A corrugated board is secured to the inner wall of a cavity wall construction to establish a defined spacing between the inner and outer walls and prevent excess mortar from bridging to the inner wall. The corrugated board has a series of spaced channels, furrows or grooves into which an anchor projects into the studs of the inner wall to secure the outer wall. The corrugated board is installed prior to the construction of the outer wall and establishes a minimum spacing or gap between the walls based upon the thickness of the board. The outer wall is constructed immediately adjacent to the outer face of the corrugated board. The grooves or channels of the corrugated board create a chamber for vapor and air circulation. The board eliminates bridging by the mortar and provides an air conduit to exhaust even the minimal amounts of vapor that will occur in the cavity.