When constructing unvented roofs with asphalt shingles in hot humid climates a vapor barrier must be installed between the asphalt shingles and the roof deck.
Vapor barrier roof hot humid climate.
In hot climates any moisture that condenses on the exterior side of the vapor barrier will be.
Classic hot humid climate wall assembly.
Asphalt shingles store water from dew condensation due to night sky cooling and rain.
The inner surface in colder climates and the outer surface in hot humid climates.
Vapor barriers are usually best installed on the side of the wall that experiences the hotter temperature and moister conditions.
The vapor retarder location specified above is specifically for hot humid climates.
In cold climates the location of the retarder is to the interior side of the thermal insulation.
In cold climates condensation is limited on the interior side of the vapor barrier as a result of installing all of the thermal insulation on the exterior side of the vapor barrier which is also the drainage plane and air barrier in this assembly.
In hot humid climates the design location and installation of the air and weather barriers are more critical than for the vapor retarder.
In very warm humid climates if a vapor retarder is used naima recommends installing it to the exterior side of the wall.