ASHRAE Journal - May 2013 - 34

Since the ECB baseline building tracks design decisions
in the proposed building, it does not truly require a level of
building energy performance, but instead can be thought of as
performance equivalency, establishing a custom energy budget for each building design.1
The Performance Rating Method, commonly referred to by
its location in the Standard, “Appendix G,” is a modiﬁcation of
the ECB method created to measure beyond-code building energy performance. The PRM may not be used currently for code
compliance. However, it rewards buildings that incorporate wise,
energy-reducing design choices that historically have not been
regulated by Standard 90.1. Instead of the baseline building being a clone of the proposed building, many of the characteristics
of the baseline building are established independently of the proposed building, to reﬂect typical design choices for the type of
building under review. The only proposed design features that are
duplicated are the size, shape, number of stories, and function.
As a result, the PRM is much closer to setting a speciﬁc level
of building energy performance. The resulting energy targets are
much more consistent and stable for a particular building type
and climate, while still using a reference baseline building to normalize for much of the uncertainty of building energy models.
The PRM credits aspects of a proposed building design
such as optimized orientation, improved selection of mechanical systems and equipment, right-sizing of mechanical equipment, efﬁcient use of wall mass, and optimized window area.
The PRM also allows credit for reductions in unregulated plug
and process loads compared to standard practice.
Standard 90.1 is under continuous maintenance, with a completely new publication every three years. Since both the ECB
and PRM performance approaches establish a baseline building
that is in compliance with the prescriptive requirements, the process of creating and modeling the baseline building changes each
time the prescriptive standards change. The PRM method is used
much more than the ECB method, because of its reference by
the popular LEED rating system. In the 2009 LEED-NC, a new
building is awarded 10 points if it can show energy cost 30%
lower than a Standard 90.1-2007 baseline. The PRM is also cited
by a number of standards and programs including ASHRAE/
USGBC/IES Standard 189.1-2011, the International Green Construction Code (IgCC), the Federal Energy Efﬁciency Standards,2
and the Commercial Building Federal Tax Deductions.3
Table 1 shows two examples of how certain aspects of a
building design are treated differently between the ECB and the
PRM baselines. Note that in the example buildings, the ECB
gives no credit (positive or negative) for selection of a particular
HVAC system over the more typical baseline system or for any
impact from the wall type selection, window area or orientation.
The PRM shows the impact of various HVAC systems, wall
system choices, orientations, and window area conﬁgurations.

Problems with the Current Performance Approaches
Too Many Performance Approach Options
The two performance approaches in Standard 90.1 combined with the adoption and use of different versions has
34

ASHRAE Journal

Compliance with 90.1
The two paths for compliance in ASHRAE Standard
90.1-2010 are the prescriptive- and performancebased paths.
The prescriptive path establishes criteria for energyrelated characteristics of individual building components
such as minimum R-values of insulation, maximum Ufactors and solar heat gain coefficients of fenestration,
maximum lighting power allowance, occupancy sensor
requirements for lighting control, and economizer requirements for HVAC systems.
The alternative to prescriptive compliance in Standard
90.1-2010 is a performance-based approach known as
the Energy Cost Budget (ECB) method. This method provides more flexibility by allowing a designer to “trade
off” compliance by not meeting some prescriptive requirements if the impact on energy cost can be offset by
exceeding other prescriptive requirements.
Using the ECB approach, a computer simulation of a
proposed building design is compared to a reference
building design (baseline) that is essentially a clone of
the proposed design with each building component adjusted to “just meet” prescriptive requirements. A building
is deemed in compliance when the annual energy cost
of the proposed design is no greater than the annual
energy cost of the reference building design. Instead
of looking at components in isolation, this method allows
recognition of the interactions of those components in
demonstrating compliance.
Regardless of which approach (prescriptive or performance) a building chooses for compliance, there are
a number of mandatory requirements that must be met
and cannot be traded off. Examples of the mandatory requirements include building envelope air leakage,
mechanical equipment efficiency, and thermostatic and
lighting controls.
resulted in a multitude of building performance evaluation
methods. Three states have codes that reference the 2001 version of ECB, four states reference the 2004 ECB, 26 states
reference the 2007 ECB, and one state references the 2010
ECB.4 LEED Version 2.2 references the 2004 version of the
PRM,5 while LEED 2009 references the 2007 version, and
LEED 4.0 is slated to reference the 2010 version.6 The Federal
Energy Management Program (FEMP) requires use of either
the 2004 version or the 2007 version of the PRM depending
on the anticipated construction date7 (Table 2).
To make matters more confusing, many of the codes or
programs add their own modiﬁcations to the standards and
modeling rules. For code compliance, Washington State uses
a modiﬁed version of the PRM8 and Florida uses a modiﬁed
version of the ECB.9 ASHRAE/USGBC/IES Standard 189.12011 references Appendix G-2010 but adds three pages of
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