During the 1970’s energy crisis building professionals and engineers started looking for ways to improve the efficiency of the building envelope. This effort paved the way for the development of air barriers. Air barrier systems are intended to control the leakage of interior conditioned air through the exterior wall system.
In a modern residential home the most common type of air barrier is an exterior house wrap or combination air/vapor barrier plastic sheet installed behind the drywall. The effectiveness of an air barrier is directly influenced by the attention to detail taken during installation. Any opening in a seam, or hole from a fastener, compromises its effectiveness.
Efficiency Improvements
One metric to judge the efficiency of a home is by its Air Changes Per Hour Per 100 sq feet (NLA). A typical 1930’s home has approximately 12.2 air changes per hour, while a 1970’s home was improved to roughly 6.9 NLA. A modern home can achieve 2.8 NLA or better. These improvements have drastically reduced the cost to condition a home both in the summer and winter.
Challenges
There are several challenges that air barriers create:
- If moisture manages to enter the wall system it can have difficulty drying quickly enough to avoid wood rot and/or mold. A properly designed, and installed, wall system takes this into account and allows moisture movement though the exterior.
- Homeowners are not always educated in proper management of moisture within the home. The result of an air tight home is that the interior humidity level can become excessive leading to a perfect environment for mold to grow.
- Reducing the air changes may also lower the quality of the air inside the building. This is a result of using household cleaners, formaldehyde in carpet and furniture, radon, and even fragrances.
A Heat Recovery Ventilator (HRV) may be installed to mitigate many of these issues but they are currently not required in the building code. An HRV brings fresh air into the house while reducing the energy lost in this process. I would highly encourage anyone building a new home to consider the addition of an HRV.
While the current wall system design is still evolving, the use of air barriers is here to stay. Due to their low cost and high return of energy savings I expect these barriers will only be improved over time. With the inevitable increase in the cost of energy, the homes of the future will need to be significantly more efficient.