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I have worked on many homes and the rule for vapor barrier was "nearest to heated space"! This is the rule for cold climate construction. It is supposed to stop migration of moisture from the heated space into the insulated space. Right? If insulation becomes saturated with water, it looses its' efficiency. Also, it will cause the framing members to rot!
My problem is: I work in an area that is cold in the winters and very humid in the summer months. The reverse occurs during summer months when there is very high humidity. The vapor barrier is to the inside, the homes are air conditioned so a condition exists where the humidity within the houses is low but the humidity outside is very high. The hot humid air penetrates the house wrap and migrates inward and condenses on the outer surface of the vapor barrier. It is acting the same as a house with an improperly installed vapor barrier (many leaks)! Now you have moisture in your wall cavity and it is short circuiting the insulation and causing rotting conditions or at the very least (mold conditions)!
Dupont distributes a product in Japan that is used as the vapor barrier. It is similar to Tyvek or other house wraps where it allows water vapor to pass but not water as a liquid. They claim that this vapor barrier will allow a certain amount of water to pass to the conditioned space and the air conditioning will remove it by dehumidification. In that way preventing moisture from accumulating within the walls.
In the United States, I have only worked in very cold climates. For the past 10 years, I have worked in Japan and this problem keeps coming up. Many companies have different solutions for the problem and get government certification for their processes. How are vapor barriers used in Mississippi, Alabama, Florida, Georgia (ie the southern states) where the winters occasionally have temperatures near 0 degrees F. but summer temperatures and humidities that are extremely high? I build American Style homes in Japan and I want to give the best product to my customer.
Does anyone know where is the proper placement for a vapor barrier in hot humid climates?