When impermeable insulation is used, the ratio of insulation outboard of the sheathing to insulation in the stud cavity should be carefully considered so as to maintain the temperature of the sheathing at relatively safe levels (above dew point) to avoid condensation.

Additionally, while not explicitly required by the codes, a relatively more permeable interior vapour barrier such as a smart vapour retarder or vapour retarder paint could be used to permit some amount of inward drying. In general, a vapour permeable exterior insulation in combination with an interior vapour barrier typically provides a lower risk wall assembly than does an assembly using impermeable exterior insulation.

See BCBC/NBC 9.25.5 for code requirements.