Where the cooling load comes from — ceiling, walls, windows, air leaks, and internal gains

What this diagram shows

A summer cross-section of a house showing the five paths heat takes to get inside on a design day. The biggest is the ceiling and roof, because a vented attic runs about 15°F hotter than the outside air. Heat also conducts through the walls, comes through the windows as both conduction and direct solar gain, leaks in with outside air (infiltration), and is added inside by people, lights, and appliances. Because the envelope’s insulation and tightness and the climate’s design temperature scale every one of those paths, two identical 2,000-square-foot houses can need very different equipment — a leaky, poorly insulated house can require twice the tonnage of a tight, well-insulated one at the same floor area and climate. Floor area only counts the surfaces.