(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.

Article Ontario Regulation 332/12 Building Code,   Information published by oncodes.ca for educational purposes only.

A vapour Barrier is required for insulation installed between the ends of floor joists in the rim joist space but does not need to be carefully sealed to the joists and floor sheathing unless it also forms part of the air barrier. It is more difficult to achieve an effective air barrier at this location because the materials must be cut and fitted between the joists (Figure 8).

Sprayed closed-cell foam is commonly used in this location because it fills the space tightly and acts as both insulation and air barrier. When also serving as an air barrier, the edges of the polyethylene covering the fitted insulation should be caulked to the framing, in order to seal the joints and prevent exfiltration. Rigid insulation is suitable in this location, and some semi-rigid or soft insulation materials that come with a reinforced aluminum backing can also be used effectively.

In the winter, the air inside the house may contain more water vapour than the outside air. This causes a difference in vapour pressure and creates a driving force that can lead to water vapour diffusing through materials into the building envelope. Many building materials are permeable to the passage of water vapour, but those classified as vapour barriers, such as polyethylene sheet, have very low permeability and are very resistant to diffusion.

Figure 8

Placement of air barrier over joist headers

Source : Canada Mortgage and Housing Corporation (CMHC)