Here is a new detail I have created and am including in all of my plans.
Dew point can be seen in this chart
Example 1 (Winter):
If 20ºC interior air at a relative humidity (RH) of 40% gets into the wall through an unsealed gap around a window, it works it's way towards the exterior and it cools down. Once it drops to about 9 ºC, it has reached 100% RH, otherwise know as rain.
Rain inside a wall is generally considered "bad". This situation will lead to mould and rotting framing. And a drop of only 8ºC can do this - not an extreme situation!
Example 2 (Summer):
Hot, humid outside summer air at 30ºC and an RH of 80% gets into a floor assembly through an unsealed rim joist. As it works it's way towards the nice 22ºC interior, it only has to cool about 2 or 3ºC before it to becomes rain inside your floor framing assembly. Again causing mould, and rotting the mud sills and rim joists, causing the floor to sag, along with the door above it, that now starts to bind and close improperly. etc. etc. etc.
Air Sealing is critical to a good quality building that will last while being mould free.