Here are four strategies that are used to keep the structure dry:

• Exterior weather and moisture protection: This strategy requires building paper, siding, flashing, gutters and other construction techniques (e.g. a drainage plane) to shed water and repel wind-driven rain. It also involves below-grade measures such as proper drainage, grade slope and water proofing to protect the foundation from groundwater leaks or from moisture movement by capillary action.
• Reducing moisture at the source: This involves producing less moisture in the first place, exhausting moist air and bringing in drier air. (For solutions to moisture problems, see Chapter 9, "Operating your house.")
• Preventing moist indoor air from getting into the envelope: This requires a vapour barrier to reduce moisture movement by diffusion and an air barrier to prevent moisture movement by air leakage.

As a general rule, the vapour barrier should be on the warm side of the insulation. In some cases, however, the vapour barrier can be located within the wall or ceiling assembly, following the one-third, two-thirds rule (1/3 - 2/3 rule). This rule requires that at least two thirds of the insulation value of the wall is on the cold side of the vapour barrier (see Figure 2-12). Because this ratio should be adjusted for houses with high interior humidity (i.e. levels exceeding 50 percent, such as those with indoor pools or open spa-pools) or for homes in extremely cold climates (i.e. northern Canada), consult with your local building authority.

• Letting the envelope dry (through) to the outside: This final strategy allows the house to deal with seasonal fluctuations in humidity and to release any moisture that does penetrate the envelope from the interior or exterior. Drying to the outside is promoted by layering materials most resistant to vapour diffusion on the warm side of the envelope and the least resistant (such as building paper) on the outside.

Some wall systems work well with a relatively impermeable insulated sheathing because the interior wall-cavity temperatures are kept high. As a precaution, when retrofitting a wall, always install code-compliant RSI levels of insulated sheathing and ensure that the interior surfaces are vapour resistant.

Some siding applications have an air space or drainage plane immediately behind the exterior finish to promote drying out of materials that have been soaked by rain, wind or solar-driven dampness. This drainage plane also provides an escape route for any moisture that has penetrated the wall cavity from the indoors. If installing insulated siding, keep a drainage space behind the insulation even though a small amount of the insulation value of the siding will be lost.