Vapor Drive
June 7, 2004
by David B. South
Water vapor molecules (or water in its gas form) try to evenly spread themselves out. If one side of a room is full of water vapor molecules, the molecules will move to the other side until the room is evenly populated. In a room this phenomenon is easily understood. It's a little more complicated in the real world.
Consider the separation of the air outside of a building from the air inside. If the air on the outside is fully populated with molecules of water vapor and the air on the inside is sparsely populated, it is easy to understand the water vapor wants to even itself out between the two air volumes. How intensely the water vapor molecules try to move through the wall is called vapor drive.
Vapor drive is greatly influenced by temperatures. Hot water vapor is more active and will tend to drive across to cooler air vigorously. For example, consider a building on a bright summer day in 100 degree heat with 80 to 90 percent humidity. Inside the building it is 70 degrees with 50 percent relative humidity. The water vapor molecules on the outside are going to try their hardest to get to the inside and even things up. If the temperature differences are more drastic, the amount of pressure or drive is increased. If the interior temperature is minus twenty as might be for an ice cream freezer, the vapor drive will be enormous.
In order to keep water vapor molecules from penetrating, we need to put a barrier between them. If the barrier is made out of a single piece of glass or a single piece of metal, the division will be nearly absolute. It is amazing how fast water vapor can get through the wood -- especially if the wood has cracks in it. This is also true of many other commonly used building materials.
If water vapor gets through the building envelope into a freezer it will
condense and freeze on the inside. As it freezes, it will build layer
upon layer until it can literally destroy the freezer. It is not unusual
to have a vapor leak into a freezer to develop ice of thickness measured
in feet.
Water vapor drive is of great concern on roofs. A small amount of dew
will fall on a roof nearly every night. During the day the sun vaporizes
the dew and can drive the water vapor molecules right into the roofing
if it is not impervious. If enough of the molecules are driven into the
roofing they will condense into droplets of water. The water droplets
will likely re-expand into a gas on a hot day and create a blister under
the roof. This allows more vapor to enter and condense as it cools off.
As this may continue day after day it may lift the roofing off the substrate.
Often these blisters will continue to grow until they eventually rupture
and the roof membrane fails.
Vapor drive is a fact we must deal with. One common way is to just ignore it. Let the vapor move back and forth through a wall or through a ceiling into the house. That isn't always all that bad. At normal temperatures, vapor drive is usually not huge. The drive often reverses. In summer, outside of the house may be very warm and humid and the inside much dryer and cooler because of the air conditioning. On the other hand, in the winter time, the outside air can be much colder and dryer, and the inside air will be warmer. Vapor drive can shift several times during a single day.
Now is where it gets complicated. Water vapor molecules will stay suspended in the air unless you cool the air off. As the air cools, it will hold or carry less water vapor. The excess water vapor molecules will coalesce and form droplets of water. We call that condensation. We often see condensation on the windows of our house. On a window it is not much of a problem. But when condensation occurs within a wall it can be a real problem.
Very few homes have vapor barriers in the ceiling. Water vapor molecules in the house will migrate up to and through the ceilings into the insulation, up through the insulation, into the attic and hopefully the air moving through the attic will carry them off. That is the reason it is imperative to have air moving above the insulation in the attic. This is most especially true in the winter time. If the water vapor is not carried off it will become saturated and droplets or condensation will occur. This condensation will then drop back into the insulation where it will seriously degrade the insulative value of the insulation. It can even wet the structure which will encourage mold, mildew, or bacteria to grow which may rot the structure.
Water vapor is becoming more of a problem as we more tightly seal our houses. When the air blows through a house it generally evens up the water vapor and we don't have much problem with it. As the house is sealed tighter and tighter against the air blowing through it, often the seal is a vapor barrier. The tendency is for condensation to form against the vapor barrier. If the house is warmer in the winter the condensation will be at the vapor barrier on the exterior of the wall which is where most air barriers are placed. If the barrier is placed on the inside the condensation may occur there. If condensation does occur, and if it is of any serious amount, it will very often follow the framing to a collection point and rot out the framing.
The next step most take is to install a vapor barrier on each side of the wall. It sounds like a good idea, but no vapor barrier is perfect. Consequently you wind up with vapor getting in between the two barriers where it is trapped. Condensation will accumulate over time and become much more of a problem than if it had a single vapor barrier.
Fortunately, in the case of the Monolithic Dome, at normal ambient temperatures the Airform and urethane foam combine to make a really good vapor barrier. This is especially true with houses, schools and churches -- buildings that operate at normal interior air temperatures. However, it is not true for freezers. They must have additional vapor proofing to protect the insulation properly.
Related Links:
- Lessons Learned by Monolithic in Exterior Coatings-- Why the Monolithic Airform makes the best roofing membrane
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