by Katie Daniel | November 11, 2016 10:02 am
By Jean-François Côté, PhD, OCQ
During the 1970s, extensive research was carried out in Canada on air leakage in buildings. Researchers at the National Research Council (NRC) demonstrated the importance of managing air leakage through the building enclosure. In the following decade, the concept of air barrier material emerged and was formally introduced in the 1985 edition of the National Building Code of Canada (NBC).
It was then known that uncontrolled air leakage increases the burden on heating or air-conditioning, and causes problems because air can transport large amounts of humidity. Building professionals adapted their designs to this new reality, but the appropriate choice and positioning of materials in assemblies brought new challenges, with the functions of air barrier and vapour barriers often misunderstood. Consequently, major failures such as mould growth in wall assemblies, rotting of structural materials, or bursting of water-saturated exterior cladding materials in winter were observed.
Starting in the mid-’90s, NBC prescribed a maximal air leakage rate for exterior wall assemblies, but did not indicate how to evaluate materials and systems to determine compliance. Interestingly, standards for air barriers appeared first at ASTM with the development of the E2178, Standard Test Method for Air Permeance of Building Materials, published in 2001. This standard rapidly became the reference for the determination of air leakage of building materials.
In Canada, the first standards related to air barrier materials and systems appeared several years later. CAN/ULC-S741, Standard for Air Barrier Materials−Specification, and CAN/ULC-S742, Standard for Air Barrier Assemblies−Specification, were published in 2008 and 2011, respectively. Some may say these are just Canadian versions of the ASTM E2178 and E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies, because they reference their U.S. counterparts for test conditions. However, that is all they have in common.
First, ASTM E2178 and E2357 standards are test methods. They describe in detail the procedure to be followed to measure the air leakage rate of air barrier materials and assemblies. Being test methods, these standards do not establish a requirement or performance level. It is therefore impossible for a material to meet or exceed the requirements of
Testing an air barrier material as per ASTM E2178 or an air barrier assembly as per ASTM E2357 provides a numerical result corresponding to the air leakage rate or permeance of the material or assembly. This result may then be used to compare various materials or assemblies, or to demonstrate compliance with building code requirements.
What are S741 and S742?
The Canadian standards CAN/ULC-S741 and S742 are specifications. Not only do they indicate how to measure air leakage of air barrier materials and assemblies (by way of references to the aforementioned ASTM test methods), but they each also include performance requirements. S741 and S742 therefore help designers and specifiers easily recognize the level of performance provided by air barrier materials and assemblies tested under S741 and S742.
The S741 standard requires the result obtained from the air leakage testing be clearly mentioned when reporting in order to facilitate comparison between materials. It is not sufficient to simply indicate “pass” on the data sheet of an air barrier material tested as per S741. For air barrier assemblies, a five-level classification (A1 to A5) is included in S742; it is based on the maximum air leakage rate measured during the test.
S741–Air barrier material testing
Today’s offering of air barrier materials is quite extensive, including products sold in sheet form (self-adhered or nailable) or liquids intended to be sprayed or trowelled. They can be manufactured from organic fibres or synthetic plastics, and be pre-assembled or site-constructed. Some rigid insulation boards can also claim to be air barrier materials.
CAN/ULC-S741 allows the testing of all these different materials to determine their performance against air leakage. This standard is intended for materials themselves, without accessories and details, irrespective of their composition. It establishes the proper test method for air leakage and air permeance based on ASTM E2178, and mandates air barrier materials being tested exhibit air leakage rates no greater than 0.02 L/s•m2 (0.004 cfm/sf). Materials must meet this requirement in both directions (infiltration and exfiltration), and the test is performed before and after a conditioning/aging period during which the material is exposed to UV radiation then subjected to heat-conditioning.
In order to meet the requirements of S741, the air leakage rate of the air barrier material after conditioning shall not be more than 10 per cent or 0.001 L/s•m2 (0.0002 cfm/sf) greater than its air leakage before conditioning. This ensures the air barrier material will maintain its integrity for a minimal period after its installation before being covered by the exterior cladding.
S742–Air barrier assembly testing
CAN/ULC-S742 covers air barrier assemblies (i.e. the combination of air barrier materials and their accessories) used in low-rise and high-rise buildings. Similar to S741, it includes test conditions to measure the air leakage rate of an air barrier assembly’s representative specimen, and defines the performance level that must be attained. Each tested assembly is then given a classification (A1 to A5) based on its air leakage rate at a reference pressure difference, measured before and after exposure to cyclic wind loading.
Testing as per S742 references the test method and procedure found in ASTM E2357. The air barrier assembly being tested is mounted in two different 2.4 x 2.4-m (8 x 8-ft) frames. The test specimens shall be representative of the air barrier assembly as constructed in the field and according to the manufacturer’s installation instructions. This includes the use of primer when applicable, proper sealing of laps, and perimeter reinforcement if required.
In the first frame, the assembly is installed without any penetration of the air barrier materials. Side- and end-laps must be present in the case of discontinuous air barrier materials (e.g. sheet-applied or panels). In the second frame, various penetrations typically found in an exterior wall are added to the assembly. These include:
All penetrations must be sealed with appropriate accessory material to maintain continuity of the plane of airtightness. Figure 1 shows a frame with all penetrations sealed and ready to be tested. Figure 2 shows two close-up images of typical penetrations.
Both frames are then tested individually. For testing, each frame is placed between two climatic chambers and a computer-controlled system allows the operator to generate a pressure difference (infiltration and exfiltration) across the frame and the air barrier assembly. The pressure difference varies from 25 to 300 Pa during the test; the air leakage rate across the assembly is measured and reported at a pressure difference of 75 Pa.
This is where S742 and ASTM E2357 part ways. After being subjected to this first test, each frame is exposed to a series of pressure differences (infiltration and exfiltration) simulating cyclic wind loads. Static pressures starting at 450 Pa (9.4 lbf/sf) and maintained during one hour, followed by cyclic pressure loads and gust loads reaching at least 980 Pa (20.5 lbf/sf), are applied to each frame. Figure 3 illustrates these pressure loads. Deflection of the air barrier assembly is measured during this portion of the test, which provides professionals with guidance on the selection of materials.
Air leakage rate measurement is then repeated twice on each frame. The first measurement is performed under typical laboratory conditions (identical to the test prior to cyclic loading). The second measurement is performed under ‘winter’ conditions, where a temperature difference is induced across the frame. In this case, the exterior side of the assembly is cooled to −20 C (−4 F) while the interior side is maintained at 20 C (68 F). For an air barrier assembly to obtain the A1 classification (i.e. the most stringent), the largest air leakage rate measured in all the aforementioned conditions shall not exceed 0.05 L/s•m2 (0.01 cfm/sf) at a pressure difference of 75 Pa (1.6 lbf/sf).
What about roofs?
The ASTM E2178 and E2357 standards were developed for wall air barrier materials and assemblies. Actually, wall assemblies most likely represent the vast majority of E2357 tests performed by laboratories. However, the key for performing air barrier systems is the continuity, which includes junctions between adjacent assemblies (e.g. wall/foundation and wall/roof junctions) and also a good roof air barrier assembly.
Before S742, designers and specifiers could not rely on a test method to evaluate the air barrier performance of roof assemblies. A significant advantage of S742 is that for roof air barrier assemblies, if the designated plane of airtightness is a low-sloped membrane roof assembly and the roof membrane serves as the air barrier material, such assembly is deemed to have an A4 classification (maximum air leakage rate of 0.2 L/s•m2 [0.04 cfm/sf]) without having to be tested.
In order to demonstrate a better classification than A4, roof air barrier assemblies for which the CSA A123.21 standard is applicable will be allowed to be tested using the parameters and procedure found in CSA A123.21. Therefore, testing will be performed horizontally (as the roof is installed normally) instead of attempting to position these assemblies vertically as required by ASTM E2357.
Canadian standards were enriched with the publication of CAN/ULC-S741 and S742. Both provide designers and specifiers with greater confidence in the expected performance of air barrier materials and assemblies used in Canada. Requirements found in these standards are coherent with those of NBC, which has integrated a reference to CAN/ULC-S742 in its 2015 edition (Division B, part 9), and with the National Energy Code of Canada for Buildings (NECB).
The 2015 NECB (Division B, subsection 3.2.4) now requires air barrier assemblies to conform to CAN/ULC-S742 with a classification of A4 or better. An option to use ASTM E2357 to meet the air leakage requirement is still present, although this option is only permitted for buildings located where the 1-in-50 hourly wind pressures do not exceed 0.65 kPa (13.5 lbf/sf), and if assemblies are installed on the warm side of the thermal insulation. The usefulness of S741 and S742 will most likely increase their presence in specifications as more professionals discover their advantages.
Jean-François Côté, PhD, OCQ, is the director of standards and scientific affairs for Soprema. A registered chemist in Québec, he chairs the CSA A123.23 technical committee on bituminous roofing materials. Côté is also an active member of ASTM International, Asphalt Roofing Manufacturers Association (ARMA), Polyisocyanurate Insulation Manufacturers Association (PIMA), and ULC. He can be reached at firstname.lastname@example.org.
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