The air/vapour barrier must die.
The terms “air barrier” and “vapour retarder” (or “vapor barrier”) are perhaps some of the most poorly understood concepts in our industry. Most people know that they are needed, but they often struggle to locate them correctly within wall assemblies. Also, the consequences of installing these materials improperly can result in failures. This general lack of understanding of the functions of these materials has resulted in simplistic rules-of-thumb that are prone to misapplication (e.g. the vapour barrier is always on the inside, the air barrier is always on the outside, and so forth). As manufacturers introduce materials with new properties and attempt to push the boundaries of building envelope construction, it is crucial that our industry agrees on terminology that communicates the specific functions and purpose of these materials to avoid confusion and costly errors. In that regard, the term “air/vapour barrier” is misleading and should be replaced with more appropriate terminology. This article will briefly explore the origins of this term, the discrete functions of air and vapour barriers in building envelopes, and the detrimental effects of incorrect terminology in construction documentation.
Air and Vapour Barriers — A brief history
Initial attempts to increase the thermal comfort of occupants in modern North American wood-frame construction date back to the 1800s. The introduction of “building paper” in the form of asphalt-impregnated felt (Hess-Kosa, 2017) — also known as sheathing membranes or weather-resistive barriers — represented an early effort to reduce wetting of wall assemblies and reduce air leakage. The industry made further advancements in the field of envelope performance in the 1930s with the rise of insulation in framing cavities and attics (Koniorczyk & Gawin, 2008). However, the undesirable effects of moisture in insulated cavities soon surfaced. The traditionally painted wood façades began to suffer from peeling, blistering and other coating failures. As is often the norm in the building industry, a “blame game” quickly ensued with the insulation manufacturers blaming the paint, the paint manufacturers blaming the insulation and the building paper manufacturers caught in the middle (Rose, 2003). It was not until the late 1930s that scientists began investigating moisture movement in building assemblies. Their findings — considered controversial or even biased by some[1] — concluded that the transfer of water vapour by diffusion (a process explained later in this article) was responsible for the paint peeling off the siding. These conclusions resulted in regulation mandating the use of low-vapour permeance membranes in construction projects in the early 1940s (U.S) and 1950s (Canada). It was the birth of the “vapour barrier”, and the industry celebrated its solution to the moisture problem by lining walls with polyethylene plastic. Ostensibly the issue of peeling paint was resolved, or at least it seemed that way.
Nonetheless, the moisture problems persisted. Further research in the mid-1980s suggested that uncontrolled air infiltration, and not vapour diffusion, was the largest contributor to moisture accumulation in cavity spaces (Quirouette, 1985). However, by that time the popularity of the now ubiquitous “6 mil poly” — backed by the standard CAN/CGSB 51.34 — Vapor Barrier, Polyethylene Sheet for Use in Building Construction — inspired methods designed to seal the already familiar vapour barrier. The purpose was to turn it into an effective air barrier.
These attempts to “seal the vapour barrier” may be characterized as the genesis of the “air/vapour barrier”, both as a concept and as a term. Conceptually, the “air/vapour barrier” was a material that purported to address both air infiltration and vapour diffusion issues. Its proponents believed that by sealing the joints of polyethylene sheets, a dual-purpose material could emerge that would ascribe the additional properties of air infiltration control to the already popular “6 mil poly”. It was perhaps this popularity that was responsible for its great appeal and wide adoption in the industry. As time passed, however, the idea of “sealing poly” was quickly abandoned, and construction practioners explored other materials to provide air tightness. It was clear that polyethylene was not a sufficiently durable material to resist the effects of wind gusts and pressures. Moreover, this lack of durability was further undermined by the intrinsic difficulties associated with installing polyethelene in a continuous fashion.
Even if the concept of the polyethylene “air/vapour barrier” slowly faded, the term endured. Coincindentally, it also appeared as if — as time passed — the general understanding of the functions of these air and moisture control elements was clouded by this conflated term. These materials that were once distinct, slowly transformed into abstract dotted lines that everyone knew were required on construction details, but no one fully understood where or why.
[1] Many argue that Frank Rowley’s theory of vapor diffusion, which led to the introduction of vapour barriers in wall cavities and ventilated attic spaces, was not grounded in sound science. This study is considered biased by many in the field, because it was funded by the insulation industry as a way to defend itself against claims that insulation was responsible for condensation in cavity spaces and peeling paint.(Rose, 2003)
Air and Vapour Barriers — What exactly is the difference?
The building envelope is tasked with performing important tasks. Its primary purpose is to protect conditioned spaces from the adverse effects of heat, air, water and vapour. In other words, it keeps the outside out, and the inside in. This is achieved through a combination of carefully selected layers designed to stop and/or drain bulk water, insulate against heat loss, prevent or reduce air leakage, and slow down vapour migration. In building science settings, these are referred to as control layers (Lstiburek, 2010). Air barriers and vapour barriers form part of these control layers.
Air barriers, as their name implies, are intended to stop uncontrolled air from leaking into conditioned spaces. This infiltration is caused by the effects of wind, the building’s mechanical systems (HVAC), and by air buoyancy forces also known as “the stack effect” (BC Housing, 2017). It is critical to stop uncontrolled air leakage, because air can act as a transport mechanism for a number of other undesirable particulates (smoke, odours, etc.). In addition, air can also carry moisture. However, the maximum amount of moisture air can carry (its saturation point) is dependent on temperature and pressure. In general, cold air is able to carry less moisture than warm air.
When saturated air encounters a surface that a cooler surface, the moisture it is carrying undergoes a “phase change” and condenses (turns from gas to liquid) on the surface. The temperature at which this occurs is known as the “dew point”. This becomes critical in building envelope design because uncontrolled air leakage can result in air entering and condensing on components of wall cavities in unintended ways leading to moisture related problems such as mold, corrosion, and the general deterioration of building materials.
Air barriers are installed to counteract these undesirable effects, and must at a minimum possess some essential properties:
· First, they must be resistant to air leakage. This resistance is quantified through testing according to industry standards (CAN/ULC S741 — Standard For Air Barrier Materials — Specification in Canada and ASTM E2178 — Standard Test Method for Air Permeance of Building Materials in the US) and is limited in the National Building Code of Canada to 0.02 L/(s·m2) of leakage at a pressure differential of 75 pa.
· Next, they need to be continuous. As shown by research, air leakage is a critical factor when it comes to ensuring proper building envelope performance. Therefore, the National Building Code mandates that materials used to preserve the airtightness of building assemblies must be installed in a continuous fashion.
· Finally, they must be act as a system. Contrary to popular belief, the air barrier does not manifest itself as a single material. Even if there is generally a primary air barrier material (usually referred to as the air barrier membrane), it is the combination of this primary material and various other components (e.g doors, windows etc.) and accessories (sealants, tapes etc.) performing in unison that truly guard the building against the deleterious effects of air leakage. This systemic approach is recognized in ULC standard CAN/ULC S742 — Standard For Air Barrier Assemblies — Specification as it attempts to mimic actual field conditions (penetrations, laps, reinforcements etc.) rather than evaluate the performance of materials in isolation (Côté, 2016).
Vapour retarders (or vapour barriers) serve a much different purpose than air barriers. They are designed to stop (or slow) the movement of water in its gaseous form from travelling through wall assemblies. This process, called vapour diffusion, is the motion of water vapour molecules from an area of high concentration (higher humidity) to an area of low concentration (lower humidity) across a gradient (vapour permeable material) — this is also known as Fick’s First Law of Diffusion. This movement is driven largely by kinetic forces at the molecular level and is governed by temperature and relative humidity, not air pressure.
Vapour Permeance (or colloquially referred to as “perm rating”) is a measure of a material’s ability to allow the passage of moisture by diffusion. This is expressed in ng/s●m2●Pa (nanogram per second per square meter per pascal) or in perms, and is often evaluated in accordance with ASTM E96 — Standard Test Methods for Water Vapor Transmission of Materials. U.S Building Codes have classified materials according to their vapour permeance on a scale ranging from “vapour impermeable” to “vapour permeable” and assigned them to “vapour retarder classes” ranging from I to III (Lstiburek, 2011). In principle, only materials that meet the requirements of a “Class I Vapour Retarder” should be called “Vapour Barriers”.
Diffusion rates through wall assemblies have been shown to be generally slow. All things being equal, breaches in the vapour barrier result in significantly lower quantities of moisture migration due to diffusion than the quantity of moisture transported through the same material due to air movement (Quirouette, 1985). As such — despite what many often assume — vapour barriers need not be continuous in wall assemblies, and may at times (depending on climate) be omitted altogether (Lstiburek, 2011).
Air Barrier, Vapour Retarder, Air/Vapour Barrier — It’s just words, what the big deal ?
The importance of effective communication in the construction industry cannot be overstated. However, barriers (no pun intended) exist that can impede or distort effective communication and result in failure or other undesirable effects. Amongst these barriers, the use of jargon is often cited as a leading cause of communication breakdown. Specifically, in the construction industry — due to its fragmented structure, technical nature, and the adversarial tendencies of its stakeholders — the lack of terminology standardization has engendered different terms and meanings that are understood differently by different people. The table below taken from Communication in Construction — Theory and practice (Dainty, Moore, & Murray, 2006) illustrates how misunderstandings can occur between two professionals that interpret technical jargon differently. Unfortunately, these complex and ambiguous terms often well-understood by discrete groups, but prone to misinterpretation by others, plague the industry.
The term “air/vapour barrier” is an example of such ambiguous terminology. The ambiguity of this term is inherent in its use of the slash ( / ) punctuation mark. In English, the slash can be used to either denote the conjunction “OR” or the conjunction “AND”. Therefore, in theory, the term “air/vapour barrier” can be interpreted as “air OR vapour barrier” or “air AND vapour barrier”. In general contexts, however, it is safe to assume that most people use it to refer to the latter. Yet, as it has been discussed previously, the term “vapour barrier” is itself a subset of the term “vapour retarder” (i.e. only a Class I vapour retarder is considered a vapour barrier). Should the term “air/vapour barrier” then be restricted to materials that meet the requirements for air barrier materials AND Class I vapour retarders? The advent of materials having various levels of permeability (e.g. spun-bonded polyolefin, silicone, vapour permeable polyurethane foam and others) clearly show that such a restriction would prove overly exclusive.
Any alternative to “air/vapour barrier” should reduce ambiguity while reflecting the current trends of the industry. At present, it is crystal clear that the benefits of controlling air leakage far outweigh the control of vapour diffusion. Therefore, “air/vapour barrier” should be replaced with a term that describes an air barrier material which also has some vapour retarding characteristics. As the permeability of materials has already been codified in building codes, it is perhaps most appropriate to borrow this terminology and apply it to our problem. I propose that “air/vapour barrier” is replaced with 4 distinct terms having the following designations and associated properties:
To conclude
It is no secret that the construction industry is plagued by a chronic productivity problem. A 2015 report by McKinsey&Company showed that while productivity rates in the manufacturing sector nearly doubled over the 20-year period spanning from 1995 to 2015, productivity rates remained painfully flat, or even worsened, in the construction industry (Sriram, Mohammad, & Nieuwland, 2015). Factors such as the fragmented structure of the industry, the cyclical nature of construction activities, and poor or inadequate communication have all been attributed to this lack of productivity growth. Moreover, these sluggish gains may also reflective of the industry’s unwillingness to embrace change.
The future is, however, not all bleak. Recent years have shown that consultants and contractors alike have embraced new technologies such as Building Information Modelling (BIM), drones, and prefabrication to increase output and reduce errors. Accordingly, the language used in construction today must express and reflect the current state of affairs. Terms and materials that no longer serve their purpose should be replaced with new ones that illustrate advancements in research and practice.
Thus, the term “Air/Vapour Barrier” should be replaced by four distinct terms namely, “Vapour Impermeable Air Barrier”, “Semi-Impermeable Air Barrier”, “Semi-Permeable Air Barrier”, and “Vapour Permeable Air Barrier” that illustrate both the air permeance and vapour permeance properties of these materials. Ultimately, the biggest challenge to any term proposed will be to overcome the inertia forces associated with the construction industry’s fear of change.
RESOURCES
BC Housing. (2017). Illustrated Guide — Achieving Airtight Buildings. Retrieved from https://www.bchousing.org/research-centre/library/residential-design-construction/achieving-airtight-buildings
Côté, J.-F. (2016). DO YOU KNOW THE CANADIAN AIR BARRIER STANDARDS CAN/ULC-S741 AND S742? Retrieved from http://blog.soprema.ca/en/do-you-know-the-canadian-air-barrier-standards-can-ulc-s741-and-can-ulc-s742
Dainty, A., Moore, D., & Murray, M. (2006). Communication in construction: Theory and practice. Communication in Construction: Theory and Practice (Vol. 9780203358). Taylor & Francis. https://doi.org/10.4324/9780203358641
Hess-Kosa, K. (2017). Building Materials: Product Emission and Combustion Health Hazards. CRC Press.
Koniorczyk, M., & Gawin, D. (2008). Heat, Air and Moisture Control in Walls of Canadian Houses: A Review of the Historic Basis for Current Practices. Journal of Building Physics, 25(4), 275. https://doi.org/10.1177/1744259107088003
Lstiburek, J. (2010). BSI-001: The Perfect Wall. Retrieved from https://buildingscience.com/documents/insights/bsi-001-the-perfect-wall
Lstiburek, J. (2011). BSD-106: Understanding Vapor Barriers. Retrieved from https://buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers
Quirouette, R. L. (1985). Building Practice Note №54: The Difference Between a Vapour Barrier and an Air Barrier, 0. Retrieved from http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/view/object/?id=db9bccc2-eff6-4249-8a3f-0d2224dc30db
Rose, W. . (2003). The rise of the diffusion paradigm in the US. In G. V. J. Carmeliet, H. Hens (Ed.), Research in Building Physics: Proceedings of the Second International Conference on Building Physics, Leuven, Belgium, 14–18 September 2003. Leuven.
Sriram, C., Mohammad, A., & Nieuwland, M. van. (2015). The construction productivity imperative. Retrieved from https://www.mckinsey.com/industries/capital-projects-and-infrastructure/our-insights/the-construction-productivity-imperative
