Understanding the Key Differences Between Air Barriers, Water Barriers, and Vapor Barriers in Building Construction

Keeping water out of your building or cabin is not as easy as you think. When thinking about the construction process some may think enclosing the building with sheathing, adding “building wrap” and then siding is enough. Making this assumption is a big mistake. There are three main types of barriers that should be considered in your wall and roof assemblies to prevent water intrusion, and collection, and allow for proper drainage. The three types are Air Barriers, Vapor Barriers, and Water Resistive Barriers. These are used in concert with your structure, insulation, and fenestration to create a functioning building envelope.

It is often said that the most important task of an Architect, aside from the health and safety of occupants, is to prevent or mitigate the damage that water can cause in a building. Water can flow through buildings due to environmental moisture, ground source water, and through airflow. You add in temperature swings of conditioned spaces and seasonal environments and water has plenty of options to penetrate the building.

Water Resistive Barriers have the most resistance to liquid water and they also breathe the least. Vapor Barriers reduce the flow of water vapor and are important where condensation may occur. Air Barriers reduce the flow of air keeping a building envelope more thermally tight.

Water Resistive Barriers

Water resistive barriers are also called Weather Barriers or Moisture barriers. They resist water but are not deemed “waterproof.” They are vapor permeable to allow trapped moisture to exit an assembly. Waterproofing is a separate type of membrane for foundations and roof assemblies to prevent water intrusion and is not permeable enough for proper vapor diffusion.

They are membranes that resist liquid water intrusion. They are used where water intrusion is likely due to environmental conditions such as underground water sources, and protection from rain, snow, and other precipitation.

Water intrusion from the liquid form can cause damage very quickly as a large amount of water can enter a broken water barrier.

Air Barriers

Air barriers can be used anywhere in a building assembly where you want to reduce airflow and leakage in or out of a conditioned space. If you are attempting to create a very airtight assembly to reduce utility costs and focus on building performance air barriers are vital. Air barriers are also important in improving air quality from exterior pollutants and radon. Air barriers should be installed in a continuous plane without gaps or holes. They require taped seams and careful coordination of fasteners used in the assembly.  

Air leakage is a prominent cause of water intrusion as it can allow a lot of water inside an assembly on a frequent basis.

Vapor Barriers

Vapor barriers stop vapor diffusion or moisture that flows from a high concentration to a lower concentration. It stops condensation from entering a wall assembly into areas where trapped water can cause damage, such as wood and insulation. There are different ratings of vapor barriers, some are called “vapor retarders” as they do not stop diffusion but retard the process. When looking at building materials be sure to select a vapor barrier rather than a vapor retarder unless you require more airflow in your assembly.

Class I vapor barrier (0.1 perms or less) 

Class II vapor retarder (0.1 < perm <1.0 perm) 

Class III vapor retarder (1.0 < perm <10 perm)

Vapor barriers should always be placed on the warm side of an assembly. In a predominantly warm climate, it will be installed on the exterior. In a predominantly cold climate, it will be installed on the interior. This will help capture moisture and condensation from pooling on the interior side of the assembly. Vapor barriers may also be required at floor slabs and underground walls where moisture may move through materials.

One important thing to remember is that air leakage is a big factor in water vapor intrusion. If you find condensation inside your assembly check for air leakage as well as proper installation of the vapor barrier. Moisture from water diffusion is much less than a break in the air or water barriers but it can still cause a lot of damage as the effects cannot be readily seen. This is where dangerous mold or rot can grow for years without discovery.

Summary

Air, Vapor, and Water Resistive Barriers must be used in conjunction with one another to create an efficient building envelope. Many products can act as one or all systems depending on location and climate. Some barriers are membranes and others can be achieved by continuous insulation.

Remember this:

Air barriers for air leakage and thermal performance as a Continuous barrier.

Vapor barriers for water vapor and condensation on the Warm Side of assembly.

Water Resistive Barrier to prevent liquid intrusion on the Exterior side of the assembly (or from where liquid water comes in).

If high-performance building envelopes are of interest, be sure to check out Passive House (Passivhaus) standards as well as WELL standards for healthy and efficient buildings.