Slowing Vapor Flow in Walls with Control Layers

Slowing the Flow of Vapor in Wall Assemblies with Control Layers


As our friend Dr. Joe Lstiburek has said before, a wall works by keeping the outside out and the inside in. 

Referencing the “perfect wall,” there is the water control layer, air control layer, vapor control layer and thermal control layer all in the right places. The vapor layer’s most important function is to be impermeable to moisture in vapor form, thereby preventing the movement of water vapor through the wall cavity to the outside and vice versa. The wall dries outward from the vapor control layer and dries inward from the vapor control layer.

But what happens if you rely only on an air barrier? Dr. Joe explores this topic discussing vapor flow through wall assemblies and what slowing the flow of vapor can do.


Detailing a continuous air barrier system


A complete air barrier system is a combination of several assemblies and components connected by multiple accessories that are designed to provide a continuous air barrier to the movement of air through an environmental separator.

High-performance air barrier systems must have the following characteristics:

  • Continuous (most important)
  • Strong enough to withstand forces during and after construction
  • Stiff to help absorb and transfer wind loads evenly
  • Durable to last the expected lifetime of the building
  • Impermeable to air flow (least important)


The reason for this order is that an air-impermeable component will be ineffective without continuity. An air barrier system must meet several requirements, but continuity is the most important and challenging characteristic to achieve in modern steel, wood, or concrete buildings.

This is why air barriers must be thought of as a system and not a product. Even though air movement into and out of the building is controlled, unrestricted air movement through the stud space can increase energy consumption and moisture risk, reduce the R-value of insulation, and create discomfort for occupants.


Get a comprehensive overview of air barrier details and materials, including how you can avoid failures and other important considerations for designers and specifiers.


About Dr. Joe Lstiburek 

Dr. Joe Lstiburek is the founding principal of Building Science Corporation, one of the most influential, innovative, and respected building science firms in North America. Dr. Lstiburek’s work ranges widely, from providing expert witness testimony to overseeing research and development projects to writing for the ASHRAE Journal. His commitment to advancing the building industry has had a lasting impact on building codes and practices throughout the world, particularly in the areas of air barriers, vapor barriers, and vented and unvented roof assemblies. His commitment to education earned him the hailing, “the dean of North American building science” by the Wall Street Journal. Dr. Lstiburek holds a Bachelor of Applied Science in Mechanical Engineering, a Master of Engineering in Civil Engineering, and a Doctor of Philosophy (Ph.D.) in Building Science. Get the full scope of Dr. Lstiburek’s work, accolades, and contributions to the industry over at Building Science Corporation.