panels, louvers, and photovoltaic panels. Infills are also referred to as spandrels or spandrel panels. Glass Float glass is by far the most common curtain wall glazing type. It can be manufactured in an almost infinite combination of color, thickness, and opacity. For commercial construction, the two most common thicknesses are 1/4 inch (6 mm) monolithic and 1 inch (25 mm) insulating glass. 1/4 inch glass is typically used only in spandrel areas, while insulating glass is used for the rest of the building (sometimes spandrel glass is specified as insulating glass as well). The 1 inch insulation glass is typically made up of two 1/4-inch lites of glass with a 1/2 inch (12 mm) airspace. The air inside is usually atmospheric air, but some inert gases, such as argon or krypton may be used to offer better thermal transmittance values. In Europe, triple-pane insulating glass infill is now common. In Scandinavia first curtain walls with quadruple-pane have been built. Larger thicknesses are typically employed for buildings or areas with higher thermal, relative humidity, or sound transmission requirements, such as laboratory areas or recording studios. In residential construction, thicknesses commonly used are 1/8 inch (3 mm) monolithic and 5/8 inch (16 mm) insulating glass. Glass may be used which is transparent, translucent, or opaque, or in varying degrees thereof. Transparent glass usually refers to vision glass in a curtain wall. Spandrel or vision glass may also contain translucent glass, which could be for security or aesthetic purposes. Opaque glass is used in areas to hide a column or spandrel beam or shear wall behind the curtain wall. Another method of hiding spandrel areas is through shadow box construction (providing a dark enclosed space behind the transparent or translucent glass). Shadow box construction creates a perception of depth behind the glass that is sometimes desired. Stone veneer Thin blocks (3 to 4 inches (75–100 mm)) of stone can be inset within a curtain wall system. The type of stone used is limited only by the strength of the stone and the ability to manufacture it in the proper shape and size. Common stone types used are: calcium silicate, granite, marble, travertine, limestone, and engineered stone. To reduce weight and improve strength, the natural stone may be attached to an aluminum honeycomb backing. Panels Metal panels can take various forms including stainless steel, aluminum plate; aluminum composite panels consisting of two thin