or smaller depth. Deflection in curtainwall mullions also differs from deflection of the building structure, whether concrete, steel, or timber. Curtainwall anchors must be designed to allow differential movement between the building structure and the curtainwall. Strength Strength (or maximum usable stress) available to a particular material is not related to its material stiffness (the material property governing deflection); it is a separate criterion in curtain wall design and analysis. This often affects the selection of materials and sizes for design of the system. The allowable bending strength for certain aluminum alloys, such as those typically used in curtain wall framing, approaches the allowable bending strength of steel alloys used in building construction. Thermal criteria Relative to other building components, aluminum has a high heat transfer coefficient, meaning that aluminum is a very good conductor of heat. This translates into high heat loss through aluminum (or steel) curtain wall mullions. There are several ways to compensate for this heat loss, the most common way being the addition of thermal breaks. Thermal breaks are barriers between exterior metal and interior metal, usually made of polyvinyl chloride (PVC). These breaks provide a significant decrease in the thermal conductivity of the curtain wall. However, since the thermal break interrupts the aluminum mullion, the overall moment of inertia of the mullion is reduced and must be accounted for in the structural analysis and deflection analysis of the system. Thermal conductivity of the curtain wall system is important because of heat loss through the wall, which affects the heating and cooling costs of the building. On a poorly performing curtain wall, condensation may form on the interior of the mullions. This could cause damage to adjacent interior trim and walls. Rigid insulation is provided in spandrel areas to provide a higher R-value at these locations. Thermally-broken mullions with double or triple-glazed IGUs are often referred to as "high-performance" curtainwalls. While these curtainwall systems are more energy efficient than older, single glazed versions, they are still significantly less efficient than opaque (solid) wall construction. For example, nearly all curtainwall systems, thermally-broken or otherwise, have a U-value of 0.2 or higher, which is equivalent to an R-value of 5 or lower. Infills Infill refers to the large panels that are inserted into the curtain wall between mullions. Infills are typically glass but may be made up of nearly any exterior building element. Some common infills include metal