Another way of looking at humidity is in the pressure of the water vapour suspended in the air. In a wall, there is a temperature gradient between the inside temperature and the outside temperature. In the steady state this is going to be a straight line. If the insulation is glass fibre, air and moisture can pass with some freedom. If hot air from inside is passing all the way outside, as the temperature drops the humidity will rise so at some point the humidity will likely reach 100% and you will get condensation. To stop this, you need vapour barriers that will stop the air inside the house flowing through the wall structure. Then, the air will stop flowing, and in theory at least, you'll get constant humidity throughout the wall, even though the temperature is dropping and there is much less absolute moisture content in the air close to the outside than there is close to the inside.
In the summer, the temperature outside is higher, so you have the opposite situation and the danger of water condensing on the way in, as the temperature drops. The humidity inside the house is higher, so the walls may get too humid at some point.
Wufi software simulates the performance of a wall structure over time, and you can see an example below, although this is not for our house!
Just like insulation slowing down the heat rather than stopping it from escaping, vapour barriers and other kinds of waterproofing do not stop water, they just slow it down. Anyone who has stayed in the rain for long enough in waterproof clothes knows this. It's fine for a while, but eventually the rain will get through. I remember my Dad discovering how waterproof his boots were after a very rainy walk around Haweswater in the English Lake District--water had got into them but it took for ever to get out again. Cheap waterproofs can also be sweat proof, so sooner or later you're going to be wet inside anyway.
Essentially our house has a couple of rain coats on. On the outside is Tyvek sheet is made by Dupont, who also make Goretex rainwear, which is designed to stop precipitation from getting in while allowing perspiration to get out. We need the same thing in a house, so that moisture does not build up within the wall structure, leading to rot. Tyvek stops drops of water from getting through, but will let water vapour pass, so the walls can stay dry.
We used Intello inside the wall structure, which allows very little moisture through in the winter, preventing condensation and rotting walls. In the summer, it opens up and lets moisture through, which allows the walls to dry out.
You can see an interesting effect below of humidity going up with temperature, instead of going down as it usually does. The first chart is the normal situation, where the relative humidity gets lower as the temperature goes up, since the absolute humidity is the same, but the air's capacity for water vapour increases. The lines at the top are humidity, the higher one outside the house, and the lower inside the house. The temperature lines are below.
The next chart shows the humidity and temperature within the wall, when the sun is beating down outside, making the temperature just inside the Tyvek high. This makes the humidity low, and as a result, humidity starts flowing outwards through the wall. We're measuring temperature and humidity in the middle of the wall, so at some point it should be possible to test how the wall performs in real conditions. After almost two years, the humidity seems to be staying in a safe range.
