Figure 1. Common places for moisture to intrude into a building.
Moisture intrusion refers to water getting into a building and is one the most significant factor affecting the durability of a building envelope, which is its ability to continue to perform its functions over time. Building moisture can cause mold, fungal growth and other microbial contamination, as well as corrosion, decay, rot and other moisture related deterioration.  These issues can pose serious health risks for occupants, and will likely lead to costly repairs to correct the problems that first allowed moisture to invade the envelope.
The basic causes of moisture intrusion into buildings, both liquid and vapor, include:
- Infiltration of rainwater/groundwater: significant amounts of moisture can enter a building through leaks in the envelope. Rain and groundwater can be drawn into a building though capillary action, surface tension, gravity, wind, and differences in air pressure.
- Infiltration of moist outside air: humid air from outside the building can be introduced by air infiltration through openings in the envelope or through the HVAC system. This is a significant problem in hot and humid climates.
- Moisture created within building: the daily activities of the occupants of a building and routine housekeeping procedures can generate additional moisture within the building itself.
- Diffusion of water vapor through building envelope: differences in water vapor pressure between the inside and outside of a building can cause water vapor to pass through the building envelope. This is a significant problem in cold climates. Small amounts of moisture within a building will condense inside the cavities of cold walls during winter months.
Prevention & Control
Figure 2. The slate provides a non-porous layer that the water can't get through, it's referred to as a 'damp proof course'.
The wall system of a building can be one the primary protections against the various forms of moisture infiltration. The components of the wall system include:
- Exterior finishes: common materials of exterior walls include stucco, wood siding, concrete, and brick. These materials can limit moisture and vapor migration into and out of the building. They also contribute to the finish function of the envelope and should be aesthetically pleasing.
- Vapor retarders: retarders include plastics (reinforced and flexible), aluminum, treated papers, and epoxies. They are not always required if other wall components can act as a sufficient barrier to vapor, such as the exterior finishes. Low porosity rock like slate can be used, see figure 2, to create what's called a 'damp proof course'. This means that there's no path (course) for the damp to get into the house.
- Air infiltration and rain barriers: play an important role in preventing air infiltration from winds and other weather conditions. Building envelope components working together can act an air barrier. The performance of the barrier greatly diminishes as the number of joints, cracks, and crevices increases.
- Insulation: special types of insulation, such as closed cell and nonhydroscopic insulation, can help reduce high moisture levels that can develop in walls. The insulation acts as a vapor barrier depending on its thickness.
- Interior finishes: the permeability, or ability of a material to allow liquids and gases to pass through it, is an important consideration for interior wall finishes. Generally, in hot and humid climates the permeability of the interior finish should be much higher than that of the exterior finish. This will allow moisture that enters the wall to migrate to the inside of the building where the moisture will be removed by the air conditioning system. The reverse is true for cold climates; the exterior finish should have a higher permeability than the interior finish.