People don't wear raincoats in Hong Kong because it's wetter on the inside than on the outside. Instead, they use umbrellas, which give them cover from the rain but allow their bodies to ventilate. For the same reason, some hats have holes in them. It has always struck me as amazing how similar the human body is to a house. Our skin is like the carefully planned shell of a house -- it won't let water in but it will let moisture (sweat) out. If we don't allow air to get to the skin to dry out that moisture, we get hot, wet and smelly. (Ever wonder how a house feels with mould or dry rot?) In addition, if we don't allow enough fresh air ventilation into our lungs -- well it's all over. Why most books leave ventilation to the last chapter I'll never understand. If we can't breath, there's no sense talking about anything else.
We hear a lot of talk about "natural ventilation" but that's really just a romantic term for a bunch of cold drafts and hot air leaks. If we want houses that are comfortable and healthy for us and free of moisture problems for the house we will have to leave "natural ventilation" to the thatch huts in the tropics and concentrate on carefully controlled ventilation here in our cold northern climate.
Here are the answers I've found to the questions people have asked me about controlled ventilation. There is tremendous confusion in this field because to some extent every house is different and breakthrough research in domestic ventilation is as recent as the 80's and is continuing.