Air and Vapor Barriers in a Cold Climate Commercial Roof—Do I Need One?
Air barrier systems in commercial buildings in cold climate zones are designed and constructed to control air and vapor transmission between a conditioned space and an unconditioned space. The air barrier system is the primary air boundary that separates indoor (conditioned) air and outdoor (unconditioned) air. The purpose of a vapor barrier, or vapor retarder, is to inhibit the migration of water molecules within the air. In the strictest sense air barriers are also vapor barriers when they control the transport of moisture-laden air.
An effective air barrier increases HVAC performance and efficiency, enhances occupant comfort, improves smoke and contaminant control, and reduces energy consumption. The air barrier in a low-slope commercial roof is an integrated system of different materials and components that control air leakage and convective heat flow through the building enclosure. Air barriers are intended to resist the air pressure differences that act on them.
A commercial roof assembly typically begins with either a fluted metal or concrete deck. For metal decks, rigid materials such as gypsum, plywood, or OSB can be installed to provide a flat and rigid substrate. Both metal and concrete deck assemblies can act as an air and vapor barrier if properly sealed with materials such as self-adhered membrane and closed cell spray polyurethane foam.
Moisture Mitigation During Construction
Although commercial roofing system assemblies are designed to accommodate moisture generated after a commercial building is completed and pressurized, moisture mitigation is also important during the construction phase.
Construction-generated moisture has not been considered broadly across the industry until recently. However, as roof failures have increased in the past decade, roofing system manufacturers are making changes in materials and assemblies to mitigate the problem. In moderate climate regions (ASHRE Zones 1-3), construction-generated moisture may not be noticed until musty air is detected or mold is discovered. In colder climates (Zones 4-8, which includes Colorado), recurring condensation from construction-generated moisture can be mistaken as roof leaks, causing roof failures to occur in as little as 4-5 years.
Concrete, a primary contributor of construction-generated moisture, holds an extraordinary amount of water even after it dries. So after the roof and insulation is installed, along with the cover board and roof membrane, and the finished structure is generally positively pressurized, the water doesn’t have anywhere to go and stays in the roof. However, an air and vapor retarder installed on the roof directly below the insulation will prevent the construction moisture from migrating into the roof assembly. A vapor retarder can be as simple as a plastic sheet but is not considered an effective air barrier. Newer self-adhered membrane products act both as air and vapor barriers, and can act as temporary roofs to dry in the building until the remainder of the roof assembly is installed. These self-adhered air and vapor barrier products have been shown to be so effective that single ply roof manufacturers are beginning to produce and market these solutions as part of their entire roof assembly.
The use of portable space heaters can also increase moisture levels during construction. Although heaters improve worker conditions, they should not be used to “dry” the construction environment. On the contrary, for each 200-pound tank of propane burned, 30 gallons of water are produced! Finally, water-based construction materials such as concrete, paint, plaster and drywall can contribute to high levels of humidity in the construction space, which could ultimately become trapped in the roof assembly.
To mitigate construction-generated moisture, buildings under construction should always be adequately ventilated, especially during concrete curing and other high moisture-related activities. Temporary high-volume ventilation and dehumidification systems will help to remove large amounts of moisture from the air.
Managing Risk at the Design Phase
There’s no doubt that an effective roof air and vapor barrier is an important component of a building envelope system. However, in today’s commercial building boom, contractors may shortcut roof assembly best practices to meet stringent deadlines and budget constraints. In cold climates, requirements for an air and vapor barrier solution should be addressed in the design and pre-construction phases of a project. This is especially true for higher value buildings with a low-slope concrete deck.
For new construction, it is the responsibility of the building’s design professional to assess the need for an air and vapor barrier; specify such materials and products in the project manual, provide details on sealing joints, penetrations, transition areas, etc.; and verify an air barrier’s compatibility with other materials. In addition, it is important for the designer to consider how energy code requirements may affect the design and installation of roof assemblies. The International Energy Conservation Code (IECC) requires that all air barrier components of each building envelope assembly should be clearly identified and/or otherwise noted on construction documents. Forty-three states, including Colorado, have adopted regulations for energy-efficient construction of commercial buildings. These regulations usually include provisions related to low-slope roof assemblies.
What Pie Can Do for You
At Pie, our Building Science Group specializes in the front-end risk management of air and vapor barrier systems to ensure that project requirements are met throughout design and construction. While the significance of effective air and vapor barrier performance is something that cannot afford to be overlooked, Pie understands that no two buildings are alike and work with our clients to set priorities and work within their budgets and the project requirements.
Our air barrier consulting and performance testing services include:
- Pre-design consultation
- Technical plan and specification reviews
- Pre-construction training seminars
- Material submittal reviews and mock-up testing services
- Quality assurance field observations during construction
- Field performance and diagnostic testing
- Air barrier testing and infrared thermography/smoke diagnostics
- Advanced air leakage detection capabilities
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