Vapor Retarder Location Cold winter climates: Indoor air substantially contains more water vapor than the outdoor air as warm air can hold more water vapor than cold air. The vapor retarder membrane - when desired - will be placed on the warm in winter side of the assembly (see illustration below). A misplaced vapor retarder promotes condensation. If a vapor retarder is placed in the wall in a cold-inwinter location, vapor will flow toward it from the interior, be prevented from continuing out of the wall, and condense in the wall. WeatherSeal Spray and Roll-On and WeatherSeal Trowel-On are made to be vapor permeable for this reason. As the water vapor migrates towards the exterior, it can get concentrated and cold enough to condense and thus wetting materials in the wall. A material that retards the flow of vapor from the warm indoor air into the wall can prevent that condensation; that is why the vapor retarder is placed to the interior side of stud cavity insulation. Hot Climates: Some wall designs in the hot-humid climate zone can benefit from lower permeance barriers on the hotin-summer side of wall. The idea is that this is the reverse of the cold climate situation. The vapor drive is from the outside toward the inside, sometimes called "reverse vapor drive". However, the potential for trapping moisture must be examined by a dewpoint analysis. WARM INTERIOR AIR COLD OUTSIDE AIR VAPOR RETARDER INCORRECTLY PLACED ON THE COLD AIR SIDE OF THE ASSEMBLY. Mixed Climates: Walls in mixed climates are exposed to vapor drives from the inside in winter and from the outside in summer. In a mixed climate, the vapor retarder placement that could be right for summer may be wrong for winter and vice versa and that is why mixed climates generally should not have vapor retarders. Dew Point Analysis Parex USA offers dew point analysis services at no charge for architects, engineers and designers who need guidance in determining the location of potential condensation (where the dew point occurs) within a wall assembly. The wall analysis is based on a variety of factors, including environmental conditions and building materials used. MA RIN E VERY COLD COLD TD HO RY MIXED HUMID MIXED DRY HOT HUMID 5