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.
1.1 Purpose
This document is intended to be a reference manual for the Appendix G Performance Rating Method (PRM) of ANSI/ASHRAE/IES Standard 90.1-2019 (Standard 90.1-2019). The PRM can be used to demonstrate compliance with the standard and to rate the energy efficiency of commercial and high-rise residential buildings with designs that exceed the requirements of Standard 90.1. Use of the PRM for demonstrating compliance with Standard 90.1 was a new feature of ANSI/ASHRAE/IES Standard 90.1-2016 (Standard 90.1-2016). The procedures and processes described in the PRM reference manual (PRM-RM) are designed to provide consistency and accuracy by filling in gaps and providing additional details needed by users of the PRM. Note that this document has been created independently from ASHRAE and Standing Standard Project Committee (SSPC) 90.1 and is neither sanctioned nor approved by either of those entities.
Interpretations to Standard 90.1-2019 Code requirements can be found out here. Users can also request official interpretations to code requirements out here. In cases where the official interpretation conflicts with interpretations provided by the PRM-RM, the official interpretations should be used.