A lot of heat is lost from buildings in winter through simple bathroom mechanical extract fans. Furthermore, people all over the UK suffer from cold drafts and heat loss caused by primitive ventilation grilles in the outside walls of their house. This thermal image of a house next to our Passfield Drive retrofit, illustrates the terrible heat loss that occurs in such houses before renovation, even though it seems that everyone in the street has tried to board and tape over the ventilation grilles! (see picture below).
(Our retrofit next door at 3 Passfield Drive does not suffer from these problems because it has passivhaus-quality heat recovery ventilation - see attachment)