What is an air barrier and how does it protect your home? Homes are very susceptible to building up humidity and condensation in closed areas. Imagine the inside of your walls. Without what’s called a vapor retarder, humidity might build up in the heat and condensation might build up in colder weather.

What Would Happen Without an Air Barrier?

Many building materials are more permeable than you think. This means that air can pass through them. Vapor – or moisture in the air – tends to condense when there’s a temperature difference between two areas. Think of having an ice cold glass of soda in the middle of the summer. Condensation beads up on the surface. In a similar way, water vapor will condense when warm vapor is drawn through a wall and hits a colder section of the wall.

If this were allowed to happen, this sustained exposure would lead to many negative consequences. Moisture inside walls would start by weakening a building’s insulation and decrease its energy efficiency. Hot and cold would exchange through the walls much more easily, both increasing energy costs and worsening your condensation problem.

Wood parts of the structure would begin to decay. Steel parts would begin to corrode. Worst of all, mold would start to grow inside the walls. Mold itself will eat up wood and some gypsum board, further weakening the home and making it more expensive to live in. That mold can become airborne and cause serious respiratory issues that require medical attention.

This is why it’s so important that homes have a good air barrier system. There are certain situations where an air barrier isn’t necessary, but this depends on many other factors.

What Is a Vapor Retarder?

A vapor retarder is a layer of the wall that blocks vapor. You may also hear it called vapor barrier. The building envelope refers to the shell that protects a home, but it too needs protection. This is what the vapor retarder does. It prevents condensation from collecting and it prevents vapor from passing into wall cavities.

Like any building material, vapor retarders come at different ratings. The rating is measured in perms, which is a unit of measurement for how permeable it is to water vapor. These materials are rated according to tests that measure how many grains of water vapor will pass through a square foot of the material in an hour’s time. For an idea of what this means, 7,000 grains of water vapor equals 1 pound.

A lower perm rating means the vapor retarder is better at resisting moisture passing through the material. Understand that no vapor retarder will stop all passage of moisture. Even the best allow a tiny bit through.

Classes of Vapor Retarder

Vapor retarders are classified into one of three classes.

  • Class I is the best, with a permeance level that does not exceed 0.1 perm. Glass, sheet metal, and foil-faced insulation sheathing are examples of this.
  • Class II vapor retarders are slightly less effective, but still pass most building codes. These do not exceed 1 perm. Unfaced expanded polystyrene or fiber faced polyisocyanurate are examples of this.
  • Class III vapor retarders are suitable for many construction situations, but will not be a good choice as a home’s air barrier. These do not exceed 10 perms. This category includes plywood and #30 building paper. Class III retarders are more useful for ventilated claddings or other situations that dry an area out but don’t serve as a more traditional air barrier.

Do vapor retarders go above this rating? They don’t. If the material measures above 10 perms, then it is permeable by construction standards.

Construction Changes by Climate

Different climates pose unique problems as the seasons change. Choice of material and placement within the wall for an air barrier in Florida will be very different from choices made in Minnesota. Let’s use these two examples to compare:

Minnesota is a colder area. Vapor retarders need to be closer to the interior because condensation is more likely to occur as warm air inside moves to hit the cold outer wall. In Florida, vapor is more likely to come in from the warm, humid air outside and hit the cooler, inner sections of wall.

In Florida, you’d want to avoid certain materials choices. Class I vapor retarders might not be as useful as Class II because of the very high humidity during much of the year. A semi-permeable vapor retarder like asphalt-coated kraft paper (a Class II vapor retarder) is ideal here.

Smart Design

Smart vapor retarders incorporate construction elements and material choice that act differently in different weather and humidity conditions. For instance, polyamide film has a Class II rating in most circumstances.

As the humidity exceeds 60 percent, the film changes permeability and allows more moisture to pass through. Because humidity inside and outside will be different, this allows building materials to dry out and pass some of their moisture into the home in especially humid conditions.

Talk to your builder about vapor retarders and what the best solution is. Many elements that go into your home – and especially material choices in the building envelope – will impact the best choice of air barrier system.

Your builder will be able to guide you to the best choices available and a range of best materials at different costs. A good air barrier that’s ideal for your climate and other material choices can vastly improve the resiliency and durability of your home, its energy efficiency, and even your health.

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