hello, i am blocking up the opening in my chimney breast, i have been told to put an air brick in it so the chimney doesn't sweat. Seal the opening with a piece of scrap metal. Partially blocked flue 3. Collapsed mid-feathers (or withes) 9. When you have such a large opening around the liner, a large amount of the hot air generated from the wood … In some cases, the build-up of heat inside the chimney and fireplace will be excessive. Allow the bricks to set. Apply a skim coat of concrete over the bricks, leaving the air bricks open. Measure your chimney opening and determine how much brick to buy.