<P> Vapor retarders slow the rate of vapor diffusion into the thermal envelope of a structure . Other wetting mechanisms, such as wind - borne rain, capillary wicking of ground moisture, air transport (infiltration), are equally important . </P> <P> The industry has recognized that in many circumstances it may be impractical to design and build building assemblies which never get wet . Good design and practice involve controlling the wetting of building assemblies from both the exterior and interior . So, the use of vapor barrier should be taken into consideration . Their use has already been legislated within the building code of some countries (such as the U.S., Canada, Ireland, England, Scotland & Wales). How, where, and whether a vapor barrier (vapor diffusion retarder) should be used depends on the climate . Typically, the number of heating degree days (HDD) in an area is used to help make these determinations . A heating degree day is a unit that measures how often outdoor daily dry - bulb temperatures fall below an assumed base, normally 18 ° C (65 ° F). For building in most parts of North America, where winter heating conditions predominate, vapor barrier are placed toward the interior, heated side of insulation in the assembly . In humid regions where warm - weather cooling predominates within buildings, the vapor barrier should be located toward the exterior side of insulation . In relatively mild or balanced climates, or where assemblies are designed to minimize condensation conditions, a vapor barrier may not be necessary at all . </P> <P> An interior vapor retarder is useful in heating - dominated climates while an exterior vapor retarder is useful in cooling - dominated climates . In most climates it is often better to have a vapor - open building assembly, meaning that walls and roofs should be designed to dry: either to the inside, the outside, or both, so the ventilation of water vapor should be taken into consideration . A vapor barrier on the warm side of the envelope must be combined with a venting path on the cold side of the insulation . This is because no vapor barrier is perfect, and because water may get into the structure, typically from rain . In general, the better the vapor barrier and the drier the conditions, the less venting is required . </P> <P> In areas below foundation level (or, subgrade areas), particularly those formed in concrete, vapor retarder placement can be problematic, as moisture infiltration from capillary action can exceed water vapor movement outward through framed and insulated walls . </P>

Why is vapor barrier on the warm side