October 1, 2021
by Rick Quirouette, B.Arch.
In 1980, the author, an officer of the Institute for Research in Construction (IRC) of the National Research Council of Canada (NRCC), was invited by a large Canadian construction company to investigate the cause of the continuing building envelope deficiencies in the construction of commercial, institutional, industrial buildings in Canada.
The deficiencies reported by the construction industry included, “… no matter how we construct building envelopes they continue to fail in early occupancy, with rain penetration, condensation damage, high energy losses, temperature control and more.”
During the six-month study, over 30 large projects under construction were visited throughout western Canada. The review eventually determined the cause of most deficiencies in the early occupancy of new buildings, a fundamental gap in building envelope science, a misinterpretation of the vapour barrier requirement as defined by the National Building Code of Canada (NBCC). The vapour barrier requirement of the NBCC were clear enough and accurate. The industry including building officials misinterpreted the vapour barrier requirement as air leakage control for the building envelope. The sad fact was no matter how well a vapour barrier was built, it could never function adequately without due consideration of the structural requirements for the air barrier system. The findings were controversial at first but were eventually incorporated in the next edition of the NBCC.
In 1985, the NBCC introduces, the ‘air barrier requirements,’ separate and distinct from the vapour barrier requirements.
SECTION 5.3 CONTROL OF AIR LEAKAGE SUB-SECTION
5.3.1. AIR BARRIERS 220.127.116.11.(1)
Where a building assembly will be subjected to a temperature differential, a differential in water vapour pressure and a differential in air pressure due to stack effect, mechanical systems or wind, the assembly shall be designed to provide an effective barrier to air exfiltration and infiltration, at a location that will prevent condensation within the assembly, through
(a) the materials of the assembly,
(b) joints in the assembly,
(c) joints in components of the assembly, and
(d) junctions with other building elements.
The introduction of this new requirement was a first in Canada and North America. It was originally slated for inclusion in the structural code of NBCC, Part 4, as a structural requirement but eventually attached to Part 5 WIND, WATER AND VAPOUR PROTECTION of NBCC. As one can see, it was rudimentary but included the essential criteria for air leakage control including barriers to:
1-“…air exfiltration and infiltration,” and
2-“…a differential in air pressure due to stack effect, mechanical systems or wind.”
Unfortunately, the application of the 1985 NBCC requirement for the air barrier system was never fully realized or correctly implemented. The problem is the lack of professional consideration for the structural requirements of an air barrier system.
The air barrier is an assembly of materials and it has a special purpose. Not only does it control air leakage, but it reduces insulation heat loss/gain, limits condensation and moisture damages, reduces rain penetration, and supports noise control and fire protection measures. It should be designed and constructed with the two following functions in mind.
The air barrier requires the selection of an assembly of materials that are air impermeable (non-porous and resistant to air leakage), is continuous over the building envelope, or compartmented, and correctly attached to secondary and primary structural components of the building envelope. For example, a metal and glass curtain wall system (shiny glass highrise building) includes a compartmented air barrier (Figure 8, page 30). Equally important, it must be as durable as the structure of the building.
The air barrier must be designed to transfer an air pressure load to the secondary and primary structure of the building. Whether a gust of wind or a long-term low-pressure difference from stack effect or ventilation, the pressure difference imposes a structural load (push and suction) on its surface area and its connections within the building envelope. The connections in turn transfer the pressure load further in the exterior wall or roof to the primary structure of the building. If any part of the attachments or support system is weak, poorly fastened, or inadequate for the structural function of the air barrier, the air barrier system will fail before the building construction is completed.
There is a belief (or misconception) a partially completed building envelope wrapped in building paper with all joints taped outside of an insulated layer will perform as an air barrier system. A building paper outboard of the insulated wall is a temporary weather barrier to protect the inner components from rain and/or snow until the remainder of the cladding system is completed. Building papers and other flexible membranes can never support the design wind loads of any building in Canada.
It has not yet been recognized by the construction industry that the air barrier system is as much a structural assembly as a window, a cladding, a roof, a column, or a beam. It must support high pressure loads, both positive and negative, without failing structurally and it must perform the task of controlling air leakage for the life of the building. The air barrier system structure cannot be designed adequately without skill and knowledge of structural design.
This article examines the structural requirements of the air barrier with respect to performance and compliance with the building code requirement 18.104.22.168.(1). It must withstand without fail “…a differential in air pressure due to stack effect, mechanical systems or wind,” and be durable for the service life of the building envelope.
This article is directed to architects and building designers to better understand the performance criteria of the air barrier design and to structural engineers willing to consider the structural challenge in the design of the air barrier system. The structural engineer should offer his services much in the same way they assist the architect to design all types of cladding systems to resist the above stated pressure loads.
Cladding and wall design evolution
Wind, rain and cold were always a challenge to architects and builders. The industry developed better solutions mostly through trial and error. There was very little building science in buildings other than structural design until the early 20th century. Construction techniques were mostly traditional and consensus based. For example Part 9 of the NBCC. Eventually, tradition could not support the necessary design evolution of the building envelope fast enough as rain penetration, condensation and energy saving problems overwhelmed the limited advances by experience and tradition. However, some building science progress found its way into early building envelope design through reasoning for example the face seal method of rain, wind and temperature control.
The cladding is an air barrier
For the longest time, the face sealed cladding of a building provided the primary defense to the elements, wind-driven rain in particular, and air leakage. In older wood framed buildings, two layers of asphaltic black paper were sandwiched between a wood siding and board sheathing. This assembly provided wind driven-rain penetration and air leakage control. More importantly it provided a structural attachment for the wood cladding, building paper, and board sheathing assembly. The wind load was transmitted directly from the siding to the construction paper to the board sheathing to the structural wood frame. This heritage assembly exhibited the characteristics of a structural air barrier assembly by reasoned experience.
In older commercial buildings with stone and brick cladding, the masonry was not vented or drained. Proper pointing of masonry was paramount to its performance. It was often constructed as a multi-layer cladding over an un-insulated exterior wall. This assembly also transmitted the wind load to the primary structure while it provided both rain penetration and air leakage control.
Building envelope problems investigated
During the 1980s and 1990s building owners faced severe building envelope problems with rainwater penetration, in-wall condensation, and material deterioration. During this period, it was determined a substantial percentage of the problems were caused by humid air leakage through exterior walls and roofs even though builders applied best practices and complied with architectural drawings and specifications.
This recognition that the air barrier component of the air/vapour barrier assembly was required to be structured or structurally supported was recognized by the Institute for Research in Construction (IRC) of the National Research Council Canada (NRC) in 1981 and introduced in the 1985 NBCC.
The material suppliers caught on quickly, but the development and innovations were focused on air leakage reduction without much recognition that air leakage is pressure driven and a pressure difference is a physical load to be considered in the air barrier system design. Designers, architects, and engineers did not respond until later years, and still without any significant structural consideration.
Air pressure difference was examined as a structural load in an early IRC/NRC research project. One such project involved a laboratory experiment of a full-scale wood-frame wall covered in polyethylene film and exposed to an increasing air pressure difference (Figure 2, page 28). The findings were interesting and disappointing. In this experiment the polyethylene film eventually failed to contain an adequate air pressure load. The polyethylene film eventually failed by bulging out between studs, tearing at fasteners, slipping at joints, and deflecting excessively. The theory the polyethylene sheet vapour retarder would perform as an air barrier was not realized. On its own, it would not support an air pressure load from a strong wind event. It failed structurally.
Roof systems are subject to similar considerations. Most wood-frame buildings in Canada support a low sloped roof. The wind pressure profile for these roofs (Figure 2) may be positive (blue) on the windward side and negative (red) on the leeward side. It is known high wind speeds can lift a roof off a typical flat roof of a commercial building. However, it can also occur on a wood-frame building with low-slope roof (Figure 2). In commercial buildings, a Factory Mutual, FM I-90 Roof Standard, specifies the roof membrane must not lift from the building at a wind speed of less than 145 km/h (90 mph) (wind uplift pressure 2155 Pa/m2 [45 lb/sf]. This becomes the design wind load for the structural air barrier system of a roof system.
A structural air barrier system
In building science and technology, it is expected the building envelope and its components will perform their functions for the life of the building. For example, insulation will retard heat flow for a long time; the cladding will control rain penetration for years; the vapour barrier will limit water vapour diffusion (water molecules passing through a material) through wall and roof assemblies; and the air barrier will contain the air of a building and prevent outdoor air from infiltrating into the building interior or out as moist exfiltration air, and support the air pressure differences applied without fail for the life of the building.
At this time, it is believed most designated air barrier systems do not perform their function for long. They fail structurally at connections and attachments. Investigation of problem building envelopes (by the author for 35 years) has found designated air barrier systems of most buildings examined were inadequate to support the wind loads and failed in the early life of the building envelope. Once failed, it is not immediately apparent as the air barrier system is generally hidden within the construction. However, it is easily recognized when other symptoms begin to appear. Symptoms such as efflorescence, icicles in winter, spalled masonry, corrosion tracks on the cladding’s surface, flooding in spring, and more.
So, what is an effective and durable structural air barrier assembly? If the air barrier must control air leakage and support air pressure difference for the life of the building, one must examine the attributes of materials assemblies that can resist air leakage to a minimum of 0.2 L/s.m2 (0.039 cfm/sf) at a pressure difference of 75 Pa (1.56 lbs) of the system (air impermeability) and provide the necessary functional and durable structural connections, to resist design wind loads and long duration small pressure loads. For discussion purpose of this article, air barriers assemblies are categorized into four groups of components. These include:
• membranes and coatings;
• rigid paneling and solid materials;
• joints and connections; and
• cladding systems.
Membranes and coatings
Membranes and coatings for air barrier construction are selected because of their air impermeability (low porosity) properties. They resist the passage of air through the material. They are flexible, easy to install, and supplied in roll form, paste, or liquid. However, by themselves they are not suitable as standalone air barrier systems. The normal air pressure loads during the life of the building will displace, tear apart, un-bond, deflect excessively, and destroy the barrier’s continuity without the necessary structural support and connections. Membranes and coatings include aluminum foil, polyethylene film, building papers, elastomeric roof, wall membranes, and more.
Rigid paneling and solid materials
Rigid paneling and solid materials fair air barrier construction may or may not be air impermeable. But their rigidity is often the required attribute to create a structured air barrier assembly. Rigid panels are generally strong enough in flexure and or compression and tension to span cavities and be attached to the structural framing. If the panel air permeability is inadequate, then by combining a rigid panel with an air impermeable membrane or coating, the assembly can exhibit the necessary properties of a structured air barrier assembly. Rigid paneling and solid materials include, but are not limited to, plywood, gypsum board, particle board, ribbed sheet steel panels, concrete shear walls, precast panels, and more.
Joints and connections in the air barrier system
Joints and connection products are unique and important. Their properties not only comprise air impermeability, but include chemical compatibility with the materials to be joined, type of structural attachment for both sides of the joint and connection, flexibility to expand and contract with live load deflections, expansion and contraction due to temperature changes, resistance to high pressure loads, and long-term low pressure loads. Corner joints in buildings can be subject to two times the wind design load experienced by the wall. Materials for joints and connections include sealants excluding non-setting sealants, membranes, sheet aluminum, and sheet steel joint components and more.
Cladding systems can and do include air barrier characteristics and air leakage control performance criteria. For example, a precast clad building using the face seal method or even with double joints at panel edges are air impermeable and can resist the local wind loads. It is inherent in the precast cladding to include this requirement which then complies with the code requirement for air leakage control.
Air pressure loads
What is an air pressure load on the air barrier system? When an air pressure (barometric +/– a local pressure variation) on one side of a building or a wall assembly is higher or lower than the pressure (barometric +/- local variation) on the other side of the building or wall, there exists an air pressure difference. This air pressure difference becomes an air pressure load when it is multiplied by the area of the air barrier system.
Typically, the barometric pressure is omitted from most analyses except for environmental pumping action. When the barometric pressure rises or falls from a sunny day to a rainy day, the pressure of the ambient air can change by as much as 5000 Pa (104 lbs/sf). This load applies to window sealed units and sealed cavities but not air barriers assemblies generally.
However, typical air pressure differences from wind, stack effect, and fan pressurization, to be considered in air barrier design can be large or small and depend on environmental or mechanical conditions imposed on the building envelope. Usually, a building may experience a wind load of 1000 to 2500 Pa/m2 (20.9 to 50.2 lb/sf) at any time during its life. Also, most commercial building and high rises in particular will experience a sustained but low pressure difference, 0 to 5 kg/m2 (0 to 1 lb/sf) from stack effect and fan pressurization.
The air barrier system within the building envelope must resist the individual and the cumulative effect of these pressure differences (the pressure load) and transmit this load to the primary structure of the building. The primary structure of the building is designed to resist all aspects of gravity and wind loads. In practice the air pressure loads on the building envelope are distributed throughout the wall or roof assemblies and inversely proportional to the leakage areas of each plane of materials. So the plane with the smallest leakage area, generally the air barrier system, (the longest brown bar in graph of Figure 3, page 28) will be exposed to the largest proportion of the pressure load in the wall assembly while the leakier planes (the vented brick wall and the indoor gypsum board) will exhibit the smaller proportions of the air pressure load. The individual pressure loads on the components plane of an exterior wall will add exactly to the total pressure difference across the exterior wall. The ideal distribution of the load would indicate a full load on the air barrier and no load on the other elements.
There are five types of air pressure differences on building components that may occur singly or in combination. They may be short duration (wind loads), gradually rising (stack effect), or sustained for long durations (mechanical ventilation). Additionally, there are two other loads to include atmospheric and thermal pumping. Each phenomenon can impose an air pressure load. It is beyond the scope of this article to provide detailed explanations of each load type, but more details can be found in the Canada Mortgage and Housing Corporation (CMHC) publication “Air Pressures and the Building Envelope”.
A structural air barrier design
An air barrier system must be strong and durable. The structural design of the air barrier system must transfer the air pressure loads to the primary structure of the building. In addition to large air pressure loads, there is load cycling which may define material fatigue during the life of the building. This is clearly explained in Underwriters Laboratories of Canada (CAN/ULC) S742, Standard For Air Barrier Assemblies–Specification.”
High pressure loads require consideration of material compressive and tensile strengths, shear resistance, flexure, and deflection limits. For low air pressure loads of long duration, the design must consider peeling of membranes, cyclic deflection fatigue, and progressive slipping of joined materials. Joints and connections must be designed to support high pressure loads (in some cases twice as large as the design wind load) in corner connections, with suitable mechanical attachments. The joint and connection details must be tolerant of live load movements, temperature expansion and contraction, and chemical compatibility.
Historically, the wind pressure load on a building was primarily resisted by the exterior wall cladding. Now, most cladding and exterior walls are of the drained and vented cavity type or pressure equalized rainscreen (PER) wall design. In these cases, the wind load transfers directly through the vents and drains to the interior elements of the exterior wall to the next most air impermeable (airtight) plane in the exterior wall. This is the air barrier plane of materials.
Examples of structural compliant air barrier systems include many precast cladding system, metal and glass curtain walls, and concrete shear walls. Other air barrier compliant parts of assemblies may include;
• the gasket and gypsum board air drywall approach (ADA) for residential wood frame buildings (Figure 4);
• the external air system element (EASE) air barrier system for the exterior side of wood framed buildings which consists two layers of vapour permeable fibreboard with a breather (vapour permeable) construction paper (Figure 5, page 28);
• other examples of composite assemblies for air barrier systems using panels and membranes may include gypsum board, polyethylene film, and gypsum board sandwiched on steel stud (Figure 6);
• precast cladding with drained and vented joint (Figure 7);
• curtain wall system with compartmented air barrier (Figure 8);
• sheet steel liners mechanically attached to girths and sub-girts with gasket joints;
• an asphalt membrane thermo-fused over a block wall is an acceptable mechanical connection to the block;
• peel and stick membranes between two structural panels; and
• many others as well.
Each of these examples will require structurally designed connections at joints between panels and at connections to other systems. The most important connections to resolve are the roof-to-wall connection (also known as the parapet), connections between different exterior wall cladding systems particularly at corners, overhangs and balconies, and other locations.
As noted, the progress in air barrier design and construction in 2021 is lopsided and incomplete. The structural design issue for the air barrier remains unresolved with a few exceptions, metal and glass curtain walls and precast cladding.
It will be incumbent on the industry of professionals, building officials and educators to take note of this serious deficiency if building envelope performance is to advance into the new century. There must be a significant change to the application of the air barrier science and technology of the building envelope to incorporate the structural requirements of the NBCC Part 5, section 5.3. on air barriers. Here are a few thoughts and ideas to consider.
Rick Quirouette, B.Arch., is a senior building science specialist with almost four decades of experience in building science and technology. He is a life member of the Alberta Building Envelope Council (ABEC) and a past-president of the National Building Envelope Council (NBEC). Operating as Quirouette Building Specialists Ltd., he can be reached at firstname.lastname@example.org.
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