CAJ

Hello, There are a few ways to do this - Block the chimney with wool insulation. If the chimney flue isn't too wide, you should be able to seal it by simply stuffing some wool-type insulation into the opening. This will allow some air to pass through (good ventilation) while preventing uncontrolled loss of warm air. Fit a "chimney balloon". As the name suggests, these are inflatable bags which can be fitted up the chimney before being pumped full of air. If positioned correctly they will seal the chimney flue. Bear in mind that they may need topping-up with air occasionally. If you have access to the roof, the chimney can be capped to create a complete and permanent seal. Further advice on draughtproofing can be found here: https://www.insulationadvice.co.uk/ia-draughtproofing