Generally, vapor barrier is a term often used when discussing humidity (or vapor pressure) where vapor (e.g. water vapor) is prevented from moving between discrete spaces or volumes. Because no material or installation is perfect, moisture will still cross the surface to be protected; the formal term is therefore vapor retarder. Similarly, air barriers are intended to eliminate air infiltration, but are more accurately entitled air retarders. A single material that is intended to reduce both air and moisture movement is therefore an air and moisture retarder.
Many people confuse vapor barriers and air barriers or assume that they are one and the same; this is often a false assumption. Many vapor barriers can act as air barriers but only if they are strong enough to sustain the air pressure loads placed upon them. Air barriers need not be vapor permeable. An example of this is a roof membrane in a flat roof application. In the case of the roof membrane the integrity of the edge condition is critical.
You should never install a vapor barrier where the barrier may trap moisture and allow mold and fungus to grow. Instead, using building wrap, which cuts air infiltration but allows moisture to escape.
You should install a thick, cross-laminated, high-density polyethylene vapor barrier under the concrete slab when building a house. Concrete keeps out most liquid water when used as floor, but it is porous, so some moisture will always wick up and transpire into the air. Here is the general sequence of tasks:
- Excavate for footings, walls, slab, and sump.
- Pour footings. Pour walls, or build concrete block walls. Include foundation vents in the walls.
- Install plastic sump in sump hole.
- Apply crushed stone before installing the polyethylene vapor barrier. This channels water to the perimeter drain and sump pump.
- Install a thick, cross-laminated, high-density polyethylene on top of the stone. Putting sand or stone on top of the vapor barrier and under the concrete creates a catch-basin for water that may remain for long periods, never draining away and gradually wicking up into the basement and creating a damp slab.
- Install a built-in perimeter drain to divert water away from the walls. (Even with the vapor barrier, water will often work its way around the edges of the slab and the vapor barrier. The purpose of the perimeter drain and sump pump is to collect this water and pump it away from the house.)
- Pour the slab.
Regarding basements and crawlspaces in general:
- You should not install vapor barrier paint on interior foundation walls of damp basements or crawlspaces. If excessive moisture enters the house through the foundation walls, you should remedy the problem at its source - from the outside.
- Most of the condensation that occurs in dirt-floor crawlspaces and basements with concrete foundation walls comes directly from the earth, even if it looks bone dry. You should always cover a dirt floor with a vapor barrier, whether the dirt floor is in a basement or a crawlspace.
- When you apply vapor barrier to the floor of your dirt basement or crawlspace, you should not install the vapor barrier directly against wood posts and studs. This will trap moisture against the wood, encouraging rot.
For a comprehensive article on vapor barriers showing that most of the info above is wrong go to: http://www.buildingscience.com/resources/3-Understanding_Vapor_Barriers.pdf
