As temperature increases, the molecules become excited and the clumps break up until individual molecules break away and become air-borne as vapor. Because the inside of a building is warmer than the outside, and warm air has a higher vapor pressure than cold air, vapor will be driven through the interior wall surface and into the wall assembly. As the difference in temperature between the interior and exterior increases so does the pressure differential, and hence the quantity of vapor which will enter the wall. Houses heated in winter or cooled in summer are the most susceptible to problems cause by vapor migration. Vapor is not a threat to wood or straw unless it condenses into liquid.
Warm air is capable of holding more water vapor than cold air.
The temperature at which condensation can occur, called the dew point, varies with the relative humidity of the air. This point is not always inside the wall, but in very hot or cold climates the dew point, and the potential for rain inside the walls, usually falls inside of a wall. Any vapor entering the wall will condense at the on the warm side of the next downstream surface from the dewpoint. This liquid may accumulate and create the conditions for dry rot to occur. Care should be taken in design construction, to minimize these pathways for moisture transport into and within the building envelope and to provide for the removal of moisture from within the wall.