Selecting and achieving the proper air barrier

All photos courtesy Sto Corp.
All photos courtesy Sto Corp.

By John Edgar
Air barriers have been a requirement of the National Building Code of Canada (NBC) for many years, but not all design professionals fully understand what is involved in specifying one. An air barrier may be a material of many functions and the choice of one over another should reflect the needs of the particular project. Historically, the requirements for airtightness have been found under NBC Part 5, “Environmental Separation.” Requirements are now being refined with the introduction of the new National Energy Code of Canada for Buildings (NECB). Air leakage, moisture management, energy conservation, and fire safety are all factors that come into play.

Based on their location in a wall assembly, many air barrier materials are also assumed to function as a water-resistive barrier (WRB). With this dual function, air barriers and WRBs must not only resist air leakage, but also prevent water penetration. In doing so, they need to remain durable and leak-proof under the stresses that may be experienced in service. A material may be airtight or leak-proof when subjected to a single test, but how will it perform when it is filled with holes from mechanical fasteners? Another important factor to consider is vapour permeability. As the insulation requirements increase, with more positioned outbound of the air barrier, vapour permeability requirements of the air barrier must be determined.

Various air barrier products—fluid-applied, polymer-based sheet wraps, and asphaltic self-adhered membranes—are being used in non-combustible construction, often without consideration of their potential impact on fire performance of a wall assembly. Selecting a material that meets all the requirements defined in Part 5 and the NECB may have unintended consequences with respect to Part 3. The best way to know if a product meets the requirements is to review the testing and code evaluation reports.

Air barriers and WRBs 
The Canadian Construction Materials Centre (CCMC) has developed a series of technical guides to evaluate materials not specifically identified in NBC. These guides outline testing requirements to demonstrate durability and compliance with the intent of the code. The development of the Technical Guide for EIFS is an example of the use of research-based test methods unique to exterior insulation finish systems.

An alternative approach would be to show equivalent or superior performance to materials that are acceptable and listed in the code. This approach can present challenges for CCMC where a product’s code acceptance is based on historical precedence and not on performance—one such example can be found with building felts.

While building felts are an acceptable material for a WRB, the ability to resist water penetration with mechanical fasteners does not currently require evaluation. This missing data makes evaluation of a new material against the performance of an existing code-accepted one difficult (and perhaps impossible) without comparing the two.

The introduction of new materials with a different approach to a common problem raises a new series of complications in evaluations. A good example is the development of the Technical Guide for EIFS with a liquid-applied (LA) water-resistive barrier. How should one evaluate a WRB that becomes part of the substrate material versus a loose-sheet material that is mechanically fastened?

For example, EIFS have unique qualities that include the ability to bond to the LA-WRB and resist the live loads imposed on the wall assembly. However, what happens to the membrane if nails ‘pop?’ What happens to sheathing joints when the wall racks and moves? What happens if the WRB is left exposed for a certain period? What happens when water makes its way back to the membrane and must be drained to the exterior?

In the case of EIFS with LA-WRBs, the EIFS industry and CCMC spent five years of research developing test methods that demonstrate durability of such water-resistive barriers. These methods were subsequently translated into Underwriters Laboratories of Canada (CAN/ULC) S716.1, Standard for Exterior Insulation Finish Systems–Materials and Systems. The collaboration between the EIFS industry and CCMC has resulted in material evaluations unparalleled in demonstrating durability and performance.

Sun Peaks Resort (pictured on page 18) is northeast of Kamloops, B.C. It includes an exterior insulation finish system (EIFS).
Sun Peaks Resort (pictured on page 18) is northeast of Kamloops, B.C. It includes an exterior insulation finish system (EIFS).

Today, various manufacturers recognize the advantages of liquid-applied membranes and have introduced products that act as standalone air barriers and moisture barriers, independent of a specific cladding. CCMC has worked with numerous manufacturers to evaluate these materials as air barriers, air barrier systems, moisture barriers, or a combination of these functions.1

Most recently, a fluid-applied moisture barrier used inbound of insulation with any cladding was evaluated. The research to develop the technical guide involved testing water penetration resistance through the membrane at mechanical fastener locations and comparing performance with materials historically accepted by NBC. The requirements included durability testing from the Technical Guide for EIFS and from International Code Council (ICC) Acceptance Criteria (AC) 212, Water-resistive Coatings Used as Water-resistive Barriers Over Exterior Sheathing. These are extremely robust requirements that go far beyond testing a single material for one or two properties.

For a specifier or a building official, this research and development with CCMC provides a process for selecting a material or system that performs in the Canadian climate. A CCMC evaluation report provides an opinion evaluated materials meet NBC intent and requirements.

Fire testing
Building codes evolve to reflect the needs and experience of the times. Today, building construction integrates very sophisticated materials into wall assemblies that are measured in millimetres. These assemblies are designed to perform more functions than walls that used to be metres thick. New codes provide basic requirements relating to health and safety, while maintaining a degree of flexibility in meeting these objectives.

Without going into a detailed analysis of codes, it is clear walls must be airtight, watertight, thermally efficient, and fire-resistive. Buildings are now defined as being of “combustible” or “non-combustible” construction. Housing and small structures can be the former—typically wood frame. Institutional and many commercial projects are required to be of non-combustible construction, which often means steel frame, concrete, or masonry, based on building height and occupancy. Combustible materials may be used in non-combustible construction provided certain testing is performed and materials meet criteria established by the codes.

Examining two fire tests used to evaluate exterior wall materials and assemblies for use in non-combustible construction provides a better understanding of how new combustible materials may affect fire test performance. The two tests are:

  • CAN/ULC S114-05, Standard Method of Test for Determination of Non-combustibility in Building Materials; and
  • CAN/ULC S134-92, Standard Method of Fire Test of Exterior Wall Assemblies.
  • CAN/ULC S114-05

This is the basic test that determines whether a material is ‘combustible.’ If a material is deemed to be combustible, then limitations for its use are outlined in NBC Part 3, “Fire Protection, Occupant Safety, and Accessibility.”

As thermal efficiency requirements are increased, foam plastic insulation has become an effective material in meeting the code’s objectives.

Foam plastic insulation is typically rated as combustible by this standard so it requires subsequent testing if used in a wall assembly. There are exceptions for other ‘minor combustible elements’ such as paint, air barrier ‘connective’ materials, sealants, wood trim, and other minor components, but the air barrier system is not exempt. NBC Article 3.1.5.5 outlines the requirement for testing in conformance to CAN/ULC S134 for assemblies with combustible materials in non-combustible construction. This is the fire test developed when EIFS were introduced.

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