{"id":93781,"date":"2019-08-21T10:04:16","date_gmt":"2019-08-21T14:04:16","guid":{"rendered":"https:\/\/www.constructionspecifier.com\/?p=93781"},"modified":"2020-01-17T09:06:52","modified_gmt":"2020-01-17T14:06:52","slug":"the-air-vapor-barrier-must-die","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/the-air-vapor-barrier-must-die\/","title":{"rendered":"The air\/vapor barrier must die"},"content":{"rendered":"

\"\"<\/a>SPECIFICATIONS
\n<\/strong><\/span><\/span>by Juste Fanou
\n<\/strong>The terms \u2018air barriers\u2019 and \u2018vapor retarders\u2019 (or \u2018vapor barriers\u2019) are perhaps some of the most poorly understood concepts in the construction industry. Most building professionals know they are needed, but often struggle to locate them correctly within wall assemblies. Also, the consequences of installing these materials improperly can result in failures. This lack of understanding of the functions of these assemblies has resulted in simplistic rules-of-thumb prone to misapplication (e.g.<\/em> the vapor barrier is always on the inside and the air barrier is always on the outside). As manufacturers introduce materials with new properties and attempt to push the boundaries of building envelope construction, it is crucial the industry agrees on terminology for communicating the specific functions and purpose of these materials to avoid confusion and costly errors. In this regard, the term \u2018air\/vapor barrier\u2019 is misleading and should be replaced with more appropriate terminology.<\/p>\n

A brief history<\/strong><\/p>\n

\"Photo<\/a>
Photo \u00a9 BigStockPhoto.com<\/span><\/em><\/figcaption><\/figure>\n

Initial attempts to increase the thermal comfort of occupants in modern North American wood-framed construction date back to the 1800s. The introduction of \u2018building paper\u2019 in the form of asphalt-impregnated felt\u2014also known as sheathing membranes or weather-resistive barriers (WRBs)\u2014represented an early effort to reduce wetting of wall assemblies and air leakage (Refer the 2017 book Building Materials: Product Emission and Combustion Health Hazards<\/em> written by K. Hess-Kosa). The industry made further advancements in the field of envelope performance in the 1930s with the rise of insulation in framing cavities and attics (For more information, read the article, \u201cHeat, Air and Moisture Control in Walls of Canadian Houses: A Review of the Historic Basis for Current Practices,\u201d by M. Koniorczyk and D. Gawin, published in the April 2008 issue of Journal of Building Physics)<\/em>. However, the undesirable effects of moisture in insulated cavities soon surfaced. The traditionally painted wood fa\u00e7ades began to suffer from peeling, blistering, and other coating failures. As is the norm in the building industry, a \u2018blame game\u2019 quickly ensued with insulation manufacturers blaming the paint producers and vice-versa, while the building paper manufacturers were caught in the middle (Consult the paper, \u201cThe rise of the diffusion paradigm in the US,\u201d by W.B. Rose, published in the 2003 book, Research in Building Physics: Proceedings of the Second International Conference on Building Physics<\/em>, edited by G. V. J. Carmeliet, H. Hens, and G. Vermeir). It was not until the late 1930s that scientists began investigating moisture movement in building assemblies. Their findings\u2014considered controversial or even biased by some\u2014concluded the transfer of water vapor by diffusion (a process explained later in this article) was responsible for the paint peeling off the siding (Many argue Frank Rowley\u2019s theory of vapor diffusion, which led to the introduction of vapor 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 insulation was responsible for condensation in cavity spaces and peeling paint). These conclusions resulted in regulation that mandated the use of low-vapor permeance membranes in construction projects in the early 1950s. It was the birth of the \u2018vapor barrier,\u2019 and the industry celebrated its solution to the moisture problem by lining walls with polyethylene (PE) plastic. Ostensibly the issue of peeling paint was resolved, or at least it seemed so.<\/p>\n

\"Figure<\/a>
Figure 1:<\/strong> Classification of materials according to their vapor permeance; referenced from a Building Science paper by J. Lstiburek.<\/span>
Image courtesy J. Lstiburek<\/span><\/em><\/figcaption><\/figure>\n

Nonetheless, moisture problems persisted. Further research in the mid-1980s<\/a> suggested that uncontrolled air infiltration, and not vapor diffusion, was the largest contributor to moisture accumulation in cavity spaces. However, by that time the popularity of the now ubiquitous \u20186-mil poly\u2019 inspired methods to seal the already familiar vapor barrier. The purpose was to turn it into an effective air barrier.<\/p>\n

These attempts to seal the vapor barrier may be characterized as the genesis of the \u2018air\/vapor barrier,\u2019 both as a concept and term. Conceptually, the \u2018air\/vapor barrier\u2019 was a material purported to address both air infiltration and vapor diffusion issues. Its proponents believed in sealing the joints of PE sheets to create a dual-purpose material ascribing the additional properties of air infiltration control to the already popular \u20186-mil poly.\u2019 This popularity was perhaps responsible for its great appeal and wide adoption in the industry. As time passed, however, the idea of \u2018sealing PE was quickly abandoned, and construction practitioners explored other materials to provide airtightness. It was clear PE 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 PE in a continuous fashion.<\/p>\n

Even if the concept of the PE \u2018air\/vapor barrier\u2019 slowly faded, the term continued to be used. Coincidentally, as time passed it also appeared as if the general understanding of the functions of these air and moisture control elements was further clouded by this conflated term. These once-distinct materials slowly transformed into abstract dotted lines everyone knew were required on construction details, but no one fully understood where or why.<\/p>\n

Differences<\/strong><\/p>\n

\"Air<\/a>
Air barriers stop uncontrolled air from leaking into conditioned spaces. This infiltration is caused by the effects of wind, HVAC systems, and by air buoyancy forces that are also known as \u2018the stack effect.\u2019<\/span>
Photo \u00a9 BigStockPhoto.com<\/span><\/em><\/figcaption><\/figure>\n

The building envelope performs very important tasks. Its primary purpose is to protect conditioned spaces from the adverse effects of heat, air, water, and vapor. 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 vapor migration. In building science settings, these are referred to as control layers<\/a>. Air barriers and vapor barriers are part of these control layers.<\/p>\n

Air barriers, as its name implies, are intended to stop uncontrolled air from leaking into conditioned spaces. This infiltration is caused by the effects of wind<\/a>, HVAC systems, and by air buoyancy forces, also known as \u2018the stack effect.\u2019 It is critical to stop uncontrolled air leakage because air can act as a transport mechanism for several other undesirable particulates (e.g.<\/em> smoke and odor). Additionally, air can also carry moisture. However, the maximum amount of moisture air can carry (its saturation point) is dependent on temperature and pressure. Generally, cold air can carry less moisture than warm air.<\/p>\n

When saturated air encounters a cooler surface, the moisture it is carrying undergoes a phase change and condenses (i.e.<\/em> turns from gas to liquid) on the surface. The temperature at which this occurs is known as the dewpoint. 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.<\/p>\n

\"Figure<\/a>
Figure 2:<\/strong> Differences between the jargon employed by architects and mechanical and electrical engineers from Communication in Construction \u2013 Theory and Practice by Andrew Dainty, David Moore, and Michael Murray.<\/span>
Images courtesy J. Lstiburek<\/span><\/em><\/figcaption><\/figure>\n

Air barriers are installed to counteract these undesirable effects, and must at a minimum possess some essential properties.<\/p>\n

Resistant to air leakage<\/em><\/p>\n

This resistance is quantified through testing according to industry standards (ASTM E2178, Standard Test Method for Air Permeance of Building Materials<\/em>). The Air Barrier Association of America (ABAA) limits the air leakage of materials used for air barrier systems in opaque wall assemblies to 00.02 L\/s.m2<\/sup> (0.004 cfm\/sf) at a pressure differential of 75 Pa (1.57 psf).<\/p>\n

Be continuous<\/em><\/p>\n

As shown by research, air leakage is a critical factor when it comes to ensuring proper building envelope performance. Therefore, most governing codes stipulate for materials that are employed to preserve the airtightness of building assemblies to be installed in a continuous fashion.<\/p>\n

\"Figure<\/a>
Figure 3:<\/strong> Proposed system for labeling air barrier materials.<\/span><\/figcaption><\/figure>\n

Act as a system<\/em><\/p>\n

Contrary to popular belief, the air barrier does not manifest itself as a single material. Even if there is
\na 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.<\/em> doors and windows) and accessories (e.g.<\/em> sealants and tapes) performing in unison that truly guard the building against the deleterious effects of air leakage. This systemic approach is recognized in ASTM E2357, Standard Test Method for Determining Air <\/em>Leakage Rate of Air Barrier Assemblies,<\/em> as it attempts to simulate a full system approach (e.g.<\/em> continuity, penetrations, and interfaces) rather than evaluate the performance of materials in isolation. Air leakage for such assemblies is limited by ABAA to 0.2 L\/(s\u00b7m2<\/sup>) (0.4 cfm\/sf) of leakage at a pressure differential<\/a> of 75 pa.<\/p>\n

Vapor retarders (or vapor 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 traveling through wall assemblies. This process, called vapor diffusion, is the motion of water vapor molecules from an area of high concentration (higher humidity) to an area of low concentration (lower humidity) across a gradient (vapor permeable material). This movement is driven largely by kinetic forces at the molecular level and is governed by temperature and relative humidity (RH), and not air pressure.<\/p>\n

Vapor permeance (colloquially referred to as \u2018perm rating\u2019) is a measure of a material\u2019s ability to allow the passage of moisture by diffusion. This is expressed in perms, and is often evaluated in accordance with ASTM E96, Standard Test Methods for Water Vapor <\/em>Transmission of Materials<\/em>. Building codes have classified materials according to their vapor permeance<\/a> on a scale ranging from \u2018vapor impermeable\u2019 to \u2018vapor permeable\u2019 and assigned them to \u2018vapor retarder classes\u2019 ranging from I to III. In principle, only materials meeting the requirements of a \u2018Class I Vapor Retarder\u2019 should be called \u2018vapor barriers\u2019 (Figure 1).<\/p>\n

\"Make-up<\/a>
Make-up of an air barrier system from the Illustrated Guide \u2013 Achieving Airtight Buildings, published by British Columbia Housing in September 2017.<\/span><\/figcaption><\/figure>\n

Diffusion rates through wall assemblies have been shown to be generally slow. All things being equal, breaches in the vapor barrier result in significantly lower quantities of moisture migration as a result of diffusion rather than the quantity of moisture transported through the same material due to air movement. As such\u2014despite what many often assume\u2014vapor barriers need not be continuous in wall assemblies, and may, at times, (depending on climate) be omitted altogether.<\/p>\n

Different meanings<\/strong><\/p>\n

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 undesirable effects. Amongst these barriers, the use of jargon is often cited as a leading cause of communication breakdown. Specifically, in the construction industry\u2014due to its fragmented structure, technical nature, and the adversarial tendencies of its stakeholders\u2014the lack of terminology standardization has engendered different terms and meanings that are understood differently across actors. Figure 2 illustrates how misunderstandings can occur between two professionals interpreting technical jargon differently. Unfortunately, these complex and ambiguous terms, often well-understood by discrete groups, but prone to misinterpretation by others, plague the industry.<\/p>\n

\"Ineffective<\/a>
Ineffective communication in the construction industry can result in failure or undesirable effects. The use of jargon is often cited as a leading cause of communication breakdown.<\/span>
Photo \u00a9 BigStockPhoto.com<\/span><\/em><\/figcaption><\/figure>\n

The term \u2018air\/vapor barrier\u2019 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 \u2018or\u2019 or \u2018and.\u2019 Therefore, in theory, the term \u2018air\/vapor barrier\u2019 can be interpreted as \u2018air or vapor barrier\u2019 or \u2018air and vapor barrier.\u2019 In general contexts, however, it is safe to assume most people use it to refer to the latter. Yet, as it has been discussed previously, the term \u2018vapor barrier\u2019 is itself a subset of the term \u2018vapor retarder\u2019 (i.e.<\/em> only a Class I vapor retarder is considered a vapor barrier). Should the term \u2018air\/vapor barrier\u2019 then be restricted to materials meeting the requirements for air barrier materials and Class I vapor retarders? The advent of materials having various levels of permeability (e.g.<\/em> spun-bonded polyolefin, silicone, and vapor permeable polyurethane foam) clearly show such a restriction would prove overly exclusive.<\/p>\n

Any alternative to \u2018air\/vapor barrier\u2019 should reduce ambiguity while reflecting the current trends of the industry. At present, we understand the benefits of controlling air leakage clearly outweigh the control of vapor diffusion. Therefore, \u2018air\/vapor barrier\u2019 should be replaced with a term describing an air barrier material, which also has some vapor-retarding characteristics. As the permeability of materials has already been standardized in building codes, it is perhaps most appropriate to borrow this terminology and apply it to the problem under discussion. See Figure 3 for a proposed system with four distinct designations and associated properties.<\/p>\n

\"Building<\/a>
Building envelope layers.<\/span>
Image courtesy J. Lstiburek<\/span><\/em><\/figcaption><\/figure>\n

Conclusion<\/strong><\/p>\n

It is no secret the construction industry is plagued by a chronic productivity problem. A 2015 report<\/a> 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. Factors such as the fragmented structure of the industry, the cyclical nature of construction activities, and poor or inadequate communication have all been suggested as contributors to this lack of productivity growth. Moreover, these sluggish gains may also be reflective of the industry\u2019s unwillingness to embrace change.<\/p>\n

The future is, however, not all bleak. Recent years have shown consultants and contractors alike embrace new technologies such as building information modeling (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 no longer serving their purpose should be retired and replaced with new ones illustrating advancements in research and practice.<\/p>\n

Thus, the term \u2018air\/vapor barrier\u2019 should be replaced by ones illustrating both the air leakage and vapor 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\u2019s fear of change.<\/p>\n

The opinions expressed in Specifications are based on the author\u2019s experiences and do not necessarily reflect those of The Construction Specifier<\/em> or CSI.<\/span><\/p>\n

Juste Fanou is the director of Jm|F Technical Documentation Solutions Inc. He is a bilingual technical documentation specialist and specifications writer with more than 10 years of experience. Fanou has assisted multiple clients in the development of front-end and technical specifications including the evaluation of material substitution proposals, and the interpretation of standards. He holds a certificate in building science, a diploma in architectural technology, and a degree in civil engineering technology. Fanou can be reached at jfanou@jmfservices.net<\/a>.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

As manufacturers introduce materials with new properties and attempt to push the boundaries of building envelope construction, it is crucial the industry agrees on terminology for communicating the specific functions and purpose of these materials to avoid confusion and costly errors. In this regard, the term \u2018air\/vapor barrier\u2019 is misleading and should be replaced with more appropriate terminology.<\/p>\n","protected":false},"author":13542,"featured_media":93782,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[520,179,449,187,264,265,489,460,28],"tags":[150,1195,1183,3339,584,1114,1071],"class_list":["post-93781","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-features","category-masterformat","category-uniformat","category-07-00-00-thermal-and-moisture-protection","category-07-20-00-thermal-protection","category-07-25-00-weather-barriers","category-b2010exterior-walls","category-b20exterior-vertical-enclosures","category-specifications","tag-air-barrier","tag-division-07","tag-moisture-protection","tag-specification","tag-vapor-barrier","tag-vapor-retarder","tag-wall-assemblies","entry"],"acf":[],"yoast_head":"\nThe air\/vapor barrier must die - The Construction Specifier<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.constructionspecifier.com\/the-air-vapor-barrier-must-die\/\" \/>\n<link rel=\"next\" href=\"https:\/\/www.constructionspecifier.com\/the-air-vapor-barrier-must-die\/2\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The air\/vapor barrier must die - The Construction Specifier\" \/>\n<meta property=\"og:description\" content=\"As manufacturers introduce materials with new properties and attempt to push the boundaries of building envelope construction, it is crucial the industry agrees on terminology for communicating the specific functions and purpose of these materials to avoid confusion and costly errors. 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