One purpose of vapor control is to prevent condensation on or elevated humidity near the surface of moisture-sensitive materials. Another purpose of vapor control in building assemblies is to allow moisture-sensitive materials to dry should they get wet. So vapor control is about controlling the flow of moisture in the vapor form: sometimes restricting the flow of vapor, sometimes promoting it.
In order to control the flow of moisture in the vapor form, it is useful to understand how vapor moves. Water vapor moves with airflow - or by convection. Water vapor also moves through air and vapor permeable materials by diffusion. As already discussed above, the magnitude of vapor movement by airflow tends to be much greater than vapor movement by diffusion.
In addition to understanding how vapor moves, it is also useful to understand the direction that vapor moves. Airflow, therefore the movement of water vapor by convection, is always in the direction from higher pressure to lower pressure. Water vapor moves by diffusion from higher concentration to lower concentration (more to less) and from warm to cold. In a typical cold climate situation, air tends to leak out of a building near the top and into the building near the bottom in winter. In summer the leakage direction is reversed. For the above grade enclosure, diffusion vapor flow tends to be to the outside (warm to cold) in the heating season and for much of the year. However, in warm weather and in homes with air conditioning, diffusion vapor flow at the above grade enclosure tends to be to the inside. For ground contact assemblies like foundation walls and slabs, diffusion vapor movement is nearly always outside to in (high concentration to lower concentration). However since ground-coupled foundation walls and slabs are usually cooler than conditioned space, water vapor will also tend to move toward these surfaces from the interior.
The strategies for vapor control are
Controlling airflow relies upon using air barrier materials that resist airflow (see discussion of Air Control above). Airflow may also be controlled by manipulating air pressures.
Airflow might also be promoted to assist in removal of moisture as in a vented attic or vented wall cladding.
Control of vapor diffusion (either restricting or promoting) is a function of the vapor permeability (sometimes referred to as simply "permeability") of materials in an assembly. For above-grade portions of the building, using materials with low vapor permeability to the typically-warmer side of moisture-sensitive materials restricts the diffusion vapor movement toward the moisture-sensitive material. Use of highly vapor permeable materials to the typically-cooler or the ventilated side of moisture-sensitive materials promotes the diffusion vapor movement away - or drying - from the moisture- sensitive material. It is wise to use a low permeability membrane or coating between any moisture sensitive materials and concrete that is in contact with the ground because the ground typically has a higher vapor concentration than the interior.
Controlling the temperature of moisture sensitive materials and assemblies is achieved by the amount and position of insulation relative to the moisture sensitive material or assembly and conditioned space. Placing insulation to the outside of moisture-sensitive materials tends to make these materials generally warmer and, therefore, dryer. Placing moisture-sensitive materials to the outside of insulation tends to make these materials generally cooler and, therefore, wetter.