Performance Codes are Actually Easier: A Different Paradigm

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Performance Codes are Actually Easier A
Different Paradigm

• 2000 National Workshop on
• State Building Energy Codes
• New Orleans, LA
• July 12, 2000
• Philip Fairey

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A Simple Rule of Thumb

• Building science is just like space science,
  only more complex.

Terry Brennen Physicist Building
Forensics Expert
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Compliance Methods
Most Codes allow two pathways

• Prescriptive - disallows compliance if any single
  component fails to meet its prescribed minimum
  requirements.
• Performance - compliance based on achieving equal
  or lower estimated overall energy use than a
  twin (reference) building.

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One Versus the Other
The pros and cons of methods

• Prescriptive
• Pro simple and easy to use and enforce.
• Con flexibility is severely limited to a small
  group of options.

• Performance
• Pro virtually no limit on available options.
• Con requires detailed estimates of energy use
  in buildings.

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Trade-off Methods
Quasi Performance Methods

• Allows user to trade a less than required
  performance in one component for an equally
  better than required performance in another
  (e.g. trading poor envelope insulation for better
  equipment).
• Most often characterized by complex, difficult to
  follow tables, nomagraphs, equation, charts, etc.

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Measuring Performance

• All performance-based energy codes are based on
  energy budgets.
• The energy budget for a building is determined
  from its reference twin.
• Different systems have different names for the
  reference twin building.
• Floridas Code Baseline building
• MEC/IECC Standard design

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The Twin Building
As compared to what?

• Same areas as the proposed building.
• Same geometry as the proposed building.
• Same orientation as the proposed building.
• Same levels of service as the proposed building.

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The Equal Service Principle
Buildings versus Lifestyle

• Codes disregard lifestyle choices, so equivalent
  levels of energy service are assumed in both
  the proposed and the reference twin buildings.
• Example The level of service for air
  conditioning is the thermostat setting.

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Floridas Energy Code

• 20 years old (or in the making).
• More than 90 choose performance-based compliance
  approach.
• Portfolio compliance is achieved on average
  Florida homes comply.
• State of Florida provides the compliance tools
  (i.e. software).

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California Code Experience

• More than 90 select performance-based compliance
  approach.
• Private vendors provide compliance software
  tools.
• Critical and necessary to provide detailed
  procedures for the verification of code
  compliance software tools(California ACM 384
  pages).

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Other Lessons

• Other compliance approaches must stem from the
  performance-based approach.
• Must have sufficient rules to construct an
  unambiguous reference.
• Reference constituting a standard for
  measuring or constructing. Websters
  Dictionary

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Chapter 4 vs. Chapter 5
Are they equivalent?

• IECC Chapter 4 performance-based approach
• Specifies the energy budget using rules for
  each component of the reference twin home.
• IECC Chapter 5 prescriptive approach
• Specifies the minimum energy performance
  requirement for each component of the proposed
  home.

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Example Chapter 5, IECC
Where the reference failed

• Windows and doors are included in calculation
  of wall requirements.
• Uw(AoUo-AgUg-AfUf -AdUd)/Aw

• where the subscripts designate
• w wall o overall
• g glass f frame
• d door

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The Window Problem
Windows are not the same as walls

• Chapter 5 includes windows and doors in overall
  wall U-value calculation, but ...
• Window U-value are not very important in hot
  climates.
• Window heat gain is not considered in calculation
  but is very important in hot climates.
• Outcome from calculation is dependent on building
  geometry.
• The result is a reference home anomaly.

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An Exercise in Futility
IECC Chapter 5 wall Uo equation

• Create a group of square buildings of various
  floor areas.
• Set window area to 15 of floor area.
• Set window U-value to 0.90 (reasonable for
  Orlando, FL, with 733 HDD)
• Solve the Chapter 5 equation to determine the
  required wall R-value for each building size.

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The Window Anomaly!
Square homes of different floor areas
2100 ft2
Window-to-floor area percent 15
3000 ft2
1000 ft2
Window U-value 0.9 Orlando HDD 733
2200 ft2
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Another Lesson

• Code compliance software tools should be
  certified (re California experience).
• Certification should ensure that homes that
  should not comply, do not comply.
• At a minimum, certification procedures should
  evaluate software tools for the following
• Reference home configuration ability
• Load prediction ability
• System performance prediction ability, and
• Code compliance determination ability.

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One More!

• Procedures for developing trade-off
  compliance approaches should be codified.
• Local jurisdictions should be able provide their
  own compliance approaches.
• Anyone can determine if a given approach complies
  with the performance requirement the principle
  of full, public disclosure.

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Code Myth No. 1

• Builders prefer prescriptive energy codes.
• Above all, builders value the flexibility to
  satisfy their customer.
• Where good compliance tools are available, the
  performance-based approach is preferred by a 9 to
  1 ratio.

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Code Myth No. 2

• Performance-based codes are too difficult to
  enforce.
• Code officials dont have to understand the
  intricacies thats the purpose of good software
  tools.
• A custom prescription can easily be reported
  for each home. With this, the code official can
  easily determine compliance.

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Compliance Summary
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Compliance Detail
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Code Myth No. 3

• Performance-based codes are too difficult to
  use.
• Ease of use is a function of the compliance
  tools, not the complexity of the method.
• PC-based compliance tools can make virtually all
  methods equally easy.

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Chapter 4, IECC
Recommended Revisions

• Remove loopholes All Standard Design home
  characteristics not specified by this Chapter
  shall be the same as in the Proposed Design
  home.
• Move minimum component requirements from Chapter
  5 (or Chapter 6) to Chapter 4.
• Refine Standard Design rules to avoid ambiguity.

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Chapter 5 (and 6), IECC
Recommended Revisions

• Require all compliance pathways to meet or exceed
  worst case performance as defined by Chapter 4.
• Codify verification and certification procedures
  for code compliance software.
• Codify allowable methods to determine customized
  prescriptive and trade-off compliance pathways at
  local levels.

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• Thankyou!