Building owners are charged with the constant battle of keeping water out of the building envelope, but sometimes the water that causes stains, mold and decay comes from within the building envelope. Consideration to design, materials and use must be given during initial construction and reconstruction projects.
Water infiltration into and within the building envelope is a prevalent condition throughout all parts of the world—so much so that the study of water infiltration is a science. Moisture can infiltrate (leak) in liquid form from the exterior into the building envelope or it can be introduced into the building envelope in its invisible vapor form and under the right conditions, condense into water molecules and cause similar havoc to that of a building leak. So what is condensation, why does it occur, and how does one control it?
Condensation is the process of water vapor in the air turning to liquid water. Condensation of water vapor occurs when the temperature of the air is lowered to the point that the air cannot hold any more moisture (the dew point). At this critical temperature, the water vapor will turn to liquid water, until equilibrium between the air temperature and the amount of water vapor that that specific temperature can hold is met.
The task of keeping the habitable space on the interior of a building dry sounds like it should be a relative easy one. Despite this seemingly easy task, there are literally hundreds of books on the subject, the building codes address it, there are lectures on it and there are engineers and contractors that specialize in it. The formula for keeping condensation from forming within a building appears relatively simple at first glance (for an assumed constant pressure): Keep the humidity inside the building envelope at a level below a saturation point for the minimum anticipated temperature in the building. So assuming that the interior environment of the building is typically controlled to 70 degrees Fahrenheit, the interior humidity must be kept below 50 percent.
This seemingly easy task is difficult because there are a number of variables that can affect the amount of water vapor in the air (humidity), as well as the temperature—they are as follows: