The Science of Wall Systems

Now that you have a general understanding of what air barriers and vapor barriers do, let’s talk a bit more about why we need them. When constructing wall systems, one can typically spend $1.50 to $2.00 per-square-foot on the barrier material that is placed on the outside of a structural wall. You may wonder if the investment is actually worth it. It is not always inexpensive to install these materials. It is, however, important. When it comes to seeking cheaper material options as part of a “value engineering” exercise, there is good reason to look elsewhere to reduce costs.

Air barriers may be required by code, they may be installed for energy savings, or they may be intended to keep buildings dry and to make structures last longer. Air barriers keep occupants comfortable and keep building owners’ bills lower.

The three core focus areas of this section will be energy savings, the effects of air movement and structural durability. Correctly designing and installing air barriers can affect all three of these aspects in meaningful ways.

Energy Savings

As mentioned earlier, air barriers cut down on heating and cooling costs for buildings by controlling air leakage. Air leakage refers to uncontrolled air movement leading to heating and cooling loss, subsequent high energy costs, and sometimes expedited water infiltration. In order to avoid air leakage, air barriers must be sealed, continuous and air-impermeable.

Hot, moist air outside can easily infiltrate an air-conditioned interior when no air or vapor barrier is in place.

Vapor drive occurs when water vapor moves from a region of high humidity and temperature to regions of lower humidity and temperature in an attempt to reach a natural equilibrium. This graphic shows vapor drive as warm, humid exterior conditions cause moist air to pass easily to the drier interior conditions through a wall assembly that contains no air barrier membrane. To picture this phenomenon another way, visualize a hot cup of coffee that cools to room temperature, hence obtaining thermal equilibrium inside and outside the cup.

Vapor drive can occur just as easily in cold weather as in warm weather if no barrier membranes are in place.

Vapor drive can also occur when we attempt to warm structures. In this scenario, water vapor moves from a region of high humidity and temperature to regions of lower humidity and temperature in an attempt to reach natural equilibrium. As you can see, air and moisture easily move out through a wall assembly that contains no air barrier membrane.

When HVAC systems run continuously in an attempt to heat or cool a building, higher energy consumption costs and diminished HVAC equipment lifespans are the result. Hot, humid air creating a vapor drive can also lead to problem areas developing within the structure, whether that air is traveling into a building or out of it.