October 14, 2016
By Peter Birkbeck, CTR
With varying degrees of detail and prescription, Canadian building codes—regardless of the model code—require all buildings to be provided with an air barrier. Where the language is more performance-oriented, such as in Part 5 of the National Building Code of Canada (NBC) and Part 3 of the National Energy Code of Canada for Buildings (NECB), key concepts such as “system” and “continuity” are introduced as well as quantitative criteria.
In emerging residential codes, these are also now becoming more precise. For example, in the energy efficiency provisions contained in NBC Section 9.36, where both material and assembly criteria are presented. (Notably Clauses 9.36.9 and 9.36.10.) Clauses in this code require a conforming air barrier assembly be qualified to CAN/ULC S742, Standard for Air Barrier Assemblies−Specification, or alternatively, with certain special conditions, ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies.
Defining an air barrier (and its benefits)
An air barrier system consists of building assemblies composed of continuously joined components. The function of the system is to provide control of air leakage into and out of the building enclosure. Such a system can be located at various points in the building, above or below grade. This typically involves an environmental separation of conditioned from unconditioned space. The primary results of the incorporation of such systems can include:
In addition to wasting energy and potentially causing damage to building components, uncontrolled air leakage has the potential to introduce outside contaminants into the building. Some time ago, researchers had previously estimated possible reductions in HVAC loads in buildings incorporating air barrier systems as up to 50 per cent in residential buildings and up to 33 per cent in office buildings. (For more on the statistic for residential projects, see “Air Tightness in New U.S. Housing,” by Sherman and Matson, which was published in the 1997 Proceedings of the 22nd Conference of the Air Infiltration and Ventilation Centre. For more on the office buildings angle, see Emmerich and Persily’s “Airtightness of Commercial Buildings in the United States,” published in the 2005 Proceedings of the 26th IEA Conference of the Air Infiltration and Ventilation Centre.)
While an air barrier component can be mechanically fastened, fluid-applied, self-adhered, an insulated or non-insulated rigid sheathing, a sealant, or sprayed polyurethane foam (SPF), exterior insulation is becoming essential to achieving energy efficiency targets in commercial and residential construction. Providing a thermal control layer outboard of the framing or substrate also provides for greater control over thermal bridging. SPF is an increasingly common way of achieving this.
Considering sprayed polyurethane foam as a major component of an air barrier system
Low-rise exterior wall air barrier systems incorporating SPF are truly multi-functional. In these systems, the SPF is self-supporting and the principal component of airtightness; it offers a high RSI-value (R-value) thermal insulation, controlling heat loss from both conductive and convective flows. In the case of medium-density sprayfoam, integral control of water vapour transmission and a resistive barrier to water is also provided.
To properly function, air barrier assemblies must have a low air leakage rate. However, they must also be continuous, buildable in the field, durable, and resistant to conditions during the service life of the building, such as pressures exerted on them by wind, mechanical systems, and related phenomena internal to the structure, such as the stack effect.
Continuity across different junctions and between dissimilar materials will normally require some other components. Sprayfoam, however, with its adhesion to most substrates and monolithic, seamless installation that conforms to the shape and surface of the substrate, can often reduce the number of individual components required.
There are numerous possibilities for auxiliary components and accessories for these systems. Common ones take the following forms:
There is a growing number of suppliers and grades of all these products. In terms of the sprayfoam manufacturer, an assembly of such identified components, installed to a specified substrate, can be considered a proprietary solution. Such a solution can be issued an Evaluation Report by the Canadian Construction Materials Centre (CCMC).
Air barrier system evaluation
Since the late 1990s, CCMC has published its Technical Guide for Air Barrier Systems for Exterior Walls of Low-rise Buildings. The resource describes all physical testing and administrative items necessary to technically evaluate and confirm the effective field delivery of an air barrier system. It is jointly issued to the proponent of the system and the third-party laboratory identified by them. The Technical Guide was among the first such comprehensive documents; it precedes the aforementioned ASTM and CAN/ULC standards.
ASTM E2357, although similar in approach, uses slightly less maximum pressures. (That is, positive and negative 300 Pa [43 lb/sf], as compared to positive and negative 500 Pa [72 lb/sf].) Assembly air leakage criteria that can be found in U.S. building codes and standards, based on testing to ASTM E2357, is often not as stringent as in Canada. Aside from some programs particular to some jurisdictions or building portfolio owners/managers, having an acceptable assembly air leakage rate of no greater than 0.2 L/s.m2 (0.04 cfm/sf @ 1.57 lb/sf) is not uncommon. (This would be a factor of approximately 10 times less airtight than the typical air barrier criteria for any individual material.)
Prior to the CCMC Technical Guide, there were only air leakage requirements for individual materials. These still appear in building codes, with the requirement typically being a sheet or panel should have an airtightness no less than 0.02 L/s.m2 at 75 Pa pressure (0.004 cfm/sf @ 1.57 lb/sf). Based on modelling, the permissible leakage rates for an assembly were established, linking the requirements to the water vapour permeance of the outermost (non-vented) layer of the wall assembly. An interior winter relative humidity (RH) of 35 per cent was assumed. For typical sprayfoam air barrier systems, this maximum permissible air leakage rate would be 0.05 L/s.m2 at 75 Pa pressure, for an outermost layer of low water vapour permeance. (In a wall assembly, such an outermost layer of low water vapour permeance would be greater than 15 and less than or equal to 60 ng/Pa.s.m2.)
For air barrier systems incorporating sprayfoam as the principal plane of airtightness, the proponent must first have a CCMC Evaluation Report or listing for the primary function of the material—that being thermal insulation. Additionally, the factory must be under control by an accredited quality assurance agency or have a registered International Organization for Standardization (ISO) 9000 series Quality Management System.
The CCMC Technical Guide outlines the rigorous testing required. For each substrate, for example, exterior gypsum sheathing, several large assemblies (i.e. 2.4 x 2.4 m [8 x 8 ft]) are constructed and subjected to air leakage testing. These assemblies variously represent:
Other components of the wall system, such as cladding and interior finish, are generally excluded.
An air leakage rating is established for the entire system using the opaque wall without penetrations as the baseline. The other two walls are required to have an air leakage rate no greater than 10 per cent of the baseline wall. The walls are subjected to both positive and negative pressure loading as per a continuous schedule, representing sustained, cyclic, and gust loads. An option of increased wind pressures is possible to qualify the assembly for geographical areas where wind design values may be more demanding.
Durability in service of all air barrier system components is an important consideration, and evaluated by the CCMC Technical Guide. Robust criteria exist for the durability for the sprayfoam, requiring a specific retention of thermal resistance (i.e. 90 per cent retention) and air leakage characteristics (i.e. less than or equal to 110 per cent of the original value) after weathering (i.e. irradiation and rain) and heat-aging exposure. The stringency of this evaluation is further enhanced by the fact the baseline condition for these characteristics is after conditioning.
All other types of components also have durability criteria, in terms of individual tests. In most cases, the tests are aggregated within a consensus material standard. No further tests are usually necessary if it can be confirmed the component meets such a standard.
Whether or not activities to install certain components are subcontracted, the sprayfoam contractor is ultimately responsible for all aspects of the air barrier system installation. To assist the contractor, the air barrier system supplier (proponent) should supply an installation manual covering all components, as well as design details describing how the components go together.
This responsibility includes quality control (QC) at different phases of the work. For instance, in systems incorporating self-adhering membrane that will later be sprayed over, tests such as adhesion tests of the membrane at pre-determined intervals would be included. Again, the installation manual should contain all the relevant quality control information, such as the proper sequencing, placement, and fastening of components.
For any required components not manufactured by the sprayfoam supplier, the manufacturers’ installation instructions should be accessed and strictly followed. It cannot be assumed the installation aspects of different sprayfoam manufacturers’ air barrier systems are the same, or in any way interchangeable. For instance, a system incorporating exterior gypsum sheathing as a substrate may have board joints pre-treated in some way with an accessory product (e.g. a tape or sealant) while other similar systems may not. Brick ties may be treated with sealant in some systems, but not in others. Further, self-adhered membrane products, common in these air barriers, may require a primer to be used. These, or any other component, may have temperature or other limitations, such as cure time or working time, which must be considered before and during the anticipated course of the project.
Essentially, the system should be constructed as detailed by the sprayfoam manufacturer (proponent). Substitutions or alternates for any component that were not qualified by the testing laboratory should not be allowed, as they could alter the performance of the system due to inferior physical characteristics or incompatibility with adjacent materials.
It is essential the sprayfoam contractor be licensed by a certification organization to install the particular brand and type of SPF specific to the air barrier system. (At the time of writing, this author noted three providers offer these services: the Canadian Urethane Foam Contractors Association (CUFCA), Morrison Hershfield, and Urethane Foam Consultants (UFC).) The grade of sprayfoam specified in the air barrier system should not be substituted, even though a particular manufacturer may offer multiple grades. The thickness of the sprayfoam, as specified in the system, must be achieved, as well as the manufacturers’ minimum declared density.
Medium-density SPF should be installed in accordance with CAN/ULC S705.2, Standard for Thermal Insulation–Rigid Spray-Applied Polyurethane Foam, Medium Density–Application. The contractor should also have received training in the installation of the entire system and credentials to evidence this should be readily available. This could also be a license issued by an accredited certification organization.
It is important the sprayfoam manufacturers’ recommendations on pass thickness be strictly followed. In accordance with Canadian national standards, sprayfoam pass thicknesses for medium-density (i.e. 2-lb or closed-cell) SPF can neither be less than 15 mm (0.6 in.) nor exceed 50 mm (2 in.).
While it is unlikely any sprayfoam air barrier system will be compliant at less than 25 mm (1 in.) thickness, SPF installed at a thicker pass than 50 mm (2 in.) could compromise the adhesion of some air barrier component products, or lead to their deformation or degradation. Notably, the adhesion of self-adhered transition membranes can sometimes be affected, due to the exothermic nature of the polyurethane reaction. However, in some cases, an initial SPF pass of minimum thickness has to be found to be of benefit in preventing this potential problem. (As these systems gain popularity, there are also new purpose-designed accessories that, for example, can assist in protecting specific air barrier system components from overspray.)
For SPF-based air barrier systems, an enhanced version of the mandatory sprayfoam “Daily Work Record” will typically be used to record the site conditions, the materials used, and any site testing. The record of the testing will include the required testing of the sprayfoam, such as density, adhesion, and cohesion, along with that for any air barrier components, such as pull-off adhesion testing. Pull-off adhesion testing is usually performed in accordance with ASTM D4541, Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers.
The minimum criteria for test frequency and what constitutes an acceptable test result for the sprayfoam, as well as for any components, may vary depending on the system. These aspects should be described in the quality control manual.
Again, CCMC evaluation reports are an initial source of information for the entire system. Important points to note in these reports are the limitations (e.g. building type and location [wind pressure values]) and the particular components, including supplier and grade.
An air barrier system must be installed as specified, with the prescribed components. Particular attention to the limitations, detailing, and sequencing of these components is essential. The CCMC evaluation report for the system is an important resource, and a credible opinion concerning its performance relative to building code requirements. The designer need not spend valuable professional time researching compatible components that will perform.
When procured, installed, and detailed as specified, sprayed polyurethane foam air barrier systems for low-rise exterior walls can result in durable, high-performance building enclosures. The sprayfoam component functions as the exterior thermal, vapour, and moisture control layer. The seamless, self-supporting, monolithic nature of SPF can contribute to achieving air barrier continuity, especially with building exteriors with complex shapes.
Systems incorporating sprayed polyurethane foam as the principal plane of airtightness often require less pre-treatment of penetrations and fewer components than other comparable systems. That can result in fewer compatibility issues and accelerated delivery of projects, often at reduced cost. (For further reading, this author suggest the National Research Council of Canada’s (NRC’s) 1997 report, “Air Barrier Systems for Walls of Low-Rise Buildings: Performance and Assessment” (40635) and its Construction Technology Update 46, “A Method for Evaluating Air Barrier Systems and Materials,” by Bruno DiLenardo. A comprehensive treatment of this topic, from design to construction, can be found in the paper, “Guidelines for Delivering Effective Air Barrier Systems,” by Kevin Knight and Bryan Boyle, published by the Ontario Association of Architects (OAA) and the Canada Mortgage and Housing Corporation (CMHC) in 2002.)
Peter Birkbeck, CTR, LEED Green Associate, is an associate specialist with the engineering division of Icynene Inc., a manufacturer of sprayed polyurethane foam (SPF) systems. He has 20 years of experience in polyurethane formulation, manufacturing, and technical support. Birkbeck is active on numerous industry consensus committees, and is a member of ASTM and the Canadian Home Builders’ Association (CHBA) Technical Research Commitee. He can be reached via e-mail at email@example.com.
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