Having an open chimney could prove costly, both in terms of health and energy bills.
Air quality adviser Breathe London says fully insulating buildings, including chimneys, not only prevents heat escaping but also saves money and reduces the amount of air pollution able to enter a house from the outside.
The organisation likens having an open chimney to leaving a window open all the time and says up to 80 cubic metres of air can flow through a chimney per hour, as much as two full rooms of air.
One such example is the Chimney Sheep, which reduces the speed of air exchanging between the inside and the outside of a property.
Made up of a thick wad of felted wool on a handle, it blocks the passage and has been found to block 94% of airflow up the chimney.
It also has the benefit of shutting out wildlife and muffling sounds from outside.