Heating and Cooling: The Basics.

Very little energy is needed to make a well-designed house comfortable. Appropriate insulation, combined with passive design principles and a draught-proofed building, can create low or even no energy requirements for heating and cooling.
Appling the principles of thermal comfort, understanding how heat is transferred from the outdoors into your home and the importance of air movement, humidity and radiant heat is important for understanding the challenge of keeping your house cool and may help to reduce energy bills, improve comfort and help the environment.
On average we use about 40% of our energy for heating and cooling, 21% for water heating, 33% for appliances and equipment including refrigeration and cooking and 6% for lighting.

Heat transfer processes to and from objects are classified into three types: conduction, radiation, and convection.

Conduction is heat traveling through a solid material. A solid (a block of metal, say) has one surface at a high temperature and one at a lower temperature, and the heat move from the higher temperature surface to a lower temperature surface by conductive energy.
On hot days, heat is conducted into your home through the roof, walls, and windows.
Heat-reflecting roofs, insulation, and energy efficient windows will help to reduce that heat conduction.

Radiation is the emission of energy as waves or particles or rays, the heat is traveling in the form of visible and non-visible light. Sunlight is an obvious source of heat for homes.
In addition, low-wavelength, non-visible infrared radiation can carry heat directly from warm objects to cooler objects. Infrared radiation is why you can feel the heat of a hot burner element on a stovetop, even from across the room. Windows will allow infrared radiation coming from warm objects outside to radiate into your home; shades can help to block this radiation. Infrared radiation will also carry the heat of your walls and ceiling directly to your body.

Convection is the transfer of thermal energy through movement of particles from one location to another; usually occur in fluids (liquids and gases). Hot air naturally rises, carrying heat away from your walls and causing it to circulate throughout your home. As the hot air circulates past your skin (and you breathe it in), it warms you.