Vapor Drive

Moveable molecules

Water vapor molecules (or water in its gas form) try to evenly spread themselves. 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 that 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.

Temperature effects

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. Water vapor molecules on the outside will try their hardest to get to the inside and even things. If the temperature differences are more drastic, the amount of pressure or drive is increased. If the interior temperature is minus twenty, as it 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 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 condenses and freezes on the inside. As it freezes, it builds layer upon layer until it can literally destroy the freezer. It is not unusual to have a vapor leak into a freezer develop ice to a 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 condense into droplets of water. The water droplets can 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 continue to grow until they eventually rupture and the roof membrane fails.

Dealing with vapor drive

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 can 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 water droplets. 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 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 carries them off.

That is why it is imperative to have air moving above the insulation in the attic. This is especially true in the winter time. If the water vapor is not carried off it becomes saturated and droplets or condensation occur. This condensation then drops back into the insulation where it seriously degrades the insulating value of the insulation. It can even wet the structure and that encourages mold, mildew, or bacteria to grow and rot the structure.

Tightly sealed homes

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 the water vapor and we don’t have a problem.

As the house is sealed tighter and tighter against the blowing air, the seal is often 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 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 accumulates over time and becomes more of a problem than caused by a single vapor barrier.

The Monolithic Dome

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.

June 7, 2004