Nowadays, with lots of places being converted to be more "energy efficient", the interventions are typically: 1) huge thickness thermal insulation on the exter

Nowadays, with lots of places being converted to be more "energy efficient", the interventions are typically: 1) huge thickness thermal insulation on the exterior 2) double/triple gaze (airtight minus the small built-in ventilations) windows 3) NO added mechanical ventilation The bets are on on how long it will take for people to realize that if they do not vent manually the house humidity/mould will form. This has already happened for houses built some 10-15 years ago (the first houses built as Class A or Class B) that - even if newly built - did not usually provide mechanical ventilation. There are on the market since a few years small "in-pipe" mechanical ventilators, including a heat exchanger, basically you make a 100 or 120 mm hole in the wall and put in it a pipe that contains the vent and heat exchanger, they are very suitable to install "as an afterthought". Trouble is that they work on "alternate cycle", i.e. they suck air from the inside for - say - 30 seconds (warming the heat exchanger), then they push air from the outside for another 30 seconds, and they may be (or become) noisy, new models have built-on timers to be able (as an example) to switch off at night if in bedrooms or switch off in the evening in the living room, etc.