A party wall is what separates your house from the neighbours house. You may have one or two party walls. When we think of heat loss, we look at our external walls. These are filled with cavity wall insulation or internal / external insulation.
Like our lofts walls lose heat. The same goes for party walls. Heat from inside our home is absorbed into the party wall, this is happening from your neighbours home too. The party wall then acts like a chimney and carries the heat up trough the roof.
On a cold winters day, when the roof is frosty, you will see the space over the party wall has no frost or snow. This is due to the party wall taking the heat from your home up and out the roof.
There are different types of party walls, some are solid brick, some are of cavity construction. There are timber party walls and solid concrete if you house is timber frame or system built.
All of these party walls can be insulated. If it is of cavity construction, the cavity can be filled however you will need to get a Party Wall Act document agreed and signed by your neighbour.
Party Wall etc. Act 1996
1996 CHAPTER 40
An Act to make provision in respect of party walls, and excavation and construction in proximity to certain buildings or structures; and for connected purposes. You can find out more about this https://www.legislation.gov.uk/ukpga/1996/40
If the Party Wall is solid (wood, concrete, brick) You can internally insulate with insulation boards such as King Span or Celotex.
As we look to make our homes more thermally fit. You could be losing 30% of the heat from you home through your party wall. This is more than heat lost through an uninstalled roof which is 25%. So a substantial amount of heat is lost through the party wall.
If you are unsure have an insulation firm come out and give you some ideas and cost. Along with keeping the heat in, it will assist in keeping noise out!