Open fireplaces will be losing you heat; they help to cool your home.
In the past when chimneys were used with fires, the chimney stack would create a draw which naturally caused warm flue gases to flow up the chimney, out through the pots and away. Now the same thing still happens as the whole height of the flue is heated from the ambient heat of the house, warm air travels up in a column and out of the chimney pots driven by what is known as “the stack effect”. If you ever get the chance on a cold, still day, stand by a flue pot and you can easily feel the warm air rising from it – it’s quite considerable. If you cover a fireplace with a thin piece of cardboard or paper, you will hear and see it flex as warm air is drawn up the chimney clearly showing how easily air moves up it by this method, on windy days it can blow both up and down the flue.
Put a ventilated rain cowl/cap on the chimney pots. Now plug the bottom of the flue with a chimney balloon or similar. DraughtBusters recommend using an old pillow in a couple of black sacks, and removing it again in the summer. Now ventilate the top part of the chimney by removing a brick or two from the chimney in the loft and replace these with an air brick or fit a plastic or aluminium louvre. This will prevent ¾ of heat loss through the flue. Take loft insulation 600mm up the chimney breast stopping it just below the air brick, and bring insulation inside the flue up to this same level. Note that the air brick/ventilator in the rooms are not now needed and can be closed off.
Most of the remaining heat loss can be reduced by pouring insulation, e.g. Polystyrene beads, vermiculite, LECA, perlite or similar in through the air brick in the attic. This helps reduce thermal thermal losses up the chimney and is easy to remove (note if an adjacent flue is still in use do not use polystyrene beads, use LECA or vermiculite as is more resistant to higher temperatures.
If the fire opening is closed off the insulation will sit on the slab. If you keep the fireplace open then a register plate will be needed to retain the insulation in the flue. All the above advice is for redundant and disused chimneys.
A note on sealing off the chimney without ventilation
If you just cap off the top and insulate the bottom, “pumping” will happen. This is where in an closed off chimney the top gets very cold, condensation occurs and the cold air falls down the chimney, warm moist air rises, this tends to dry the chimney breasts and indeed the house but dumps water in the form of condensation in the top of the chimney, saturating it until it can take no more at which point it starts dripping or running down the flue. This is akin to tropical rain forest except colder and not good. Adding loose fill insulation will not reduce this pumping action. Chimneys can act like dehumidifiers with or without air movement condensation is drawn to the coldest points, the pot and the flaunching and can literally rain or run down the flue. An air vent does not stop this but it stops it being quite so bad and then once the sun comes out and warms the loft (most days) the whole thing dries out with warmed dry air flowing up the flue. This isn’t covered in literature, but in reality happens a lot if chimneys so please keep yours well ventilated.
Our advice for these is to seal them up internally, seal at first floor ceiling level and ventilate from there up, insulating them is an option but not necessary. This applies to all chimneys on internal walls, i.e. fully within the building, we do not believe that an air brick is any longer required at the bottom, not having one will both reduce draughts and save energy, as explained you must ventilate the top part of the chimney above your first floor ceiling.