The British Columbia Building Code | Section 5.5. | Vapour Diffusion
Division B: Acceptable Solutions Part 5 – Environmental Separation
British Columbia Building Code 2018 Division B
Section 5.5. Vapour Diffusion
5.5.1. Vapour Barriers
5.5.1.1. Required Resistance to Vapour Diffusion
(See Note A-5.5.1.1.)
1) Where a building component or assembly is subjected to differentials in temperature and water vapour
pressure, the properties and position of the materials and components in those components or assemblies shall be
such that they control vapour diffusion or permit venting to the exterior so as to minimize the accumulation of
condensation in the building component or assembly.
2) Except as provided in Sentence (3), a vapour barrier shall be installed to provide the principal resistance to
water vapour diffusion.
3) A vapour barrier is not required where it can be shown that uncontrolled vapour diffusion will not adversely
affect any of
a) the health or safety of building users,
b) the intended use of the building, or
c) the operation of building services.
5.5.1.2. Vapour Barrier Properties and Installation
(See Note A-5.3.1.2.)
1) The vapour barrier shall have sufficiently low permeance and shall be positioned in the building component
or assembly so as to
a) minimize moisture transfer by diffusion, to surfaces within the assembly that would be cold enough to cause
condensation at the design temperature and humidity conditions, or
b) reduce moisture transfer by diffusion, to surfaces within the assembly that would be cold enough to cause
condensation at the design temperature and humidity conditions, to a rate that will not allow sufficient
accumulation of moisture to cause deterioration or otherwise adversely affect any of
i) the health or safety of building users,
ii) the intended use of the building, or
iii) the operation of building services.
(See Note A-5.5.1.2.(1).)
2) Coatings applied to gypsum board to provide required resistance to vapour diffusion shall conform to the
requirements of Sentence (1) when tested in accordance with CAN/CGSB-1.501-M, “Method for Permeance of
Coated Wallboard.”
3) Coatings applied to materials other than gypsum board to provide required resistance to vapour diffusion
shall conform to the requirements of Sentence(1) when tested in accordance with ASTM E 96/E 96M, “Water
Vapor Transmission of Materials,” by the desiccant method (dry cup).
Division B: Acceptable Solutions Part 5 – Environmental Separation
British Columbia Building Code 2018 Division B
Section 5.5. Vapour Diffusion
5.5.1. Vapour Barriers
5.5.1.1. Required Resistance to Vapour Diffusion
(See Note A-5.5.1.1.)
1) Where a building component or assembly is subjected to differentials in temperature and water vapour
pressure, the properties and position of the materials and components in those components or assemblies shall be
such that they control vapour diffusion or permit venting to the exterior so as to minimize the accumulation of
condensation in the building component or assembly.
2) Except as provided in Sentence (3), a vapour barrier shall be installed to provide the principal resistance to
water vapour diffusion.
3) A vapour barrier is not required where it can be shown that uncontrolled vapour diffusion will not adversely
affect any of
a) the health or safety of building users,
b) the intended use of the building, or
c) the operation of building services.
5.5.1.2. Vapour Barrier Properties and Installation
(See Note A-5.3.1.2.)
1) The vapour barrier shall have sufficiently low permeance and shall be positioned in the building component
or assembly so as to
a) minimize moisture transfer by diffusion, to surfaces within the assembly that would be cold enough to cause
condensation at the design temperature and humidity conditions, or
b) reduce moisture transfer by diffusion, to surfaces within the assembly that would be cold enough to cause
condensation at the design temperature and humidity conditions, to a rate that will not allow sufficient
accumulation of moisture to cause deterioration or otherwise adversely affect any of
i) the health or safety of building users,
ii) the intended use of the building, or
iii) the operation of building services.
(See Note A-5.5.1.2.(1).)
2) Coatings applied to gypsum board to provide required resistance to vapour diffusion shall conform to the
requirements of Sentence (1) when tested in accordance with CAN/CGSB-1.501-M, “Method for Permeance of
Coated Wallboard.”
3) Coatings applied to materials other than gypsum board to provide required resistance to vapour diffusion
shall conform to the requirements of Sentence(1) when tested in accordance with ASTM E 96/E 96M, “Water
Vapor Transmission of Materials,” by the desiccant method (dry cup).