How Water Vapor Affects A Building Structure

Water vapor can have harmful effects on buildings and structures. Water vapor can damage buildings in the form of dry rot, corrosion, and mold growth. In recent years, scientific studies have been conducted on how water vapor affects structures and what can be done to make structures more resistant to its effects.

What is Water Vapor And What Are Its Effects?
Water vapor is the gaseous form of water in the air that creates condensation in the exterior and interior of buildings. Water vapor can pass through walls or is spread by air currents. Materials can be either porous or non-porous to water vapor. Porous materials include wood, insulation, and other building masonry materials that readily absorb water vapor and once overloaded, create liquid and condensation. This creates a perfect storm of conditions for rot and mold.

Where Does Water Vapor Occur?
  • How Does It Get In?
  • Vapor Drive
  • Perm rating
Condensation can be found in walls, roof cavities, and the interior windows of structures. In general, the most common places condensation develops are within the wall, under wallpaper, the ceiling, and around window areas. Outside, condensation can build up on the roof and the exterior walls of a building. The most common way water vapor gets in is by air leaks found on a structure, such as wall openings. But it can also be spread by mechanical means like air conditioning units. Also, the more humid the climate you live in, the more water is found in a home or building. Diffusion is another method water vapor creates condensation. Diffusion happens when water molecules move from a high moisture level to a low moisture level.

When water vapor passes through a fixed surface like a wall, the force of the water molecules is called the vapor drive. The greater the concentration of water molecules and the more extreme temperature difference, the greater the vapor drive. Vapor drive causes condensation to occur on cool surfaces. Each part of a building and its materials will have a different resistance to vapor drive. This is called a perm rating.

How To Keep Water Vapor At Bay
Perm is short for permanence and it uses such factors, as permeability and thickness of materials, for resistance to vapor drive.
  • Perm Ratings
  • Air Barrier And Vapor Barrier Explained
  • Types Of Air Barriers
  • Types Of Vapor Barriers
A Perm Rating of a Class I vapor material stops water vapor while Class III is considered permeable to water. Since water vapor is transported by air and vapor diffusion, there are two methods to stop water vapor.

Air barriers protect buildings from moisture transported by air while vapor barriers stop water vapor from vapor diffusion. Air barriers come in different forms and shapes and enclose and seal all six sides of a structure to protect it and control air leaks. They can range from spray-on foam to flexible wraps. They are usually placed in the exterior side of a building because it allows for an easy set-up and less complications. However, air barrier should always be placed where there are high amounts of water vapor in the building. That is because wetness from diffusion is highest in those areas as it moves to a lower moisture levels. Vapor barriers are usually installed and restrict water diffusion through the building. Examples of vapor barriers include membranes, coatings, and foam insulation.

Water vapor creates problems in homes or structures including dry rot and mold. It is important to remember there are two ways to stop and restrict water vapor. A vapor barrier stops water diffusion and an air barrier stops the flow of water vapor in the air; they are often used in conjunction with one another.

This article is composed by and offered as a public service by Jimmy K.
Jimmy K is the Independent Owner and Operator of

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