General

Chapter 4 presents actionable design guidance important to an effective and durable moisture control strategy for above-grade building assemblies—roofs and exterior walls. Nearly all matters addressed in this chapter are climate-dependent solutions because moisture is largely a climate-dependent durability hazard. Section 4.2 focuses on control of rainwater and Section 4.3 focuses on control of water vapor diffusion.

While simple in concept, the achievement of durable, moisture-resistant buildings is challenging because it is often the “little things that make a big difference”; and, many of these “little things” are hidden after construction is completed or may not be obvious to those with a casual understanding of modern construction materials and methods. For example, some of the most important flashing elements are hidden after siding is installed. Also, water vapor diffusion relates to intrinsic or “hidden” properties of the materials which may vary significantly even among similar-looking material types. Thus, durability as it relates to moisture control must involve a well-informed and integrated design and construction approach (refer to text box). Quality control during construction is also important to supplement normal building department inspections, which may tend to focus on building safety issues and therefore may not address the “little” details that can make a big difference for durability.

Why is the guidance in Chapter 4 so important? Failures to appropriately address the effects of rain and water vapor have contributed to varying degrees of moisture durability problems—including rot, corrosion, collapse, insect infestation, and mold—in as much as ¼th of the housing stock in some of the most vulnerable regions of North America.5, 6, 7 Particularly severe and accelerated moisture durability failures have occurred when good practices for control of rain and water vapor have not been properly integrated into the design and construction of a home.8 Therefore, moisture management is not just a matter of building durability; it also is a risk management concern with serious potential liability implications (refer to Chapter 2).