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Water Migration

Q: Can you explain the principles of controlling water migration through exterior walls?

A: This is a fairly complex topic and one that has caused significant changes in the model building codes. Not understanding these principles and, therefore, not designing for them or executing them in installation, can lead to degradation of water sensitive building materials and an unhealthy interior environment.


First, it is important to understand some building science essentials. Water comes in three states: liquid, solid and gas. It is water in its liquid state that can cause considerable damage if it enters a wall cavity. Water vapor by itself is not that harmful in the cavity. It is when water vapor is allowed to change into a liquid state when problems arise.


Natural convection drives air from an area of warmth to a colder region. Similarly, air containing a high concentration of water vapor will be driven to air with a lower concentration of water vapor. These two dynamics literally force water vapor through walls. The final key building science phenomenon is that air can hold only so much water vapor, and that maximum amount precipitously drops at lower temperatures. At these lower temperatures, the water changes from the vapor state into a liquid state. One strategy gaining acceptance is to maintain a temperate environment within the wall cavity, which greatly minimizes the potential for condensation. This is done by using continuous insulation on the exterior side of the framing. This use of continuous insulation is a requirement in the International Energy Conservation Code.


The direction of the temperature drives changes with the season. In summer it is warmer outside the building, and in winter the temperature is warmer in the interior of the building. This limits the ability of controlling water vapor through the use of materials that don’t allow water vapor to pass through them. A perm rating is a single number rating system that quantifies how well a material resists the flow of water vapor through it. The lower the perm rating, the more it resists this flow. At a certain level the material is considered a vapor retarder. While it would seem desirable to then place a vapor retarder on both sides of the exterior wall, that is not the case. This is termed a “double vapor retarder,” and it will trap any water in its liquid state that gets into the wall cavity.


A good source for understanding the design and installation of exterior walls is ASTM E2266, Standard Guide for Design and Construction of Low-Rise Frame Building Wall Systems to Resist Water Intrusion. There are a few terms that need to be defined to clearly understand these principles. The first is “cladding,” which is defined as the “outermost component of the wall system.” This leads to the two primary theories in wall design relating to water management. The first is a “barrier wall.” A barrier wall in the guide standard is defined as a wall system that is “intended to manage all precipitation at the exterior surface of the cladding and associated sealants and flashings.”


The second theory in the standard is the drainage wall. The concept in this case is that the cladding stops the majority of the water but “incorporates means for dissipating water that may circumvent the cladding.” This is where the model building codes require the use of a water-resistive barrier. The International Building Code mandates the use of this barrier. Along with the barrier there must be a means to remove the water in its liquid state and channel it away from the building. This is accomplished with strategically placed flashing and weeps. The caution here is that the self-adhering flashing must be compatible with the WRB for long-term in-place performance.


The interface between building components such as wall to window requires special detailing and care in installation. This is true for both air and water leakage. Although beyond the scope of this article, air barriers are equally important and a code requirement. The water and air barrier may be the same material. Both must be continuous throughout the exterior envelope. Special detailing around all punched openings such as doors and windows is the norm. This entails wrapping the WRB over the sheathing, over the cut edge of the sheathing, and on to the web of the framing member. This barrier must also be continuous where the framed wall intersects a structural concrete column or floor system.


Controlling moisture migration through an exterior wall is of critical importance. It can be accomplished by understanding the building science that controls the migration along with an understanding of the regulating codes and standards. The successful contractor understands both of these and exercises attention to detail during the construction phase of a project.

Robert Grupe is AWCI’s director of technical services. Send your questions to, or call him directly at (703) 538.1611.

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