Vapour Diffusion

Section 5.5 Vapour Diffusion

5.5.1 Vapour Barriers

5.5.1.1. Required Vapour Barrier

1)  Except as provided in Sentence (2), where a building component or assembly will be subjected to a temperature differential and a differential in water vapour pressure, the component or assembly shall include a vapour barrier.

2)  A vapour barrier is not required where it can be shown that controlled vapour diffusuion will not adversely affect any of

a)  the health or safety of the 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 A-5.3.1.2 in Appendix A)

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 would not allow siffucient accumulation of moisture to cause deterioration or otherwise adversely affect any of

i) the health or safety of the building users
ii) the intended use of the building, or
iii) the operation of building services.
(See Appendix A.)

9.25.4 Vapour Barriers

9.25.4.1. Required Barrier to Vapour Diffusion

1) Thermally insulated wall, ceiling and floor assemblies shall be constructed with a vapour barrier so as to provide a barrier to diffusion of water vapour from the interior into wall spaces, floor spaces or attic or roof spaces.

9.25.4.2. Vapour Barrier Materials

1)  Except as required in Sentence (2), vapour barriers shall have an initial permeance not greater than 45 ng/(Pa-s-m 2 ).

2)  When used where a high resistance to vapour movement is required, such as in wall constructions that incorporate exterior cladding or sheathing having a low water vapour permeance, vapour barriers shall have a permeance not greater than 15 ng/(Pa-s-m 2 ).  (See Appendix A.)

9.25.4.3. Installation of Vapour Barriers

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. (See Apendix A.)