This detail is taken from the following Best Practice Guide:
Wood-Frame Envelopes in the Coastal Climate of British Columbia

Rim Joist — Exterior Air Barrier Approach — Detail 4 EABA

Frame Shrinkage

A 6 mm gap is recommended between sheathing panels at the level of the top plates. The size of this gap has been based on the assumption that the structure has been constructed with wood members with a moisture content of 19 per cent or less.

Placement of the cross-cavity flashing at the level of the rim joist effectively creates a control joint to accommodate the shrinkage of the rim joist and plates.

Venting

At this time there are two schools of thought as to whether the top of the cavity should be vented at the level of the cross-cavity flashings. Venting the cavity at the cross-cavity flashing will increase the volume of air that passes through the cavity and hence will increase the potential to dry moisture from the cavity. However, venting at the top of the cavity may, under some circumstances, result in a negative pressure in the cavity which may draw water into the cavity, or alternately wind-driven rain may enter under its own momentum.

If the top of the cavity is to be vented, the blocking used to support the flashing should be discontinuous and the flashing should project 12 mm from the face of the cladding.

Cross-Cavity Flashing

The joints in the cross-cavity flashing should be designed and constructed to minimize the potential for water penetration. Joints at inside and outside corners should be standing seam. Joints in the straight run sections should be S-lock. Alternatively, as shown in this detail, a self-adhered membrane flashing may be used below the metal flashing and simpler lap joints may be used. Utilizing a 2:1 slope on the cross-cavity flashing reduces the potential for water to leak through at the joints.

Alternative Detail

The alternative detail illustrates a second method of achieving airtightness at the rim joist. Implementation of this detail alleviates the problem of the header wrap being damaged during construction. This detail could be utilized if the header wrap is not installed before the upper level wall framing is constructed.

No vapour barrier is required when spray-in-place urethane foam insulation is used.

Rim Joist — Exterior Air Barrier Approach — Detail 4 EABA

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