Requirements of the enclosure do not change between retrofit and new construction. In both cases, the building enclosure must perform four critical control functions:
High performance building enclosures perform these control functions exceptionally well.
The order of these enclosure control functions is important. Water represents a very destructive force for buildings and is also the cause of many indoor air quality problems. If water is not controlled, the other control functions become irrelevant.
Air control is critical because if we want to condition the indoor air, we first need to contain it. Airflow is a very effective way to move heat energy and moisture. It does not make much sense to worry about heat loss through conduction if we don't have a good handle on airflow. Similarly, since airflow through a small hole can move much more moisture than can diffuse through a large surface area of material, air must be controlled to avoid the moisture problems we might otherwise associate with water vapor.
It is important to control water vapor because at temperatures colder than the dew point, water vapor can condense into water or ice leading to damage, mold or decay. Elevated humidity (water vapor pressure) can also create conditions favorable to mold growth and other decay/contaminant vectors. We must control water vapor to avoid conditions of elevated humidity in the interior environment or within walls, roofs, and floors.
Once we have an enclosure that controls water and air and manages water vapor, then and only then does it makes sense to focus on reducing the conductive heat flow through the use of insulation.
Another control function that applies to some parts of the building enclosure is:
Animal pests can represent a risk to life safety and to durability of the structure (e.g. by degrading structural elements or corrupting the safety measures in the electrical system). Pests can also represent contaminant vectors that impact the health of people. Pests can cause bypasses in the enclosure control functions (e.g. nesting in cavities created by removing insulation). In many ways, critters can be a real nuisance for a building and its human occupants. There are certain elements of the building enclosure that are more vulnerable to damage from animal pests or that represent access points.
Understanding these control functions is essential for the successful design and implementation of high performance retrofit. It is also useful to know that existing low performance homes may be able to get away with performing some of these control functions poorly. Or, problems related to poor control functions might be masked by other issues in the building. In high performance, there is less room for error.
A brief discussion of each of the control functions is provided below. A visual key and labels are used to identify these control functions in the illustrations throughout Part 2 of this guide.