The building envelope consists of the walls, windows, doors, roof, floor, and foundation. This exoskeleton does much more than merely contain the conditioned air for the comfort of the occupants. It is also a significant contributor to the heating and cooling load. Heat moves through each of these elements, into the building in the summer and out of it in the winter.
The higher the temperature difference, the greater the thermodynamic flow of heat. The quantity of heat is also dependent on the quality of the insulation: Better insulation increases the thermal resistance of the building envelope. Many projects focus on the thermal principles of building design as cheap, practical ways to lower energy consumption.
A vapor barrier is an integral part of the building envelope. This is an impermeable membrane that prohibits the trespass of outside humidity into the house in the summer (and vice versa in the winter). Humidity never stops trying to equalize, inside vs. outside. The higher the humidity difference across a building element, the stronger the force for humidity to cross the barrier.
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Excerpted from Going Green, available as an eBook at this list of eBook retailers online.