Low-Slope Roofing—Air Barriers and Vapor Retarders

Roof assemblies have more roles than simply keeping water out. Understanding control layers is key to successful design of modern roofs.
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Sponsored by GAF
By James R. Kirby, AIA, and Thomas Taylor, PhD

When and Where Are Air Barriers Needed?

Designers should always consult local code to determine whether an air barrier is needed for a particular project. But, under the widely adopted IECC 2018 and ASHRAE 90.1 2016 state that a building enclosure is required to function as an air barrier for new construction except in climate zone 2b. However Chapter 1 Scope and Administration, Section 101.4.3 references additions, alterations, renovations or repairs. It reads as follows:

    “Additions, alterations, renovations or repairs to an existing building, building system or portion thereof shall conform to the provisions of this code as they relate to new construction without requiring the unaltered portion(s) of the existing building or building system to comply with this code.”

Section 101.4.3 also has a list of exceptions where compliance with the current code is not necessary provided the building's energy use is not increased. Exceptions 4 and 5 apply specifically to certain roof recovering situations:

    “4. Construction where the existing roof, wall or floor cavity are not exposed.

    5. Reroofing for roofs where neither the sheathing nor the insulation is exposed. Roofs without insulation in the cavity and where the sheathing or insulation is exposed during reroofing shall be insulated either above or below the sheathing.”

Exceptions 4 and 5 suggest the following:

  • If a reroofing situation involves the tear-off of an existing roof system, i.e., exposing the roof deck and installing a new roof system, the new roof system will have to comply with the enclosure requirement of the current code.
  • If a reroofing project involves a roof re-cover, i.e., the existing roof system remains in place, the new roof system does not have to comply with the current code, but can’t create a situation where the building will consume more energy as a result. Consider the following as an example where a roof recover could consume more energy than prior to recovering:
    1. The existing roof membrane is run up a parapet wall and tied into the wall air barrier on the exterior side, creating a continuous air barrier across the entire enclosure.
    2. The existing membrane is cut back to allow the new, recovering membrane to terminate on the inside of the parapet wall with a new term bar.
    3. The new, recovered membrane may now not be tied into the air barrier system for the rest of the building. This void in the air barrier system can lead to additional energy consumption and a new pathway for moisture laden air to condense within the wall or roof assembly.

    For this example, it’s important (and may be required by code) to ensure the roof recovering maintains the continuity of the air barrier system after the new membrane is completed. It is recommended that designers discuss air barrier requirements with local code officials prior to reroofing projects.

Air Barriers and Parapet Walls

While many building designers successfully use the approach of extending the roof membrane up and over a parapet wall as shown earlier, with good sealing to the wall air barrier, this method can have issues. The space within a parapet wall has very little, if any, air circulation and can get very cold in northern climates during winter. Condensation could form which might then migrate downwards, potentially damaging the wall assembly.

While it’s not commonly done, one possible option that helps reduce condensation risks within parapets, is to continue the roof membrane down and under the parapet. Again, as with the first option, it must be securely joined to the wall air barrier. This alternative also requires a little more pre-planning and coordination across trades to make sure a stripped-in membrane is installed prior to the assembly of the parapet wall. The two approaches are shown conceptually below:

As will be shown later, an alternative approach is based on the use of a vapor retarder.

Tighter Buildings, but what about Indoor Air Quality?

As buildings get tighter, there can be concerns about indoor air quality. A detailed discussion on this topic is beyond the scope of this article, however there are at least two general approaches that are used. The first is to ensure that a commercial building’s HVAC system allows for sufficient fresh air to be drawn into the building. This task is usually assigned to the HVAC design engineer who follows standards such as ANSI/ASHRAE 62.1, Ventilation for Acceptable Indoor Air Quality, to determine how much make-up air to deliberately draw into the system.

Secondly, tighter buildings can have indoor air quality issues if the materials used to finish and furnish the interior off-gas odorous and potentially noxious chemicals. For that reason, many interior designers are selecting materials after careful review of Environmental Product Declarations (EPD), Health Product Declarations (HPD) and similar.

Air Barriers and the Construction Process

An air barrier is one of the few building systems that involves the coordination of many different trades. For successful installation of the air barrier each trade needs to understand which materials have been designated as being part of the air barrier. Before construction starts, a pre-construction meeting should occur with all parties involved with the design and installation of the air barrier system. Typically, this would include:

  • General Contractor and air barrier, roofing, waterproofing, insulation, drywall, window, concrete, carpentry and masonry subcontractors.
  • Building Owner or owner’s representative.
  • Developer
  • Architect
  • Air Barrier Manufacturer

A pre-construction meeting is a good opportunity for the roofing contractor to be involved with the air barrier discussion which should include:

  • Review of the drawings and specifications, especially the transition and tie-in areas.
  • Review of the materials and accessories that covers which party is responsible for each material/system and any compatibility issues.
  • Discussion of the sequencing of the work of all trades in order to determine a construction schedule.
  • Discussion of coordinating an assembly mock-up, if required for the project


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Originally published in September 2021