Air movement accounts for more than 98% of all water vapor movement in building cavities. Air naturally moves from a high-pressure area to a lower one by the easiest path possible-generally, through any available hole or crack in the building envelope. The remaining two percent of moisture movement is by heat transfer and diffusion. Sealing air pathways is one of the most important methods of reducing moisture in the home.

The laws of physics govern how moist air reacts within various temperature conditions. The study of the properties of moist air is technically referred to as “psychrometrics.” A psychrometric chart is used by professionals to determine at what temperature and moisture concentration water vapor begins to condense. This is called the “dew point.” By learning how to determine the dew point, you will better understand how to diagnose moisture problems in a house.

Employing an unvented attic space may require designing the attic/roof space as conditioned space, similar to that required when creating habitable space in the attic. There are several sources with more information on unvented attic designs. Traditional attic ventilation remains a cost-effective, though imperfect, solution for moisture control. In colder climates, roof ventilation serves to remove humidity and condensation from the roof/attic space, and helps to prevent the chronic formation of ice dams at the eaves. Attic spaces and roof cavities should be ventilated in accordance with minimum requirements of local building codes.

Roof overhangs and projections, such as porch roofs and overhanging upper floors, provide a primary means to deflect rainwater away from building walls. Thus, the potential for water penetration through siding, windows and doors is minimized. Because the protection of roof overhangs increases with increasing overhang width, larger overhangs than those recommended in this section may be important in the consideration of weather-resistant wall-barrier design.

While roof overhangs and porch roofs protect building walls from impinging rain, gutters serve to protect building walls and foundations from roof water runoff. Roof gutters, downspouts and leaders or diverters form the initial components of a drainage system for the building and site. A proper design of gutters and downspouts for water-shedding (steep-slope) roof systems should be looked for during an inspection.

Due to the potential for water accumulation within the wall cavity, a water-resistant membrane must be installed behind any exterior siding and veneer. Under certain conditions, it is permissible to eliminate the membrane in detached accessory buildings, or where the siding, finish materials or lath provides the needed protection.

Face-Sealed: This type of WRE relies exclusively on the ability of the outer surface of the wall and joints around penetrations to deflect water and prevent it from penetrating the wall surface. If a defect in the wall surface or joint detailing (such as caulk) exists or occurs over time, then water will penetrate and potentially accumulate in the wall, causing damage to any moisture-sensitive materials within the assembly. One example of this type of system is known as conventional or barrier EIFS (exterior insulation finish system). However, building standards only allow the use of a new type of drainable EIFS (i.e., drained cavity) on residential construction.

Rainscreen: A rainscreen can be considered an incremental improvement over the drained-cavity approach. This type of WRE is uncommon in the U.S. but has been used to some extent in Canada to address severe climate conditions. By the addition of some details to help reduce air-pressure differential across the cladding system during wind-driven rain events, water penetration into the drainage cavity is further limited. At a minimum, this approach involves use of an air barrier behind the cladding to resist wind pressures. Thus, wind pressure across the siding (which is vented and not airtight) is reduced and is less likely to result in water being driven through the siding due to pressure differentials across the siding. Also, the cavity between the cladding and water/air barrier must be compartmentalized by use of airtight blocking or furring at corners of the building, as a minimum practice. This feature prevents pressure differences on different surfaces of the building from “communicating” through a continuous cavity behind the cladding, which can cause unintended pressure differences across the cladding that drive rainwater through the cladding into the drainage cavity. Because many of the required components of a basic rainscreen system are already present in a simple drained-cavity wall system, drained-cavity systems are generally considered a more practical alternative for typical applications.

Relying on window and door products that are labeled according to standard test methods does not necessarily guarantee that water leakage will not occur through frames into walls. Frames that rely on seals and sealants at internal and exposed joints will eventually leak water, as these joints fail over time. The life expectancy of window and door units may vary widely, from 10 to 50+ years, depending on unit type and materials, exposure, maintenance, types of seals and sealants used at joints, and other factors. Frames that rely on “welding” of joints rather than sealants will generally provide a longer moisture-resistant service life.

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