I have found the best way to seal an unused fireplace flue is to install a piece of slate or thin stone on top of the unused flue tile using masonry caulk to adhere the stone.
How to seal an unused chimney top.
Installing the chimney cap.
Does an unused chimney need ventilation.
How to permanently block a chimney.
If your damper is near the firebox you can use insulation to seal the chimney.
But for a masonry chimney that has a clean out in the basement tape that clean out air tight as well because it can be a source of warm moist air getting inside the closed chimney.
Capping the flue is one of the best ways to close up the top of the chimney as it will still allow air to get in and out preventing condensation from building up.
Closing down an unused chimney.
Plug and seal air tight the horizontal line at the level of the vapour barrier in the house and cap the top.
Sealing and insulating the clay flue at the top and bottom of the stack will ensure the job will yield these desired results.
The long term savings associated with sealing the chimney has already been mentioned.
Even if you are not comfortable with climbing up to the roof and doing it.
How to temporarily block a chimney.
This way if the fireplace is ever to be used in the future the piece of stone can be removed easily without causing any damage to the flue.
Wait for at least 10 to 15 minutes before applying the second coat and once again begin from the bottom.
Gradually move to the top of the chimney.
When retiring a chimney many chimney experts recommend using roxul a hydrophobic mineral wool insulation which means that it will dry out if it ever gets wet.
The top 5 chimney killers.
Sealing your older unused chimney should not be a complicated task as long as attention is paid to prevention of air intrusion as well as environmental and pest control.
If you want an easy way to temporarily block and unused fireplace flue to stop a draft you should consider installing a top sealing damper.