Buildings designed for passive solar heating with natural sunlight to light a building’s interior incorporate large south-facing windows, skylights, and building materials that absorb and slowly release the sun’s heat. Incorporating passive solar designs can reduce heating bills as much as 50 percent. Passive solar designs can also include natural ventilation for cooling. Windows are an important aspect of passive solar design. In cold climates, south-facing windows designed to let the sun’s heat in while insulating against the cold are ideal. In hot and moderate climates, the strategy is to admit light while rejecting heat. Interior spaces requiring the most light, heat, and cooling are located along the south face of the building, with less used space to the north. Open floor plans allow more sun inside.