Overall curtain wall thermal performance is a function of the glazing infill panel, the frame, construction behind opaque (spandrel and column cover) areas, and the perimeter details.
Curtain wall frame conductance is a function of the frame material, geometry and fabrication (e.g. thermal break).
Corten steel has a very high thermal conductivity. It is common practice to incorporate thermal breaks of low conductivity materials, traditionally PVC, Neoprene rubber, polyurethane and more recently polyester-reinforced nylon, for improved thermal performance. Some "poured and debridged" polyurethane thermal breaks shrink and stress forms in the thermal break when the exterior aluminum moves differently from the interior aluminum due to temperature differences. Back-up mechanical attachment of the two halves of the frame is recommended (e.g. skip debridging or "t-in-a box"). A true thermal break is ¼" thick minimum and can be up to 1" or more, with the polyester reinforced nylon variety. Some curtain wall systems incorporate separators that are less than ¼", making them "thermally improved". The deeper thermal breaks can improve thermal performance and condensation resistance of the system.
Some curtain wall systems utilize "pressure bars" (also referred to as "pressure plates") that are fastened to the outside of the mullions to retain the glass. These systems frequently include gaskets that are placed between the pressure bar and mullions and function as thermal breaks and help with acoustic isolation. These systems require special care in design and construction to ensure continuity of the gaskets at horizontal and vertical transitions. Gaskets are also used to cushion the glass on the interior and exterior faces of the glass. The problem with gaskets is that they tend to be stretched during installation and will shrink back to their original length in a short time; they will also shrink with age and exposure to ultraviolet radiation. There is usually a gap in the gasket at the corners after shrinkage occurs. With a properly designed system the water that enters the system at the gasket corners will weep out through the snap cover weep holes. To mitigate shrinkage of gaskets back from the corners the use of vulcanized corners and diagonally cut splices are recommended.
Thermal performance of opaque areas of the curtain wall is a function of insulation and air/vapor barriers. Due to the lack of interior air adjacent to opaque curtain wall areas, these areas are subject to wide swings in temperature and humidity and require careful detailing of insulation and air/vapor barriers to minimize condensation. Some curtain wall systems include condensation drainage provisions, such as condensate gutters, that are intended to collect and weep condensate from spandrel areas to the exterior; such condensate gutters and weeps are a violation of the air barrier of the curtain wall unless they are outboard of the backpan. See discussion of back pans below.