Houses built between certain time periods were built with a cavity between the inner and outer brick. This gap was supposed to act as a “thermos” style barrier with the cold. New technologies have now been developed to fill that cavity with insulation that would outperform the thermos style barrier.
There are various styles of cavity and various levels of insulation installed on build. Ranging from no insulation at all to polyboard or partial fill cavities. Each of these can be improved on and further reduce the amount of heat lost through the walls of your home.