> As someone who has built my fair share of houses and worked on many more over the last 25 years, I have to agree that our ability to seal air better than ever is causing problems, especially if you don't have forced air heating and cooling running all the time. People just completely ignore or haven't ever thought about room ventilation, positive or negative room pressures, and where and how to ventilate to the outside. In older houses there was always air leaks, small, but they let the house breath, especially if you had a central fireplace and your house was at negative pressure. I have seen many people purposefully over seal their house in ill conceived ways, to try and improve insulation, but end up causing ventilation problems with mold being especially common. Materials and vents that should let air pass through are commonly replaced with impermeable plastics and sealing tapes, many times in conjunction with a misunderstanding in expansion and contraction joints that have small gaps.

Sorry, but this is just wrong and is the type of "accepted wisdom" that holds back construction, particularly here in the UK.

Repeat: ventilation and permeability are different things.

Yes, permeability can achieve some ventilation but it comes at the cost of:

1) Not being reliable; it is dependent on pressure deltas around the dwelling.

2) Being potentially deleterious to the fabric of the building where escaping warm air cools and water condenses on surfaces in the fabric.

3) Being wasteful of energy.

The solution is to separate the two:

1) Seal the fabric totally, bearing in mind vapour permeability. The seal should be on the warm side of the insulation.

2) Install a ventilation system which provides reliable fresh air and, ideally, exchanges the heat leaving the building with the fresh air coming in.

We can no longer rely on folksy wisdom. Let's engineer our houses.