Air moves from areas of higher pressure to areas of lower pressure. A common example of negative pressure is a process area with several exhaust fans running to remove smoke or fumes with no or lack of replacement air (Make-up Air –typically heated) creating a vacuum in that space. Air follows the path of least resistance so it usually comes in an open doorway creating a huge draft even when it's not windy outside. Although an Air Barrier will offer some resistance to the incoming air (reducing the Barrier's efficiency depending on strength of negative), the best means to offset negative air pressure is by adding sufficient make-up air.
Other building conditions such as large elevator shafts can create a stack effect which is similar to negative pressure.
* Enershield efficiencies are based on a balanced air system and Enershield will not be held responsible for diminished efficiencies due to a facilities negative air issues.