Heat flows in three distinct ways. In a wall, for example, heat can move in one, two or three ways at the same time.

• Conduction: Heat can be transferred directly from one part of an object to another part by molecules bumping into each other. For example, heat from a cast iron frying pan is transferred to the handle and eventually to your hand. Some materials conduct heat better than others, depending on their structure. Insulation works by reducing heat flow with tiny pockets of air, which are relatively poor conductors of heat.
• Convection: The movement of air or a fluid such as water can transfer heat. In an uninsulated wall space, for instance, air picks up heat from the warm side of the wall and then circulates to the cold wall, where it loses the heat. The mixing of warm and cold air also transfers some heat. Cold convection currents are often misinterpreted as air leaks around windows.
• Radiation: Any object will radiate heat in the same way as the sun or a fire does. When standing in front of a cold window, you radiate heat to the window and so you feel cold, even though the room temperature may be high.

In most houses, radiation accounts for less than 10 percent of heat loss and most of that loss will be associated with windows. Conduction and convection are the main causes of heat loss; convection is the main culprit when the house is leaky.