It is most important, first of all, to keep the sun off the building. Buildings should be designed as selfshading. Usually, horizontal elements on the southern side and vertical elements on the east and west side of the building are beneficial architectural elements to shade the building. Light colors also work well to reflect heat from the exterior skin. As the thermal heat penetrates the skin of the building working its way to the interior, insulation is the main defense to stop this heat gain.
Insulation is part of the design of the exterior wall and the ceiling below the attic space.
Some insulated walls are necessary to separate nonconditioned spaces. Insulation works wells to stop thermal heat, but is ineffective against radiant heat. A radiant barrier is a mirrored metallic material that reflects the radiant heat back to the exterior. A radiant barrier is more effective when used in conjunction with an air space. Radiant barriers should be used in the attic and in all exterior walls.