Specify a Climate-Appropriate Exterior Wall Covering Method

Exterior wall covering materials and methods may look the same on the outside, but they may not perform the same even though they may be considered equivalent from a building code perspective. Exterior wall covering assemblies can include features behind the siding that either improve or diminish a wall’s ability to protect a building from rainwater intrusion and moisture accumulation. The importance of this consideration varies depending on climate. For example, some regions of the U.S. experience as much as 40”/year of rainfall impinging on vertical surfaces such as walls of buildings! Thus, it is appropriate to consider that walls should perform like a vertical roof surface in deflecting and draining as much as 20 gallons per square foot per year of rainwater away from a building.

This section presents a simple procedure for selecting a durable and climate-appropriate method for constructing an exterior wall covering assembly—something that U.S. building codes do not address—to ensure risk-consistent performance in all climate conditions. Other important considerations include the proper use and integration of flashing methods (Section 4.2.8) with a variety of water-resistive barrier (WRB). Proper specification of wind-driven rain resistant window and door products is also important; this topic is addressed in Chapter 8 as it is especially important in areas subject to natural hazards such as tropical storms and hurricanes, although severe thunderstorms can also cause similar conditions but with a much shorter duration.

There are essentially four methods for constructing a weather-resistant exterior wall envelope as illustrated and described in Table 4–6. Each of these methods of construction provides different levels for managing or resisting rainwater by relying, to differing degrees, on one or more of the following three principles:

Deflection—Use of cladding to deflect water away from protected building components including the underlying water-resistive barrier layer.
Drainage—Use of a water-resistive barrier (WRB) integrated with other wall components and flashing to create a continuous drainage plane that directs water that leaks behind the cladding (not deflected by the cladding surface) to drain downward and out of the assembly.
Drying—Use of a vented air-space behind siding to promote evaporation of water that is not immediately drained or which is absorbed.

A fourth principle that always applies is related to the durability of the materials used for the various components of an exterior wall covering assembly as well as the protected structural material within the wall. Specification of building materials that are tolerant of moisture and resistant to degradation (rot or corrosion) will lessen the consequences of any moisture that defies the first three principles. Differences in the application of these principles are largely responsible for differences in the performance and durability of exterior walls. Other determining factors include severity of wind-driven rain climate, roof overhang shielding of the wall, and site wind exposure—all of which are addressed in this section.

The design process below can be used to assess and specify an appropriate method of construction for an exterior wall covering assembly (see Table 4–6) based on wind-driven rain climate, roof overhang protection, and wind exposure—the key factors governing the severity of rainwater loads experienced by walls.

Step 1: Assess Site Climate Condition

Climatic conditions are categorized on the basis of the potential for wetting of walls, especially wetting from wind-driven rain. The exposure categories are:

• Severe—climates that cause frequent wetting due to wind-driven rain, such as coastal climates and areas prone to frequent thunderstorm events.
• Moderate—climates that cause periodic exposure to wind-driven rain.
• Low—climates that are relatively dry with little rainfall or wind-driven rain.

The above classifications are intentionally subjective, as there are no clearly defined criteria in the United States for assessing wind-driven rain and its effects on building wall systems. However, a wind-driven rain map is provided in Figure 4–10 as a means of classifying the severity of a local climate based on the above categories.

Step 2: Assess Building Exposure

The terrain surrounding a building impacts its exposure to wind-driven rain. The ratio of roof overhang width to the height of the protected wall below also alters the exposure of a given building wall to weather and wind-driven rain. Wide roof overhangs relative to wall height effectively reduce the exposure. Similarly, increased shielding of the site against wind tends to reduce the effects of climate.

Table 4–7 may be used to determine a building’s exposure level, based on the climate condition determined in Step 1, the roof overhang ratio, and the wind exposure. The exposure level then leads to a basis for selecting an appropriate exterior wall covering assembly method in Step 3. The exposure levels in Table 4–7 can also be used on a smaller scale to get a sense of the exposure for particular faces of a building or even for specific envelope elements like a window. Understanding the exposure in this manner can course decisions on flashing details, potential use of greater overhangs, and areas to focus attention for construction quality control.

Step 3: Select an Exterior Wall Covering Assembly Method

Based on the building exposure level determined in Step 2, use Table 4–8 to select an appropriate exterior wall covering assembly method based on relative performance expectations. Alternatively, other factors may be reconsidered in the building and site design to improve protection from rain, such as the use of larger overhangs to protect walls.