I have studied and dealt with the movement and control of water vapor for many years in both industrial and residential settings. I have discussed this subject with engineers, professors, contractors, carpenters, home owners, and friends and family. The one conclusion I have arrived at is that no two people agree.
I have read many publications that discussed the topic and no one explains it accurately. Let me try.
First we must have a short lesson on the physics of water vapor. The air around us contains moisture. We hear how much every time the weather man says what the relative humidity is. He may say "35%" is the relative humidity. That means that at that temperature, the air is holding 35% of what it can hold of water. As the temperature of the air increases its ability to hold water also increases. The dew point is another term the weather man uses. This is closely related to relative humidity. If the air in the above example is cooled, it's relative humidity increases until it reaches 100%. The temperature at which this happens is the dew point temperature. If the air temperature falls any lower, some of the moisture that is in the air changes from vapor to either liquid or solid. This could be rain, snow, dew, frost, or even ice.
Before we go further, we need to talk a little about thermodynamics. That is the movement of heat energy. It is a law of nature that heat energy travels toward a colder place. Also we need to know about vapor pressure. High temperature, high humidity air wants to go to low temperature, low humidity air. The greater this difference, the greater the vapor pressure. This vapor pressure is towards the low temperature or lower energy area.
We all know what insulation is, Right? It is the resistance to heat flow. It does not stop it. It only slows it down.
Lets discuss two more items; house wrap and vapor barrier. House wrap slows air infiltration into the house. It is not a vapor barrier. It typically is placed on the outside of the house just under the siding. A vapor barrier is just what it says, a barrier to vapor transmission. Usually a 4 or 6 mil poly is used as a vapor barrier. This barrier needs to be placed in the warm side of the insulation. That is usually just under the sheetrock on the inside of the outside walls and ceiling.
O.K. With these items in mind, lets look at a home in the winter time. The inside conditions are 70°F and 45°F dew point. The outside conditions are 0°F and -10°F dew point. Because the inside air has more energy it wants to get to outside where the energy level is lower. The insulation tries to slow this movement. Keep in mind this movement is very slow. As the air moves, it carries with it the water that is in vapor form. As this air moves through the insulation its temperature drops. It continues to drop until it reaches the outside of the wall. At which time it is the same temperature as the outside air (0°F).
If the insulation is not protected by a vapor barrier, on the warm side, here is what happens. As the air moves through the insulation it cools. When the air temperature inside the insulation reaches 45°F or its dew point, the water begins to condense out and the insulation gets wet.. As the temperature continues to fall below freezing, the condensing moisture is now frost. When the insulation gets wet or frosted, its insulation value drops. Then the liquid water in the insulation turns to ice. The insulation value continues to drop and the cycle continues. This water and ice can cause lots of problems.
If the above insulation was protected by a vapor barrier, the water vapor and the air could not penetrate into the insulation. Heat would still transfer to the outside, but the moisture would remain inside. Therefore the insulation would remain dry and its value would remain the same.
Two questions!! 1. Why are attics ventilated? 2. What happens to the moisture in the house?1) Many people believe attics need to be ventilated to remove the moisture coming up through the ceiling. This is false. The reason is to keep the roof as cool as possible. The ventilation is to remove the heat energy that is coming up through the ceiling, not the moisture. If this heat energy is not removed, it will heat the roof and melt the snow that is sitting there. This melting snow runs down the roof slope until it freezes along the eve and forms an ice dam. This, as we all know, is an undesirable condition. It is very important that the attic space is ventilated properly.
2) The moisture in the interior air must be controlled. If it is not,
the relative humidity will increase until the cooler surfaces like windows,
water pipes and toilet tanks will begin to "sweat". This can lead to many
unwanted problems.
The moist air must be removed and replaced with dry air. An effective
way to do this is to install a heat exchanger. This device exhausts the
moist air and replaces it with outside drier air. In the process of doing
that it removes much of the heat in the warm moist air and puts it into
the incoming cold dry air. This gets rid of the moisture and saves energy
that would be needed to heat the incoming outside air. This system is usually
controlled by time and relative humidity inside the home.
To summarize;
We must control the moisture in the inside of our homes by using vapor
barriers to prevent insulation and other home damage. We must control the
relative humidity of the inside air by properly exhausting the moist air
and replacing it with dry outside air.
We must ventilate our attic spaces not to control moisture but to control
roof temperature.
Thank you for your time.
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