Homes that are built to last don't happen by accident. The designers, builders, and contractors who work on them understand that there are generally three things that make houses fail:
- Water
- Moisture
- Humidity
Seriously, water, in all its various forms, is the biggest culprit in the failure of houses. The two main forms in which it interacts with houses are bulk water (liquid & ice) and water vapor. Some of the ways to deal with bulk water are:
- Roof overhangs. These keep water off the walls and away from foundations.
- Gutters. When they get the water at least 5 feet away from the foundation, they're most effective.
- Grade. The ground around the house must slope down from the house.
- Drainage planes. Behind the cladding on the walls, the drainage plane allows water that gets in to drain away instead of penetrating into the building materials. Flashing details (using building materials to channel the flow of water) around windows and doors are critical.
- Capillary breaks. An impermeable material between foundation and framing or between footings and foundation walls stops the wicking of water upward through the house.
Water vapor is big issue as well -- You never want to trap it in building assemblies. That means, don't put plastic in your walls. Over the life of the house, water will find a way into most places. If it gets in and can't get out, you end up with rotting wood, termites, mold,...
Vapor barriers aren't really necessary (except in very cold or very hot, humid climates) because most of the water vapor that gets in does so through infiltration, not diffusion. The way to keep water vapor from being a problem, therefore, is a little different than what used to be prescribed. Forget the vapor barrier -- Seal the air leaks.
The two big rules for dealing with moisture are:
1. Don't let water in.
2. For water that does get in, allow the building assemblies to dry out.