9.25.4.

(1) Thermally insulated wall, ceiling and floor assemblies shall be constructed with a vapour barrier sufficient to prevent condensation in the wall spaces, floor spaces or attic or roof spaces.

(1) Vapour barriers shall have a permeance not greater than 60 ng/(Pa∙s∙m²), measured in accordance with ASTM E96, “Water Vapor Transmission of Materials”, using the desiccant method (dry cup).

(2) Where the mild climate indicator, determined in accordance with Sentence 9.25.1.2.(6)v1, is greater than 6300, vapour barriers shall be designed according to Part 5, where,

(a) the intended use of the interior space requires the indoor relative humidity to be maintained above 35% over the heating season and the ventilating and air-conditioning system is designed to maintain that relative humidity, or

(b) the intended use of the interior space results in an average monthly indoor relative humidity above 35% over the heating season and the ventilating and air-conditioning system does not have the capacity to reduce the average monthly relative humidity to 35% or less over that period.

(3) Where the mild climate indicator, determined in accordance with Sentence 9.25.1.2.(6)v1, is less than or equal to 6300, vapour barriers shall be designed according to Part 5, where,

(a) the intended use of the interior space requires the indoor relative humidity to be maintained above 60% over the heating season and the ventilating and air-conditioning system is designed to maintain that relative humidity, or

(b) the intended use of the interior space results in an average monthly indoor relative humidity above 60% over the heating season and the ventilating and air-conditioning system does not have the capacity to reduce the average monthly relative humidity to 60% over that period.

(4) Where polyethylene is installed to serve as the vapour barrier, it shall conform to 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 CAN/CGSB-51.33-M, “Vapour Barrier, Sheet, Excluding Polyethylene, for Use in Building Construction”.

(6) 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”.

(1) Vapour barriers shall be installed to protect the entire surfaces of thermally insulated wall, ceiling and floor assemblies.

(2) Vapour barriers shall be installed sufficiently close to the warm side of insulation to prevent condensation at design conditions.