UPDATED on September 18, 2013
By now, almost all builders know the importance of installing a water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. (WRB) behind siding. Most types of siding leak, so it’s a good idea (and a code requirement) to install a WRB to protect your wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen.
from any wind-driven rain that gets past the siding.
A WRB can be vapor-permeable, like Tyvek, or vapor-impermeable, like foil-faced polyisocyanurate. As long as the wall assembly is designed to dry out when it gets wet, either vapor-permeable or vapor-impermeable WRBs work well.