Does the building code require a vapor barrier when I use insulating sheathing?
Yes, in certain cases. The addition of insulation boards on the exterior of the assembly helps reduce the potential for condensation occurring in the wall assembly. If enough insulation is added to the outside, then a vapor retarder on the inside is not necessary. Also, it is good practice to allow a wall assembly to be able to dry to at least one side, and since many insulation boards can be classified as vapor retarders, removing the vapor retarder from the inside allows increased drying of the assembly to the inside and improves the performance of the wall. The code recognizes this and addresses it in an exception which allows for the vapor retarder to be removed where it can be demonstrated to the satisfaction of the building official that the absence of a vapor retarder will not damage the materials or adversely affect the performance of the insulation due to moisture or it freezing. However this still requires some calculations to demonstrate that the potential for condensation i
