass, and figured glass.

Roll-up shade. A window shade installed on inside of building that rolls up around a cylindrical holder at the top. These shades serve to maintain privacy, reflect some solar radiation, and reduce convection flow when fully extended. Also known as a roller shade.

Roof window. A fixed or operable window similar to a skylight that is placed in the sloping surface of a roof. See also Skylight.

Rough opening. The framed opening in a wall into which a window or door unit is installed.

R-value. A measure of resistance to heat flow of a material or construction (insulating ability). The higher the R-value, the better the insulating effect and the lower the rate of heat flow.

Thermal barrier, thermal break. An element, made of a material with relatively low thermal conductivity, which is inserted between two members having high thermal conductivity in order to reduce the heat transfer. Such elements are often used in aluminum windows.

Thermal conductance (C). The same as thermal conductivity except that thickness is “as stated’ rather than one inch.

Thermal conduction. The mode of heat transfer through a material by molecular contact. Heat flows from a high-temperature area to one of lower temperature.

Thermal conductivity (k). The heat transfer property of materials, expressed in units of power per area and degree of temperature (e.g., Btu-per-hour per inch of thickness per square foot of surface per one degree F temperature difference).

Thermal emissivity. Similar to thermal emittance, except that the suffix “-ivity” refers to a property of general material, while “-ance” refers to a specific material with a certain thickness, surface finish, etc.

Thermal emittance. The ability of a surface to emit long-wave radiation relative to that of a perfect black body. Also known as the long-wave infrared emittance. A perfect black body has an emittance equal to 1.0, while a perfect reflector has an emittance equal to zero.

Thermal mass. The mass in a building (furnishings or structure) that is used to absorb solar gain during the day and release the heat as the space cools in the evening.

Thermal radiation. The heat transfer by radiation from surfaces at or near the room temperature (i.e., wavelengths in the range 2.5 – 50 microns). It is often referred to as far IR radiation or long-wave IR radiation.

Thermal resistance. A property of a substance or construction which retards the flow of heat; one measure of this property is R-value.

Thermal shock. A rapid change in temperature imposed on a glass body.

Thermal stress. Stress caused by the temperature differential across a glazing layer; e.g. for a tinted or switchable glazing in its darkened state, the sunlit side of the glazing will be hotter than the reverse side.

Thermochromic glazing, thermochromics. Glazing which changes its thermal, solar, and visible transmittance in response to its temperature. Because of absorption, the temperature of the glazing may differ from the ambient temperature.

Thermogram. An image of an object taken with an infrared camera that shows surface temperature variation.

Tinted glass. Glass that is colored by incorporation of a mineral admixture, by surface coating, or by the application of retrofit film. Any tinting reduces both visual and radiant transmittance.

Toplighting. Lighting from skylights, roof monitors or clerestories. Toplighting historically has been utilized when floor areas are too large to be illuminated adequately by sidelighting.

Translucent. Permitting light to pass through, but with differing degrees of obscuration and diffusion.

Transmission. The quantity of heat flowing through a unit area due to all modes of heat transfer induced by the prevailing conditions.

Transmittance. The percentage of radiation that can pass through glazing. Transmittance can be defined for different types of light or energy, e.g. visible light transmittance, UV transmittance, or total solar energy transmittance.

Transom window. The window sash located above a door. Also called transom light.

Transparent. Permitting light to pass through with clear vision.

Transparent Insulation Material (TIM). A generic name for a class of glazing materials having high visible transmittance and very low thermal transmittance. Includes so-called geometric media (honeycomb structures, aerogels, etc.). Some TIMs are translucent (diffusely transmitting) rather than transparent.

Triple glazing, Triple-pane glass. A window with three panes of glass or two outer panes of glass with a suspended plastic film in between. The layers are separated by two gas-filled spaces (usually Argon or Krypton) to increase energy efficiency and provide other performance benefits.

UBC. Uniform Building Code.

U-factor. The heat transmission in unit time through unit area of a material or construction and the boundary air films, induced by unit temperature difference between the environments on each side. A measure of the rate of non-solar heat loss or gain through a material or assembly. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value. Also known as U-value.

U-Factor (total). The area-weighted average thermal transmittance of a complete window, including center-of-glass, edge-of-glass, and frame U-factors.

Ultraviolet Radiation (UV). Extremely short wavelength invisible radiation at the violet end of the visible spectrum. UV rays are found in everyday sunlight and can cause fading, chalking of dark paint finishes, or other damage. Extreme UV exposure can cause certain plastic materials to distort and can cause sunburn.

Ultraviolet transmittance-weighted. A measure of non-visible solar transmittance between 280 and 380 nanometers in wavelength.

Vacuum glazing (window). See Evacuated glazing.

Vapor barrier. A membrane or coating which resists the passage of water vapor from a region of high vapor pressure to low pressure, more accurately called a vapor retarder.

Vapor retarder. A material (usually in the form of a membrane or coating) that reduces the diffusion of water vapor across a building assembly, from a region of high vapor pressure to low vapor pressure.

Venetian blind. A light-controlling shading device consisting of overlapping thin, horizontal slats which can be raised or adjusted from wide open to closed positions by varying the tilt of the slats.

Vinyl. See PVC.

Vinyl-clad window. A window with exterior wood parts that are covered with extruded vinyl.

Visible light. The portion of the electromagnetic spectrum yielding light that can be seen. Wavelengths range from 380 to 720 nanometers.

Visible Transmittance (VT). The fraction of visible radiation transmitted by a glazing system between the limits of 380 and 770 nanometers (0.38 – 0.77 micrometers). It is weighted according to the photopic response of the human eye (V-lamdba curve) and is expressed as a number between 0 and 1. Also known as visible light transmittance (VLT).

Visible reflectance. The measured amount of energy in the visible wavelength range that is reflected by a window system; it is expressed as a percentage.

Visible spectrum. That portion of the total radiation that is visible to the human eye and which lies between the ultraviolet and infrared portions of the electromagnetic spectrum. The colors associated with the visible spectrum range from violet, indigo, blue, green, yellow, orange, through red.

Visual comfort. A set of qualities associated with the amenity of a window, such as freedom from glare and excessive contrast.

V-lambda curve. A bell-shaped function describing the relative response of the human eye to solar radiation as a function of wavelength under bright light conditions. Also known as photopic response function.

Warm edge. Term used to describe technology that uses insulating spacers to achieve better thermal performance of an insulating glass unit, particularly evident in the increase of edge surface temperatures on the indoor side in the winter.

Weatherstrip, weatherstripping. A strip of resilient and flexible material for covering the joint between the window sash and frame in order to reduce air leaks and prevent water from entering the structure. Also, the process of applying such material.

Window. The frame, equipped with sash(es), ventilator(s) or louvers, if any, and their fittings, which, when glazed with glass or substitute for it, closes an opening for the admission of air and/or light in the wall of a building. (From the old Norse word “vindauga,” which is formed from “vinder,” wind, and “auga,” eye. Therefore, a window is an “eye for the wind” or “wind-eye.”)

Window-to-wall ratio (WWR). The ratio of the total area of a building facade which is occupied by windows (glass area and frame).

Window unit. A complete window with sash and frame.

Window wall. A metal curtain wall of the commercial type in which windows are the most prominent element.

Wind pressure. The pressure produced by stopping the wind velocity; the main cause of air infiltration.

Wire glass. A glass with inner wire mesh for strength and fire-retardant qualities.

Xerogel. A type of transparent insulation material similar to aerogel but simpler to manufacture. It has both a higher visible transmittance and higher thermal conductivity than aerogel.

Zenith. The point on the skydome directly overhead, a 90 degree solar altitude.