Take a typical building. Build it in any of four cities. Build it to any height from one to 32 stores. Use walls that are predominantly glass as in Figure 2 or predominantly concrete (gray or white). How will these choices affect the heat loss or gain- and consequently the cost of operating the building? A study by Engineering Research Associates of San Francisco provided the answers given here. The main conclusions are: substantial savings in owning and operating the heating and cooling systems can be realized by reducing the amount of window area; a 16 story glass wall building of gray concrete with 64 percent window area takes about 31 percent more energy to cool and about 24 percent more energy to heat than a 16 story concrete building with 32 percent window area; This comparison is not affected appreciably by building height; the location of the building does not affect the comparison appreciably; and the capacity of the cooling system needed for a building with white concrete walls can be at least two percent less than for one with gray concrete walls. In addition to the savings realized by using less window area in buildings, occupants also gain in comfort. Those who must sit near windows gain considerable radiant heat form sunlit glass. On cold, dark days the reverse is true. The human body radiates heat energy to the glass thus making the occupant uncomfortable. Because concrete panels have considerable mass they serve as energy storage reservoirs, producing a beneficial time lag in the balancing or averaging of peak temperatures. This results in more comfort and economy.