Prevents Blockage of Air Flow GreenGuard Universal Attic Rafter Vents are designed to fit both 24in. center and 16 in. center rafter spacings. A convenient perforation in the center of the vent permits it to easily be snapped into two smaller vents for use in 16″ spacings. When properly installed between rafters, GreenGuard Universal Attic Rafter Vents prevent clogging of the path from soffits to the upper attic when insulation is blown in.