There's a lot of confusion about whether to use a vapor barrier and where to put it. In this article, building science blogger Allison Bailes helps sort things out by explaining the physics of what happens in the wall depending on climate and where the vapor barrier is located.
The role of a vapor barrier is to prevent water vapor from encountering a cold surface inside an exterior wall. If the vapor barrier is installed on the wrong side, or if it has holes in it, the resulting condensation can cause the wood to decay and supper mold growth.
Understanding how moisture behaves is important to remodelers who want to prevent condensation and enhance the ability of walls to dry out should they get wet.