This product stops a vast majority of back drafting and loss of room air up the chimney, but it is not gasketed. As such, there is not a perfect air seal. To achieve a perfect seal, a rope gasket would need to be added to the perimeter of the cover. Another option is to install a top mounted fireplace damper, such as the
Lock-Top 8 in. x 13 in. Energy-Saving Chimney Damper. Top mount dampers are by far the most effective energy savers, as they seal against a level silicone gasket that is part of the damper assembly, as opposed to the uneven surface of a cast iron damper or the veneer surface of the fireplace facing.