9.25.4.2.Vapour Barrier Materials

(1) Except as provided in Sentence (2), vapour barriers shall have a permeance not greater than 60 ng/(Pa×s×m2) measured in accordance with ASTM E96 / E96M, “Standard Test Methods for Water Vapor Transmission of Materials,” using the desiccant method (dry cup). (2) Thermally insulated foundation wall assemblies are permitted to be constructed with variable-permeance vapour barriers having a permeance not greater than 60 ng/(Pa×s×m2) using the desiccant method (dry cup) and greater than 300 ng/(Pa×s×m2) using the water method (wet cup) measured in accordance with ASTM E96 / E96M, “Standard Test Methods for Water Vapor Transmission of Materials.” (See Note A-9.25.4.2.(2)) (3) Where the intended use of the interior space will result in high moisture generation, the assembly shall be designed according to Part 5. (See Note A-9.25.4.2.(3)) (4) Where polyethylene is installed to serve only as the vapour barrier, it shall comply with Clause 4.4, Thermal Stability, and Clause 5.7, Oxidative Induction Time, of CAN/CGSB-51.34-M, “Vapour Barrier, Polyethylene Sheet for Use in Building Construction.” (5) Membrane-type vapour barriers other than polyethylene shall conform to the requirements of CAN/CGSB-51.33- M, “Vapour Barrier, Sheet, Excluding Polyethylene, for Use in Building Construction.” (6) Membrane-type vapour barriers other than polyethylene that are susceptible to deterioration under prolonged exposure to direct ultraviolet radiation shall (a) be covered, or (b) only be installed in locations that are not exposed to direct ultraviolet radiation after the completion of construction. (See Note A-9.25.4.2.(6)) (7) Where a coating is applied to gypsum board to function as the vapour barrier, the permeance of the coating shall be determined in accordance with CAN/CGSB-1.501-M, “Method for Permeance of Coated Wallboard.” (8) Where insulation functions as the vapour barrier, it shall be sufficiently thick so as to meet the requirement of Sentence (1).