If you’re building a house these days, you can be pretty sure it’s going to be well insulated and pretty airtight.
What this means in practice is that you generally need some way of getting stale air out and fresh air in. One way is using a combination of trickle vents above windows and extractor fans in bathrooms, but a newer way is adding an MVHR (mechanical ventilation and heat recovery) system.
This system has a combination of input and output points around the house, connected by ducting to a central unit. Input points extract air out of rooms such as kitchens and bathrooms, while output points force fresh new air into rooms such as bedrooms and other living areas.
The heat recovery part retains the heat inside the house by extracting it from the stale air and using it to heat the fresh air as it comes in. This part can be bypassed in summer when the fresh air is already warm.
We’re now in the process of having the ducting fitted, which is the first service generally to be fitting inside the ceiling, followed by water pipes and electric cables. The ducting is a new, flat style which leaves more room for other services and insulation, and each input/output point has a short, flexible foil section eventually leading to a discreet vent in the finished ceiling.