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:
- galvanized steel ducts;
- junction boxes; and
- brick ties.
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 email@example.com.