Spray Foam in the Building Envelope >> Difference Between Air, Vapor & Water Resistive Barriers

Difference Between Air, Vapor & Moisture Barriers

Vapor Barriers are not to be confused with an air barrier. A vapor barrier is designed to restrict the flow of water vapor through a material, just the same as a air barrier material restricts the flow of air through a material.

Vapor barriers or vapor retarders are intended to control the rate of diffusion into a building assembly.  As a vapor barrier they will control the rate of moisture flow where they are placed. Therefore the vapor barrier does not have to be continuous, does not have to be free of holes,  does not have to be sealed, etc.  A hole for example in a vapor barrier will simply mean that there will be more vapor diffusion in that area compared to the other areas where the vapor barrier. 

All materials are permeable, even steel.  The main issue is the degree of their per rating. 
Water vapor permenance is measured by the amount of water which will work its way through a material.

  • Normally reported in ng/(Pa•s• m2) units of measure
  • Most regions require a vapor barrier with a maximum water transmission rate
    of 60 ng/(Pa•s•2)

Much work is being done and much discussion is being held on whether vapor barrier should be used at all and if they are used should the water transmission rate be. There is discussion on the need to allow buildings to dry. Keep in mind that during this time period where vapor barriers are being discussed you still need to meet the local building code which may demand it even when Building Science would indicate that it should not be used.

Water vapor is most often transported becusae of air leakage.  It is normally dealt with by installing a proper air barrier.

Vapor barriers are designed to be installed on the warm side of the insulation.

Water Resistive Barriers are materials which are primarily designed to be used to keep liquid water from entering the building enclosure. Water resistive barriers are specifically designed not to be a vapor barrier. The minimum water vapor permeance for a water resistive barrier is 300 ng/(Pa•s• m2). Water resistive barriers are combined with flashing and other materials to ensure that there is a shingled assembly to direct liquid water which passes on the cladding system to be directed to the exterior.

Water resistive barriers are designed to be installed on the cold side of the insulation.

Combined air barriers, vapor barriers and water resistive barriers can be provided in a single material. There are also vapor permeable air barriers, and there are water resistive barriers which are not air barriers. Please understand the three separate functions and then determine whether the material you choose provides more that one function and then you need to decide whether you will design your building so it actually performs more that one function.

As an example, you can have two, three or even four air barrier materials in a wall assembly but it will depend on which material you have chosen and how you have connected the air barrier materials together.

Gypsum wallboard, polyethelyne film, exterior grade drywall, self adhered membranes and spray polyurethane foam insulation are all air barrier materials in a specific wall assembly but you need to choose which one is the air barrier and then ensure that you have the air barrier material chosen has been connected.