BOOST air to the rooms you need it BLOCK air in unoccupied rooms SAVE MONEY- heat and cool only the rooms you use NO AIR GAPS- The only cover that conforms to the vent damper creating a COMPLETE SEAL WARNING Not all metal vents are magnetic. Not for aluminum or high fin vents. See listing pic 7 for details PLEASE NOTE- This item is made for STEEL Vents only. It will not work on aluminum, plastic or other non-ferrous materials. If in doubt, use a small magnet to test your vent before ordering.