Описание, для справки:

"Short circuiting of ventilation air occurs when ventilation air enters and leaves a space or duct before it has a chance to mix well enough with room air to do the job it was intended to do—that is, to adequately dilute pollutants."
http://www.buildingscience.com/documents/insights/bsi-016-ventilation-top-ten-list

Short circuits occur when air is caused to go around and away from its original and intended path, resulting in areas of the attic being bypassed. Air flows to the path of least resistance. In a properly designed and installed attic ventilation system, the air will flow from intake vents to exhaust vents, flushing out the warm, moist air along the entire underside of the roof.
The most common example of short circuiting ventilation is when more than one type of exhaust vent system is installed. If both ridge vents and roof louvers are installed, for example, one of these two exhaust-type vents would end up acting as an intake vent, leaving large areas of the attic unventilated and increasing the potential for weather infiltration problems.
http://www.roofingcontractor.com/articles/90692-ventilation-short-circuiting-how-short-circuiting-happens-and-how-to-correct-it