I've had the pleasure of hearing Maria Spinu teach twice now and it has been a treat. She's a building scientist originally from Romania; and although her material is technical it is fascinating and oh so important to the composition of buildings.
The presentations I heard were particularly focused on the passage of water (liquid and gaseous) through walls, though both looked at different aspects of the problem. This was partly because one presentation took place in Colorado Springs and the other in Houston and as we know these are quite different sorts of climates—one dry and the other hot and humid.
By far the largest percentage of water has the potential to come through a wall as liquid rain. The solutions to this are not on a microscopic level and are generally understood in the building industry: keep water flowing down and away from the building. Tuck flashing in from underneath so that no liquid pools behind it, slope window sills down towards the outside of the wall, fill in gaps between building materials. Maria categorized liquid water infiltration as caused by gravity, capillary action, or rain driven.
The remaining water that will make it through a wall is vapor. And 98% of this vapor is driven in or out quickly by pressure differences. The remaining 2% happens more slowly due to the balancing of humidity differences and is called "Water Vapor Diffusion." Water driven by pressure through barriers is called "Air Transported Moisture."
Why is any of this a problem? In Colorado we worry less about mold and rot than many other parts of the United States do, but less carefully formulated walls can still cause water to accumulate in a dry climate. If the inside of a house has moister and warmer air than the outside, than the water vapor will be sucked through the walls outside to balance pressure and moisture differences. Short of making a vacuum-sealed chamber there is no getting around this: water vapor travels through many building materials. And as it is drawn from the warm inside toward the cold outside, the water vapor will cool and turn into liquid water. At this point it will either need to drain or evaporate again to escape. Usually this happens somewhere inside the wall.
In climates where the outside is always warmer than inside (air-conditioned buildings in South Florida) or vice versa (heated buildings in much of Canada) it is good to control where this water vapor changes to liquid by putting a layer in the wall that the vapor can't pass through. The liquid will collect on the side of this layer and of course drainage should be provided down through the wall.
But in "mixed" climates (most of the United States) where some of the year is hot and some cold putting in a layer like this will cause trouble. Either side could become a dewpoint (where water changes form) and providing adequate drainage on both sides is nearly impossible. Also, a vapor barrier will prevent liquid from re-evaporating and diffusing towards the drier side of the wall. Maria states that this that it is better to make sure that the entire wall is permeable so that this can happen. The wall can dry itself out. Something to watch out for: a number of building materials can act as unintentional vapor barriers and keep condensation in the wall.
Thanks Maria! Knowing how building science works helps us to build a better house.