Air & Vapor Infiltration in Masonry Assemblies
by Charmaine Tenwinkel, CDT, W.R. Grace & Co.
to be effective, an air and vapor barrier must be a continuous layer, carefully applied at wall openings, waterproofing, and the roof assembly
Air leakage is a major cause of exterior and interior wall deterioration, high energy costs and poor control of interior humidity. Air leakage is damaging because it holds and transports moisture so well. The combination of moist air, a pressure differential and even a small wall opening may result in extreme moisture accumulation in a wall system.
How Condensation Occurs
When warm humid air is chilled by a cold surface, condensation occurs. To prevent condensation, the wall surface must not be allowed to fall below the dew point of the room air. Water vapor will pass through building materials depending upon the materials' vapor permeability and the vapor pressure differential. The passage of water vapor is not harmful in itself unless the temperature drops below the dew point during the passage and condensation occurs.
Accumulation of water in CMU, mortar and brick can lead to efflorescence or frost formation and potentially deteriorate the wall. Accumulation of water on the wall interiors can cause excess humidity, mold and mildew, occupant health and discomfort issues, and deterioration of the building materials.
How Air & Vapor Barriers Work
In general, when building conditions are conducive to water vapor condensation, a vapor barrier is used on the warm surface of the wall structure. In cold climates, typically the barrier placement is inside the insulation layer towards the building interior while in warmer climates, the barrier is placed outside of the insulation layer towards the building exterior.
By definition, a vapor barrier will not transmit more than one grain of water vapor (480 grains = 1 oz) per square foot of wall surface per hour under a vapor pressure differential of one inch Hg of pressure. Air and vapor barriers are designed to reduce to a minimum, the entrance of water vapor into the wall and thus reduce the vapor that might condense by reaching its dew point.
Although all buildings are potential candidates for air and vapor barriers, typically these materials are used in structures with a critical need for the control of air and moisture movement. Such structures include museums, computer facilities, libraries, science or research buildings, hospitals, or other structures where the performance of the HVAC system is critical to maintain specific humidity and air movement levels for the building contents and/or occupants. It is important for the design or HVAC engineer to identify the average temperature and precipitation ranges for the building's geography and to determine where the dew point will fall in the wall system for placement of the air and vapor barrier. The building materials, particularly the insulation materials, and HVAC loads will impact the ultimate design of the wall system and the placement of the air and vapor barrier.
For an air and vapor barrier to be fully effective, it must be applied as a leakproof continuous layer and carefully tied into wall openings, the structural waterproofing and the roof assembly to prevent vapor leakage. Typically the vapor barrier is used on the exterior of an insulated cavity wall to prevent the flow of vapor into the back-up wall and consequently the building interior. It is important that the vapor barrier be fully sealed around brick ties and other penetrations.
Choosing An Air & Vapor Barrier
There are many vapor retarders available, but few materials perform as both an air and a vapor barrier. Air and vapor barriers must meet several requirements for proper performance:
While many plastic materials will act as a vapor retarder, most have not been tested in wall assemblies for reduction of both air and vapor infiltration. There are currently two types of products used in commercial construction that have been tested in masonry wall assemblies to meet both needs. The more traditional material is a self-adhered, 36 mil rubberized asphalt sheet bonded to a 4 mil cross-laminated polyethylene cover. These materials have been used for over 20 years, particularly in Canada where the use of air and vapor barriers is dictated by code. They have the advantage of sealing around penetrations and self-healing from minor cuts. They can be installed in sections when the back-up wall and veneer are being built consecutively.
Another successful material on the market is a water-based, latex rubber liquid product that is sprayed onto the back-up wall. It has the advantage of sealing around brick ties and other penetrations in a quick installation with less labor than the sheets, but the back-up wall needs to be constructed prior to the veneer for this product to be cost effectively installed.
Both product types have been fully tested in masonry cavity wall assemblies to ensure both air and vapor barrier performance through a typical wall system.
No material will perform unless the details are well thought out and can be constructed as drawn. The sequence of construction is also important in achieving the vapor barrier integrity. The entire wall assembly should be designed as a unit, and the designer must consider the means by which the wall will be connected to the foundation, roof, windows, doors, and all other openings. The HVAC engineer should play a role in helping the designer determine the best material, placement, details and performance. Pre-job meetings, mock-up panels and discussions on construction sequence by all parties are the best assurance of the right material in the right place at the right time.
The owner's occupants, the building contents and the long term performance will all benefit from the previous steps taken in the design, engineering and construction phases to prevent the infiltration of air and vapor into the completed structure.
© 2000 Charmaine Tenwinkel,