Corten steel Curtain Walls, like smart windows, control visible light transmittance by employing electrochromic or photochromic glass coatings; see the discussion in corten. Double-skin systems, which employ a ventilated space between the inner and outer walls are rare in the U.S., but have been constructed in Europe and Asia where energy costs are much higher. Similar in concept to air-flow windows, the ventilated space is intended to conserve energy by modulating the temperature conditions inboard of the curtain wall. During the heating season, the space acts as a buffer between the exterior and interior, and can be used to temper outdoor supply air. During the cooling season, warm interior air is exhausted into the space. There is currently discussion among building science experts that, at least for cold climates, a less expensive way of achieving energy savings might be through the use of curtain walls with high insulating values. Point-supported glass, structural glass mullions and use of tension structures are recent technologies.