Photo courtesy of Shaw Roofing

Building science is, in essence, the study of the impacts of heat, air, and moisture on the building enclosure. Through the study of building science, the design professional learns how to prevent damage from air and moisture infiltration, and how to improve the overall energy efficiency, durability, and long-term performance of buildings. Rather than viewing the elements of a building as a collection of individual and unrelated components, ensuring performance of the building enclosure requires a systems perspective. Avoiding risks in a building structure requires understanding how the increasingly complex systems required in a building come together, stay together, and perform together.

The Fundamental Science of Control Layers

It is an inescapable and universal truth that natural forces are always at work on buildings. The fundamentals of building science are found in the Second Law of Thermodynamics:

• Heat will flow from warm to cold;
• Moisture will flow from warm to cold;
• Moisture will also flow from wet to dry;
• Air flows from an area of higher pressure to one with lower pressure; and
• Gravity acts down.

Image courtesy of GAF|Siplast

Energy, air, and moisture will flow between two zones until a state of equilibrium or balance is achieved. Understanding the preferred path of movement is the first step in knowing how to control the environment within a building enclosure.

What are Control Layers?

Control layer is the term used for the parts of the building enclosure that are designed to stop external elements from moving uncontrolled through a structure. There are four main control layers in a building: they address liquid water, air, heat, and water vapor. Whether the control layer completely stops the movement of the elements or slows down the movement depends on the materials used and the needs of the building.

In essence, the four control layers can be split into two main categories: those that manage moisture and those that manage energy efficiency. Moisture management works to prevent liquid water, water vapor, and condensation from impacting the building. Managing energy efficiency focuses on the movement of air and heat through the building enclosure. All of the control layers need to work together to achieve maximum effectiveness.

Moisture Management

Of the many challenges a building enclosure faces, water represents the greatest continuous threat. The presence of water in unwanted locations within the building enclosure can contribute to rot, decay, and biological growth. There are four primary ways moisture makes its way into a building and they all need to be considered when designing a building’s enclosure: flow of liquid water, capillary suction, air-transported water vapor, and diffusion.

The most obvious and impactful way is liquid water, or water from rain or snow. Buildings must be designed to efficiently manage or shed water by moving the water out and away from the structure. This is the primary role of the water control layer in the enclosure assembly.

Next is capillary action. This is how water moves through pores and small cracks, sometimes acting against gravity, and seeping into masonry, foundations, and any other tight spaces in the building.

The last two—air-transported water vapor and diffusion—can be the hardest to see. Moisture vapor transported in either manner can cause damage when it is allowed to condense or become liquid in a place where the liquid water cannot be managed. The materials and designs used to control air intrusion and reduce vapor diffusion are often less obvious. They must interact with the other building control layers, such as the air and thermal control layers, in order to perform.

The importance of a continuous air control layer to manage uncontrolled airflow, and the moisture it carries, cannot be overstated. Gaps and discontinuities in the air control layer within the building enclosure allow air and its associated moisture to enter the assembly. Air leakage can carry as much as 100 times the amount of moisture through a hole in the building enclosure when compared with the transport from vapor diffusion moving through the building materials. The amount of moisture air will hold is dependent on its temperature. Warm air can hold more moisture than cold air. When warm, moist air encounters a cold surface, the air will cool and the excess moisture that the air can no longer hold will condense on the cold surface, causing a problem if the water cannot be managed at that location.

Vapor diffusion through building materials can be less impactful because water vapor moves much slower in this manner. All materials used within the building enclosure have a permeance and will slow the movement of moisture vapor that is moving through the assembly by way of diffusion. Understanding the permeance of materials and how easily moisture vapor may flow through them—or not—is the first step in preventing issues caused by moisture moving in this manner.

Image courtesy of GAF|Siplast

Moisture management strategies.

Using the roof-to-wall interface as an example to illustrate moisture management strategies, the first line of defense is to drain water away by directing water to roof drains and out from the wall. The next step involves venting or using airflow to pull moisture vapor out of the building enclosure assembly. The final moisture management step is managing diffusion: allowing diffusion to dry the assembly in at least one direction, accommodating any incidental moisture in the building materials to leave the assembly. The process of diffusion is slow and should be a strategy only for incidental moisture. Ensuring moisture vapor does not become condensation and liquid water where it cannot be managed in one of these ways becomes the responsibility of the thermal control layer.