Your building's 'envelope' is made up of windows, outside doors, walls, the foundation, floor, roof, and perhaps skylights. The envelope shields the carefully controlled, temperate indoor environment from the sometimes harsh outdoor environment. If the envelope works well as a barrier and as a filter, you will use less energy in your lighting and HVAC systems to control the light, temperature, humidity, and fresh air levels. The basic objectives of the envelope are to stop infiltration, reduce heat transfer, control humidity, and control sunlight.