High humidity environments can present real challenges while you're designing your wall system. When both high heat and high humidity are present, it can lead to even more construction complications. If you're looking to build a wall, especially in what you know to be a moist environment, there are a few basic things you need to know about vapor barriers.
1. Moisture should always be a consideration.
Just about every place on the planet experiences some amount of moisture. Short of constantly building in the Atacama Desert, you're going to build walls that deal with water, and you need to be ready to take your individual environmental circumstances into account to minimize the structural impact of water.
2. Vapor barriers are about more than balance.
When it comes to installing vapor barriers, many builders fear overdoing it and ending up with too much wall drying. The barrier level needs to keep air and water from seeping into the cladding of your wall, but many worry that high permeability barriers like DuPont ™ Tyvek ® building wrap allows rain to be drawn straight into it as the cladding heats. It's an understandable fear, but not a justified one: tests have shown that high permeability barriers can still be suited to humid environments.
3. Permeability isn't really a major issue.
Designed in conjunction with the Building Science Corporation of Waterloo, Ontario, and Somerville, MA, the Relocatable Building Enclosure Test Station (RBETS) is a program that has been testing wall performance in the hot-humid climes of Tampa, FL, for over ten years now. Different combinations of wall assemblies are monitored for humidity, moisture, and temperature over time, and subjected to extra water.
Interior vapor barriers with different levels of permeability were installed on different RBETS walls to test how much of an impact permeability made on managing vapor drive. To the surprise of some, the tests revealed that there wasn't much of an impact at all. By the end of testing, no optimum range of vapor permeability could be determined, simply because there was no real difference between vapor barriers with high or low permeability.
4. The real culprit isn't everyday humidity.
The RBETS tests show that even without an interior vapor barrier, a well-built wall is likely to hold up perfectly against moisture, drying completely even in the cooler winters. A bigger issue? Leaks, which cause a lot more damage than the normal amount of moisture driven into the cladding. When water pours in between the wall and the roof, inside and outside drying is crucial. When you're thinking about moisture, let permeability take a back seat to managing bulk water.
In the end, the science is sound: no matter how humid your climate is, nothing is more important for a moisture barrier than being able to handle the rain and moisture leaking in beyond daily atmospheric conditions. Spend less time worrying about the red herring of vapor permeability when you're building your wall system, and instead, make sure it's ready for hard rain.