Back to top
August 2, 2016
Quality, Environmental, High performance & sustainability

The role of vapor retarders on building performance

It doesn't have to be ice, a leak or a flood that causes significant damage to your building and operations. Moisture, even in its smallest form, can have devastating results if not properly controlled. In addition to property and structural damage, moisture is a breeding ground for mold, mildew and insects, which bring with them exposure to health-related issues.

In this post I will discuss the nearly invisible state of water – vapor – and how we can control its ability to infiltrate our walls and affect the health of our buildings. Vapor is water that is suspended as particles in the air, and it's this water, which we can't see, that is ever present and the most challenging to control.

There will be water

We may as well face the fact that moisture will enter the building. Exterior building assemblies are not water tight. 

Understanding how water moves and how building envelopes interact with HVAC systems and the enviroment allows architects and engineers to specify the right mechanical systems and manage moisture intrusion through the building envelope design. For contractors, it's important to ensure that just as there is a way for water to enter the building, there is also a way out.

So how does water vapor move? Generally, water vapor travels from the warm side of building to the cold side of the building and from high pressure to low pressure.

Hygrothermal map - Building Sciences
Hygrothermal map - used with permission from

The role of the vapor retarder

The main function of a vapor retarder is to control water in its vapor form. The installation of a vapor retarder, particularly in our cold climate is extremely important. Installing or not installing a vapor retarder in the correct location is the first defense in building a durable and healthy wall system.

The goal of installing a vapor retarder is to keep the interior warm, moist air from getting to a cold exterior surface. Vapor retarders were originally intended to prevent assemblies from getting wet. However, they often prevent assemblies from drying out.

The difference between an air barrier and vapor retarder

Confusion on the issue of vapor retarder and air barriers is common. Air barriers are vapor permeable, which allows vapor to pass through. An air barrier, such as Tyvek©, is usually required per building and energy codes and is NOT a vapor retarder. When the vapor retarder is located on the exterior side of the wall, the vapor retarder will act as the air barrier and the vapor retarder.

Note: Barriers/Retarders have a Perm rating, and ratings 10  and over are considered permeable (air barrier). The higher the number, the more vapor will pass through the surface.

Where and how to install a vapor retarder

A custom plan should be developed that takes into account all building materials and systems that impact or are impacted by moisture content. Air-conditioned enclosures, below grade spaces and an exterior vapor retarder change the building conditions. 

For specialty buildings, such as a pool house, sauna and refrigeration warehouse, the vapor retarder should be located based on computer analysis of the wall assembly. Why? Depending on its location in the wall, vapor retarder could trap the moisture or allow the moisture to move freely throughout the facility, which would cause extensive moisture damage of the facility.

Double vapor retarders, or vapor retarders on both sides of an assembly, should never be installed - in order to facilitate assembly drying in at least one direction.

Wall detail
Wall detail

Monitoring for early signs of moisture

The best way to control moisture in your building is to monitor the conditions proactively. How do you know if you could have a moisture problem? Look for the following signs:

  • Condensation on cold surfaces, such as windows and pipes.
  • Condensation on the top and bottom floors of a building – in attics, basements and crawl spaces.
  • Rust stains and corrosion on metal materials.
  • Warped materials, rot or cracks at the foundation, ceiling or roof line and where systems come together.
  • Gaps and holes in sealants.
  • Water spots in the roof or windows.
  • Leaks around plumbing and equipment/appliances, which can contribute to moisture in the air.
  • Inefficient ventilation and exhaust systems.
  • Increase in number of insects.
  • Damp or wrinkled paper and damage to equipment.
  • Visible growth or persistent odor of mold or mildew.
  • High level of reported asthma and health issues by building occupants.
Damage due to air leakage
Damage due to air leakage

What to do if you suspect you have a moisture problem

The major issues are usually caused when people fail to act. 

When it comes to water, it's important to involve the experts. Identifying the season, time of the day, weather, temperature and humidity in the space will help the architect or engineer start a more thorough investigative process for the condition that you are experiencing.

The most effective action is prevention. Implementing the proper moisture controls upfront, such as installing a vapor retarder in the right location, means a higher quality building that creates the conditions for higher performance – of your building, the systems and the people who use it.

By understanding the role of protective materials and systems and how they function separately and together within a wall system, you can make smart choices that optimize the function of your facility and ensure its durability over a longer period of time. Why is this important? First, it's healthier for the building and its occupants. Second, it's cheaper to maintain and prevents costly fixes.

Wall mock up
A mock up helps evaluate performance and ensure proper installation

At Pepper, we implement quality control measures during planning and implementation to advise you on the best solutions for your building's performance. If you have a question about your existing facility or upcoming project, please contact us. We can help.

About the Author

Corey Zussman

Corey Zussman, AIA, NCARB, ALA, RBEC, RRC, REWC, RWC, RRO, CDT, CQM, CxA+BE, BECxP, CABS, LEED® AP BD+C, Level 1 ThermographerDirector of Quality Management, Illinois & Wisconsin

Corey Zussman is a 26-year veteran in the construction industry. Since 2012, he has helped guide the leadership, design and implementation of Pepper's quality management systems and continually assesses and improves the effectiveness of our processes and standards. As director of quality management, Corey visits Pepper jobsites to plan, oversee and promote construction quality directives. He serves as a resource to all project teams throughout the preconstruction and construction process. 

Corey holds his Master of Business Administration with a specialization in Quality Management and his Bachelor of Architecture with a minor in Construction Management and Business Management from the Illinois Institute of Technology. His industry involvement includes being an active member of AIA and Construction Specifier Institute, RCI and ASQ – and he frequently lectures on quality issues. He is a licensed architect and has his NCARB certification. Corey is a Registered Exterior Wall Consultant, Registered Waterproofing Consultant, Registered Roof Observer and is a certified document technician. 

Corey is a frequent speaker and instructor at industry-related events, such as AIA - Chicago, American Contractors Insurance Group (ACIG), BEC-Chicago (Building Envelope Committee), CAC-RCI (Roof Consultants Institute – Chicago Chapter),  ALA Midwest conference, CSI Chicago, ABAA national conference & trade show and 2016 CSI Construct. He has also served as a consultant to other general contractors and architects nationwide. Topics include critical transitions of AVB, sealants, metal framing, exterior skin, plumbing, HVAC, sprinkler, electrical and proactive construction quality planning.

Read more about Corey.