Insulation makes homes and buildings more comfortable and energy efficient, but it may also create another problem - condensation. Mold grows and wood rots where condensation occurs repeatedly or chronically. Vapor barriers help prevent these outcomes by introducing a moisture- and air-proof or resistant barrier or retarder between areas of sharply differing temperatures, most often at outside walls and attics.

Condensation occurs on surfaces that are much colder than the surrounding air. The cold sweat that forms on an icy drink in summer also occurs where heated indoor air meets the cold temperatures of winter. Conversely, it can also form where air-conditioned air collides with the humid outdoor atmosphere of summertime.

Vapor barriers are made of a number of different materials, such as plastic sheeting, treated Kraft paper, closed-cell foam insulation, even metallic foil. They are sold both separately and as part of insulation products.

Proper barrier installation is often a source of confusion. Does it face the inside of the home, with insulation against the elements, or is it better facing the outdoors? The correct answer depends on the climate where you live. Generally, where the need for winter heat is greater than the need for cooling in summer, vapor barriers should face indoors toward the living space. Where cooling needs are more important than winter heat, the opposite is true: the barrier belongs against the outside surface, with the insulation on its inside.

To be fully effective, a moisture barrier must be continuous, with no holes or tears in the material. For the same reason, the barrier should be taped around any protrusions such as electrical boxes and pipes.

[For more information on HHI see insulation, vapor barrier, and vapor retarder.]