Where should the vapor barrier be located with regard to insulation, for a home in a tropical environment that has no conditioned space?
fiberglass batt, high ambient humidity, rafter cavity
If your home has no conditioned space, then it does not need a vapor barrier since the inside and outside are the same environment.
Critical when you have a conditioned space
But I will assume you are going to create a conditioned space by insulating and mechanically conditioning your home, and then the placement of the vapor barrier becomes critical.
There are two central issue to keep in mind when deciding where to place any vapor membrane:
- warm, humid air can condense on a cooler surface and create liquid water, which you definitely do not want inside of a wall assembly, and
- the correct product is not an air and vapor barrier such as plastic film, but a vapor ' diffusion retarder'.
When cooling your home is more common than heating
In a warm, humid climate such as Hawaii, where cooling the interior of your home is more common than heating, a vapor barrier should be on the warm, or exterior, side of the wall assembly.
If it is placed on the interior of the wall, underneath the interior wall covering, and your home is cooled with an air conditioner, then warm humid air will make it through the insulation, condense against the vapor barrier, and the resulting water will cause fungus and rot inside the wall.
For more information, read this article on the US Department of Energy's website.