ASHRAE Journal - May 2013 - 36

Design Parameter

Proposed Design

ECB Baseline Design

PRM Baseline Design

Example Building 1

HVAC System

Water-source heat pump with
condenser loop served by natural
gas boiler and ﬂuid cooler.
Efﬁciencies as designed.

Water-source heat pump with
condenser loop served by natural
gas boiler and ﬂuid cooler. Efﬁciencies
from prescriptive requirements.

Packaged variable air volume (VAV)
with hydronic reheat provided by a
natural gas boiler. Efﬁciencies from
prescriptive requirements.

Walls

Concrete masonry unit (CMU)
with interior insulation.
U-value as designed.

CMU with interior insulation.
U-value from prescriptive requirements
for mass walls.

Steel frame. U-value from prescriptive
requirements for steel frame walls.

Orientation

Long axis running east/west.

Average of four rotations of 90 degrees.

Window-to-Wall Ratio

20%

31%

Example Building 2
HVAC System

Packaged rooftop heat pump.
Efﬁciencies as designed.

Packaged rooftop heat pump. Efﬁciencies from prescriptive requirements.

Packaged VAV with electric
resistance reheat and parallel fan-powered terminal units. Efﬁciencies
from prescriptive requirements.

Walls

Wood framed. U-value
as designed.

Wood framed. U-value from prescriptive
requirements for wood frame walls.

Steel frame. U-value from prescriptive
requirements for steel frame walls.

Orientation

Long axis running north/south.

Average of four rotations of 90 degrees.

Window-to-Wall Ratio

40%

31%

Table 1: Comparison of ECB and Appendix G, baseline design assumptions for a 40,000 ft² (3716 m2) office building.
modiﬁcations. For Federal tax incentives, the rules are really
convoluted. The modeling must be completed in accordance
with a mixture of the 2004 version of the PRM with some
rules from the 2004 California Nonresidential Alternative Calculation Method Approval Manual, but the baseline building
is deﬁned by the prescriptive requirements of Standard 90.12001. Table 2 shows various uses for different vintages and
modiﬁcations of the two performance paths in Standard 90.1.
By contrast, the test procedures for air conditioners, water
heaters, boilers and other equipment typically change very
little as the standards for these equipment types become more
stringent. Whole-building performance is far more complicated than that of individual pieces of equipment, yet we modify
the whole-building test procedure almost continuously, making it very difﬁcult for software developers and energy modelers to stay abreast.

Lack of Standardization Limits Software Development
It is easy to see why building performance assessment is
confusing. Software developers who want to automate the
process of baseline-building creation have more than a dozen
Standard 90.1 versions and performance options to deal with,
making it cost-prohibitive to create software to serve all these
purposes. This is probably one of the main reasons why the
tools to implement the performance approaches of Standard
90.1 are so sparse.
Software developers are not the only members of the
building industry burdened by these complex requirements.
Building modelers and reviewers (code ofﬁcials and program
implementers) need to become experts on all the subtle dif36

ASHRAE Journal

ferences of these approaches to judge compliance or award
incentives. A single project that needs to achieve code compliance, LEED certiﬁcation, and a federal tax incentive would
need three separate baseline building models.
For another perplexing example, envision a LEED project
that demonstrates it is 30% better than Standard 90.1 using the
PRM, but can’t cite that as complying with the standard. Try
explaining these nuances to a building owner when trying to
justify higher consulting fees.

Baseline is a Moving Target
Both performance paths in Standard 90.1 are based on a
baseline building that meets the prescriptive requirements.
This presents two main problems. The ﬁrst is that it becomes
difﬁcult to compare the performance of buildings of different
vintages or establish a deliberate improvement in performance
requirements. If a building is 30% better than the 2004 version
of Standard 90.1, how does that compare to a building that is
15% better than the 2007 version? Does the building that is
15% better than the 2007 Standard even comply with the 2010
Standard? LEED Version 2.2 awarded 10 Energy and Atmosphere, Credit 1 points for a 42% improvement compared to
the Standard 90.1-2004; in LEED 2009, 10 points are awarded
for a 30% improvement relative to Standard 90.1-2007. Looking ahead, the soon-to-be-released LEED Version 4.0 is expected to award 10 points for a 24% improvement over Standard 90.1-2010.
Figure 1 shows that, based on the average energy use of
buildings modeled to comply with each version of Standard
90.1, it is actually easier to achieve 10 energy points in
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