Let's first understand how heat is transferred. It moves from warm to cold in three ways:

1. Radiation – electromagnetic waves traveling through space, such as from the sun or a radiator.

2. Convection – moving air. This is the reason for warm air leaks and cold air infiltration through cracks, seams and other gaps in the building envelope. Warm air rises upward to the attic or up the chimney and pulls in cold air through the basement or gaps in windows and doors, etc. on lower floors.

3. Conduction transfer of heat though a solid object, moving from warm to cold. Certain substances, like glass and metal, are very good heat conductors while other substances, such as cellulose, soy, hay or other materials used to make insulation, are good insulators. The degree to which a particular object resists the conduction of heat is referred to as its R-value, and the higher the R-value the better the resistance/insulation.

Insulation primarily addresses the heat radiation and conduction issues and will also help with convection. Sealing cracks, gaps and the like, addresses convection issues. By now it's probably obvious to you that properly controlling heat loss and gain involves paying attention to all three aspects of the heat equation; only dealing with one aspect, such as caulking windows, without giving attention to the others, such as adequately insulating your attic, will only be a band-aid with limited results, and making a radical change, like replacing y our furnace with a high-efficiency model, will only result in limited savings if your efficiently-produced heat is flowing out of your leaky windows