Membrane roof systems serving as air barriers

by Katie Daniel | November 29, 2017 2:56 pm

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By Joan P. Crowe, AIA
For commercial buildings, membrane roof systems can serve as part of an air barrier system. While this is common practice, questions still arise on how to design, specify, and install an effective air barrier system. To assist readers in grasping air barrier concepts, this article covers the basic building science principles, discusses code requirements in commercial buildings, and explores how continuous air barrier concepts relate to membrane roofs.

Air barrier basics
The primary function of an air barrier is to prevent or restrict air leakage through a building’s envelope. Air barriers are intended to control air flow from the exterior to the interior of a building and vice versa.

An air barrier needs to be installed continuously on all sides of a building (i.e. ‘wrapping’ the entire building thermal envelope as depicted in Figure 1). For an air barrier to function properly, it has to:

An air barrier is not a single product or material. Rather, it is a combination of materials assembled and joined together as a system to provide a continuous barrier to air leakage through the building envelope. Its effectiveness can be greatly reduced by openings and penetrations, even small ones. These openings can be caused by poor design and workmanship, damage from other trades, improper sealing and flashing, and mechanical forces, along with aging and other forms of degradation.

The National Research Council of Canada (NRC) collected research data that illustrates how even small openings can affect overall air leakage performance. For example, only about 0.3 L (one-third of a quart) of water diffuses through a continuous 101 x 203 mm (4 x 8 in.) sheet of gypsum board during a one-month period even though it has a high permeance.

However, if there is a 645 mm2 (1-si) hole in the same sheet, 28.4 L (30 quarts) of water can pass through the opening as a result of air leakage, as illustrated in Figure 2. This example illustrates air leakage can cause more moisture-related problems than vapour diffusion.

Air barrier vs. vapour retarder
When discussing air barriers and building envelopes, another important topic comes to mind—vapour retarders. There is often confusion between air barriers and vapours retarders. A vapour retarder is frequently called a ‘vapour barrier’ and this contributes to the mix-up. The purpose of a vapour retarder is to minimize or reduce water vapour diffusion into a low-slope roof or wall system. In other words, it is used to prevent the formation of condensation in a low-slope roof or wall system.

Generally speaking, a vapour retarder is used where a building’s interior humidity is expected to be relatively high, and the building is located in a cold climate. It is commonly installed on the warm (interior) side of a roof or wall. In a low-slope roof assembly, the vapour retarder is normally installed under the primary roof insulation. Therefore, one often sees it installed directly on a roof deck (e.g. concrete or wood deck) or on a continuous substrate (e.g. gypsum board and wood panels) installed directly over a metal deck. Conversely, in certain warm, highly humid places, vapour retarders are installed on, or near, the exterior side of roofs and walls of air-conditioned buildings.

There is often confusion between air barriers and vapour retarders. The purpose of a vapour retarder is to minimize or reduce water vapour diffusion into a low-slope roof or wall system. In other words, it is used to prevent the formation of condensation in a low-slope roof or wall system.

Code requirements
Air leakage has been a concern in the construction industry for many years. It can affect indoor air quality (IAQ), energy efficiency, and occupant comfort, as well as contribute to moisture and condensation damage of the building envelope. Therefore, energy codes have added air barrier-related provisions to address air leakage concerns.

Air barrier requirements can be found in the 2011 and 2015 editions of the National Energy Code of Canada for Buildings (NECB). This article explores the most recent editionspecifically, the ‘prescriptive path’—to show compliance and commercial building requirements for new construction.

For the 2015 edition of NECB, the prescriptive air barrier requirements are in section 3.2.4, Air Leakage, under Division B-Acceptable Solutions, Part 3-Building Envelope. This section is as follows:

3.2.4. Air Leakage General
1) The building envelope shall be designed and constructed with a continuous air barrier system comprised of air barrier assemblies to control air leakage into and out of the conditioned space. Opaque Building Assemblies
1) All opaque building assemblies that act as environmental separators shall include an air barrier assembly conforming to Sentence (2) or (3).
2) Except as provided in Sentence (3), air barrier assemblies shall
a) conform to CAN/ULC-S742, “Air Barrier Assemblies – Specification,” and
b) have an air leakage rate no greater than 0.2 L/(s·m2) at a pressure differential of 75 Pa.
(See Note A- and (3).)

3) Air barrier assemblies are permitted to be tested in accordance with ASTM E 2357, “Determining Air Leakage of Air Barrier Assemblies,” to meet the air leakage requirement stated in Sentence (2), provided
a) the building is erected in an area where the 1-in-50 hourly wind pressures do not exceed 0.65 kPa, and
b) the air barrier assembly is installed on the warm side of the thermal insulation of the opaque building assembly.
(See Note A- and (3).)

In short, NECB offers two options to show compliance—air barrier assemblies shall conform to CAN/ULC-S742 or be tested per ASTM E2357. CAN/ULC-S742 is a specification and addresses air barrier assemblies (i.e. the combination of air barrier materials and their accessories). It also indicates how to measure air leakage of air barrier materials and assemblies, and performance requirements.

ASTM E2357 is a test method and describes the procedure to be followed to measure air leakage rates of air barrier materials and assemblies. Based upon the results of the measurements, the procedure then assigns an air leakage rating for the air barrier assembly.

At the same time, it is important to verify which energy code has been adopted. Jurisdictions do not necessarily adopt the latest issued codes. It is also critical to keep in mind jurisdictions may add or delete portions of the code or have local amendments.

Keeping the air barrier continuous
An air barrier needs to be installed continuously on all sides of a building. In order to achieve this, the wall air barrier should be sealed to the roof air barrier. Figures 3 to 5 are conceptual drawings to help illustrate a continuous air barrier at a parapet wall condition with a membrane roof system.

Figure 3 is where the wall air barrier is a membrane or coating system installed on the structural backup wall and the membrane roof system serves as the roof air barrier. In this example, the membrane roof system is sealed to the wall air barrier to keep the air barrier continuous.

Figure 4 is where an exterior insulation sheathing board serves as the wall air barrier. This type of construction is where a layer of ‘continuous insulation’ is desired on a wall system. In this example, the membrane roof system is sealed to the exterior insulation sheathing board to keep the air barrier continuous.

Figure 5 is where the wall cladding serves as the wall air barrier. In this example, the membrane roof system is sealed to the wall cladding to keep the air barrier continuous.

For new construction, it is the responsibility of the building’s design professional to determine the need for an air barrier and provide details on sealing joints, penetrations, and transition areas, as well as verify an air barrier’s compatibility with other materials.

Also, one must bear in mind that the opportune time to address air barriers is in the design and pre-construction phases of a project. In other words, the time to discuss air barriers is before construction begins, not during.

Joan P. Crowe, AIA, is manager of codes and regulatory compliance at GAF. Her responsibilities include monitoring building codes, standards, and regulations, providing technical assistance to the sales and marketing departments, and producing technical documents. She holds a BS degree in architectural studies and a master’s of architecture from the University of Illinois Urbana-Champaign. She can be reached at[4].

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