Metrics

For whole buildings, there are two broad performance objectives of interest: Energy Performance and Indoor Environmental Performance (e.g. IAQ and comfort). Each objective can be represented by various performance metrics, which are simply defined as a quantification of the performance of the relevant components or systems.

To understand what a metric is, consider a manufacturer that produces a pen. One relevant metric in this case might be how long a line the pen can produce until it runs out of ink.

Three other examples, but in terms of building performance, are duct leakage, which is a metric that represents the airtightness of a duct system; specific leakage area, which is a metric that represents the airtightness of the building envelope; and house depressurization, which is a metric that represents the backdrafting potential for combustion appliances. These latter three metrics each has implications in terms of energy and indoor environmental performance. However, the importance of a particular metric to each performance objective may be weighted differently, and therefore each must be able to stand on its own.

To assure whole-building performance, it is also necessary to consider the relationships between metrics for components and systems, due to interactions between systems and components. For example, it is necessary to quantify duct leakage, specific leakage area, and house depressurization to understand the impact that duct leakage flows can have on combustion safety in tight houses.

Diagnostics

Diagnostics are usually defined as relatively quick short-term field procedures involving measurements and perhaps analyses to evaluate performance metrics for a system or component under functional test or actual building site conditions.

While it is also possible and sometimes preferable to evaluate metrics using data taken over an entire season, time limitations make it impractical to collect and analyze such long-term information during residential commissioning. Such limitations will be largely dependent on the value of the commissioning process to the involved parties. However, for an existing house, commissioning can often use readily available historical data either as part of diagnostics or to set norms.

To understand what a diagnostic is, consider once again the pen. The diagnostic to quantify the line length metric might be using a tape measure to determine how long a line the pen was able to produce. Once this diagnostic measurement quantifies the line length metric, the value of the metric can be compared with a norm to determine whether the pen’s performance is acceptable or not.

From the building performance examples above, consider duct leakage. A possible diagnostic is to use airflow measuring equipment that creates and measures pressure differences, which can then be used in subsequent computerized analyses to calculate the duct leakage.

Norms

A metric itself does not indicate good or bad performance. However, when quantified, each metric forms the basis for developing the norms against which component or system performance can be compared. As with the metrics, the norms will vary depending on the objective of the commissioning. They will also depend on the stage of the house in its life cycle.

To understand what a norm is, again consider the pen. A norm in this case might be the length of a line of ink produced by a reference pen, it might be an average of the length of lines drawn by several pens, or it simply might be the general length of line that the user wants it to produce.

For the metrics related to building performance, consider that various building standards specify requirements for maximum duct leakage, for minimum or maximum specific leakage area, and for maximum house depressurization levels. An example is the California Title 24 norm that duct leakage be 6% or less of the total airflow through the air handler.