The modeling procedures in the 2016 standard are fundamentally different from previous versions in that the baseline building is fixed to be roughly equal to Standard 90.1-2004, and this will not change over time. The 2004 version of the standard has been used for more than a decade as a benchmark by the US DOE for evaluating the stringency of more recent versions of Standard 90.1 and now it is used as a stable baseline for all performance calculations. Prior to 2016, the Appendix G baseline building changed with each version of Standard 90.1 and sometimes with each addendum. This created much confusion for software developers, energy modelers and program administrators. An unnecessary amount of time was spent creating the baseline building and verifying its correctness.
With the new procedure, the baseline building does not change. Instead, as the standard becomes more stringent, a greater level of improvement over the fixed baseline is required. Energy cost is still the metric for evaluating energy performance. The 2016 standard defines a new term, the performance cost Index (PCI), as the ratio of the proposed design energy cost to the (roughly) 2004 baseline building energy cost. A PCI of 1.0 would be a building with performance identical to the baseline. A PCI of zero would be a zero-energy-cost building.
(Equation 1.1-1)
$$PCI = \frac{Proposed Design Energy Cost}{Baseline Building Energy Cost}$$
The target PCI or PCIt is the level that must be achieved to comply with the standard and this target will become lower with each subsequent version of the standard. A PCI target can also be determined for previous versions of the standard and PCI targets can be associated with performance levels for beyond code programs. The target PCI for 90.1-2016 code compliance is a function of the building type, the climate zone and the amount of unregulated energy projected to be used by the proposed building. Unregulated energy use is neutral for 90.1 code compliance. Section 4.2.1 of Standard 90.1-2016 has procedures for determining PCIt. The PCIt for other standards and beyond-code programs are specified in different ways, but in most cases building type, climate and unregulated energy are factors.