Image 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.

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.

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.

Energy efficiency management

Managing energy efficiency of buildings relies upon both preventing uncontrolled air movement and resisting heat flow. Adding more insulation to the thermal control layer could seem to provide the answer to improving energy efficiency. However, simply adding more insulation to a building envelope soon reaches the point of rapidly diminishing returns. The real culprit behind poor efficiency is air movement. 

Airflow through the building envelope is the most effective method of carrying heat and moisture into or out of a building. In order for air to flow in or out of a space, there needs to be a pressure differential across the boundary and an opening for the air to move through. Buildings become pressurized due to wind, stack effect, and heating, air conditioning, and ventilation (HVAC) systems. Wind exerts pressure on the building enclosure causing positive pressure on the windward side and negative pressure on the opposite side. Stack effect is caused by warm air rising, increasing pressures higher up in the building. Any discontinuities in the air control layer, no matter how small, will provide the opening for air along with heat and moisture vapor to move through the enclosure.

Air leakage has been identified as a significant, quantifiable contributor to poor building energy efficiency. According to “The Hidden Science of High Performing Building Assemblies,” Environmental Building News November 2012, air infiltration and exfiltration make up an estimated 25-40 percent of total heat loss from a building in a cold climate. Buildings in hot climates are not immune to the impacts of air leakage. In a hot climate, air leakage makes up 10-15 percent of total heat gain, increasing the energy costs to cool the interior. Losing up to one-third of conditioned air to uncontrolled airflow has a weighty impact on the operation of a building and is a significant preventable cost. Moist, hot air entering the conditioned space not only requires extra energy to cool, it also must be dehumidified. If it is not dehumidified, humid air can condense when it contacts a cooler surface, leading to damaging water buildup. These increases in energy usage are why the International Energy Conservation Code (IECC) has required the use of air barriers in building enclosures since 2012.

Control layers that address airflow not only create energy savings but also help to mitigate moisture accumulation. This interrelationship of air and moisture, and how they are controlled, leads to the importance of the interactivity of control layers. 

Working Together = Success!

Given the interwoven concerns of managing air and moisture, control layers are better viewed as interconnected layers and assemblies and not just a single product or material. The enclosure is better defined as a combination of materials assembled and joined together in a system, providing a continuous barrier to water penetration, air leakage, moisture vapor intrusion and thermal movement through the building envelope.

The relative importance of each control layer is based upon the magnitude of impact it has on the performance of the building enclosure. The liquid water control layer must be addressed first. Uncontrolled liquid water in a building is extremely deleterious and failures here render other control strategies less consequential. Second in importance is the air control layer, due to its interrelationship with heat and moisture. The thermal control layer is next, as it regulates where condensation from leaked air could potentially occur. Finally, water vapor management strategies provide the final layer protection.

Any of these barriers’ effectiveness can be greatly reduced by discontinuities, even small ones. These openings can be caused by poor design, damage from other trades, improper sealing and flashing, mechanical forces, aging, and other forms of degradation. Thankfully, there are ways to avoid these discontinuities and set up a project for success.

Andrea Wagner Watts is the Building Science Education Manager for GAF, engaging with industry professionals to provide guidance, technical support and education for roof and wall assemblies. With more than 15 years of experience in the industry, Andrea strives to improve the overall performance of the building enclosures through application innovation, product development and building science research. Andrea has published on building science, assembly interfaces, durability and resilience and holds multiple patents. She serves as an executive board member of ABAA, is the co-chair of their Technical Committee and chairs the ASTM E06 Task Group on air barriers.

Building Enclosure  |  BuildingEnclosureOnline.com  |  Fall 2023

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