Fireplace - how to cover control air flow up the chimney

[Kirk R.]

I have a flu that does not seal well on a fireplace.  I have a gas log that I am pretty sure does not need a vent.

I can feel a draft up the chimney.  What would be an inexpensive way to seal it?  I still would like to be able to light the gas fire every once in a while.

[Roy N.]

@Kirk R. 

If you are burning gas/wood/coal, etc, then you need both a vent (traditionally air from the room) and an exhaust (the chimney) - this is the draught you feel.   Modern gas stoves and fireplace inserts are sealed (airtight) with respect to the living space (room) and vent their combustion air from outside - typically a separate pipe through the side of the building or a centre pipe within the exhaust flue (either through the side of the building or as a sleeve in an existing chimney.

The cheapest way to stop air and heat from being drawn up your chimney is to decommission your fireplace; stuff insulation in the top of the firebox against the closed damper; then tape a sheet of 6mil poly (vapour barrier) under the insulation being certain to create a seal with the sides of the firebox all the way around the perimeter of the barrier.

Since you still want to use your fireplace periodically, but do not wish to incur the expense of installing an airtight insert, you could still attempt to plug the top of the firebox when you are not using the fireplace.   To do this you could cut a piece of rock wool insulation (ether batt or board) to tightly fit into the top of your firebox - I would leave a piece of {bright coloured} string or chain hanging down to remind you the flue is blocked.  When you want to use the fireplace, remove the insulation.  When are have finished and the firebox has cooled, close the damper and replace the insulation.   It will not be as airtight as above, but better than your current situation.