Vapor Barrier?

The issue of air and vapor barriers surfaced yesterday at one of my residential projects currently underway here in Massachusetts. There was concern expressed about the lack of a vapor barrier shown in the drawings, specifically polyethylene sheeting, on the inside face of the exterior walls. The concerned party went on to explain that every project he had ever worked on had a polyethylene vapor barrier installed on the interior face of the exterior walls. That last pronouncement just about ruined my day because I didn't realize there still existed still such confusion in the US about exterior wall systems. Well, let me say this about that.

Conventional exterior wall framing.

Mass masonry wall systems, which are exterior walls made entirely of solid masonry* with no cavity or air space, are purposely left out of this discussion and will be addressed in another post at a later date. Most residential construction uses a cavity wall system; this is the type of wall in which one sees vertical wood or metal studs spaced at regular intervals before their exterior side is covered with a rigid sheeting material like plywood and subsequent to that, some kind of finish system. One of the most popular finish systems in the US today is spun-bonded polyethylene, sometimes called house wrap, vinyl siding and some kind of trim. Insulation is usually installed between the vertical members, or studs, and then a vapor barrier and finished wall board of some type is installed on the inside face of the exterior walls. Thus we have, from outside to inside: siding, polyethylene, rigid sheathing, studs and insulation, a vapor barrier and finished wall board--6 categorical layers.

This type of system is ubiquitous in the US and is usually acceptable to most building departments. It is also the cheapest to build due to its popularity; contractors can find cheap labor who know how to put it together, and the material demand allows for an economy of scale in unit pricing. As an efficient building envelope, if it is carefully built and all the layers are installed to exact tolerances, it is mediocre at best. Usually it provides a poor building envelope, and this is due to what's commonly called crack loss. 

* I include adobe, rammed earth, precast and any other type of non-cavity wall system here.