Not clear on which aspect of a fireplace you want to be efficient. As for allowing cold air to penetrate the room when the fireplace is not being used, no fireplace is efficient in this aspect. Masonary and metal allow the coldness to emigrate into the room. The flue allows a good amount of air to seep past.Glass doors are a must, but they will not stop the air totally. No matter which you choose, be prepared to block the whole opening even further with a large piece of clear plexiglass. If you cut to fit properly, it will not be noticeable. As for heating, with at least two fans in operation it will heat a room easily within an hour. The fans are placed inside the bottom grates and blows the air up and around the fireplace. For this reason the bed of the fireplace needs to have hot embers on the bottom since this is where the heat comes from. Turning the fans on as soon as you light a fire does no good. I have heard of Vermont Castings but not the other one. I would be leary of anything called Kozy. I cannot tell you good or bad about either.