The “vented airspace” refers to spaces which have the ability to move air from the space that will be vented to the outside. This is a common practice in attics where the building codes require that the space be “vented” and they specify the amount of air movement by a percentage of the ceiling area (1/150 or 1/300 depending on the vapor barrier).

For more information, see WCD #6 – Design of Wood Frame Structures for Permanence. Download free here.