Facility Maintenance Decisions

The purpose of a vapor retarder is to prevent the movement of water vapor into a roofing assembly. When installed properly, vapor retarders can play an integral role in the successful performance of a roofing system. In certain climate zones and in building environments where moisture drive is an issue, these membranes mean the difference between a functioning roofing system and one plagued with issues. Because of their very low permeability, many asphaltic membranes can be used as vapor retarders. The versatility of asphaltic membranes lends themselves to be ideal solutions for slowing moisture movement into a roof assembly.

An effective vapor retarder limits, or eliminates, moisture movement into the roofing assembly from the interior of the building. This helps prevent condensation from occurring inside the roof system, which may lead to failure of the system or other issues such as:

· Mold or fungal growth

· Leaks within the building

· Deterioration of the adhesion of the system

· Failure of any of the individual components, particularly the insulation or coverboard

There are many situations where vapor retarders should be used, but one general rule, according to the Modified Bitumen Design Guide of the Asphalt Roofing Manufacturers Association (ARMA), is that, “A vapor retarder should be considered when the outside mean temperature is below 40°F (4°C) and the expected interior relative humidity in winter is 45% or greater, or when high interior humidity is anticipated due to occupancy-generated moisture.”

Many different building amenities (such as the presence of a swimming pool or other high-humidity-producing operations ― cooking or washing) or construction processes that produce large amounts of moisture inside a building can create the need for an asphaltic vapor retarder. The ultimate need for a vapor retarder should be determined by the architect and/or engineer in accordance with current engineering practices and vapor theory, based on data provided by the building owner.

Recently, with the rise of reflective, single-ply membranes, a new design consideration for vapor retarders has come into context, especially in northern climates. Because they are not fully bonded to the insulation or coverboard layer, mechanically-fastened single-ply membranes allow water vapor to come in direct contact with the underside of the roofing membrane. If the membrane temperature is below the dew point, it can cause the vapor to condense on the underside of the roofing membrane. A vapor retarder placed below the insulation layer can help keep water vapor from infiltrating the roofing system and can prevent this situation from becoming a significant issue. This condition has been the subject of several recent industry-related articles and is being widely discussed within various industry technical committees. Traditionally installed asphaltic systems are fully adhered, so this condition typically does not occur on an insulated asphaltic roofing system.

In addition to understanding when a vapor retarder may be appropriate, it is also crucial to understand the proper application of the vapor retarder. Ideally, the vapor retarder would create a continuous seal below the insulation layer. This means that the vapor retarder should follow much of the same rules as the roofing membrane. It should be properly sealed around penetrations, pipes, supports that go through the roofing system and other critical details. It should be sealed to the walls and potentially around the insulation layer. If additional penetrations, such as mechanical fasteners, are to be driven through the vapor retarder, a self-sealing vapor retarder should be considered. The concept is to keep the vapor retarder continuous across the roof. Finally, the insulation above the vapor retarder should then have a high enough R-value to keep the temperature at the vapor retarder above the dew point temperature to prevent condensation. These design aspects are critical to the proper functioning of the vapor retarder.

Once the need for a vapor retarder has been established and additional application details have been considered, the bituminous market offers many varieties of vapor retarders that can be installed with multiple application methods. The key to choosing a vapor retarder is to select one with a low permeability, as measured by its “perm” rating. The porosity of a material to the passage of water vapor is measured in perms, which is defined as the number of grains of water vapor that will pass through one square foot of the material in an hour when the vapor pressure differential between the two sides is equal to 1 inch of mercury (0.49 psi). To be an effective vapor retarder, a material should have a permeance of less than 0.5 perms. Individual manufacturer data should be reviewed to determine if specific products meet this requirement, but many asphalt-based products have perm ratings very near zero and qualify as vapor retarders when properly installed.

Asphalt-based products provide many options for the installation of vapor retarders. These options include hot-asphalt applied, cold-application adhesive, heat welded or self-adhered. Smooth-surfaced SBS base sheets or BUR ply felts can be adhered in hot asphalt. This application can be very cost effective and ideal for systems on large, open projects or on projects that are already using hot asphalt. Smooth surface SBS or APP membranes can also be installed in cold adhesive or heat-welded applications. Cold adhesive can work very well for projects that have limited access or are sensitive to hot asphalt or the use of torches on the roofing system. Heat-welded membranes also offer a solution for projects with tight access or for projects with tight timelines as the membrane becomes instantly bonded. Each of these application methods can offer unique solutions to both designers and contractors.

As building codes drive the use of vapor retarders further south, these types of membranes will continue to be an integral part of the roofing assembly. Bituminous membranes provide unique opportunities to choose a vapor retarder product that can meet the many design challenges faced by any roofing project. With their versatility of application, years of performance history and reliability, SBS, APP and BUR membranes can provide a unique solution to vapor retarders for nearly any situation.

To learn more about asphaltic vapor retarders in roofing systems, visit the ARMA website www.asphaltroofing.org.

Nathaniel Martin, Portfolio Manager, Johns Manville – Low Slope Committee Chairman, Asphalt Roofing Manufacturers Association (ARMA)