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2021 IECC Sections R405-R406

Lesson 9: Performance and Energy Rating Index Paths

IECC R405-R406 (IRC N1105-N1106) · 14 practice questions

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Reading

Lessons 2 through 8 covered the prescriptive path: fixed R-values and U-factors from Table R402.1.3, met component by component. The 2021 IECC also allows two alternative ways to show a house complies, and both let a designer trade a weaker item in one place for a stronger one somewhere else, something the prescriptive table never allows. R405 is the simulated performance alternative, which compares a computer model of the proposed house against a computer model of a code-minimum reference house. R406 is the Energy Rating Index, which uses the same kind of energy model that a HERS rater already builds for a rating, but expresses the result as a single score. GCHS plans reviewers see both paths on production-builder projects and on above-code custom homes, and knowing what still has to be checked by hand, regardless of what the software says, is the point of this lesson.

Energy Rating Index scale and the Climate Zone 2 target A horizontal scale from 0 to 100 showing 0 as a net-zero energy home, 100 as the energy performance of a 2006 IECC reference home, a lower-is-better direction, and a marker at the Climate Zone 2 maximum score of 52, with a further marker at 49 after the 2021 edition's 5 percent additional efficiency reduction. Energy Rating Index (lower score = less energy use) 0 = net zero energy home 100 = 2006 IECC reference home CZ 2 target: 52 Table R406.5, base ERI path 49 after 5 percent reduction (R401.2.5) A house still must clear the mandatory items and the envelope backstop no matter the score
The ERI score is a single number, but it is not the only thing an inspector checks.

R405: the simulated performance alternative

R405, called the Total Building Performance Option in R401.2.2, lets the designer show compliance by energy modeling instead of by the prescriptive table. The analysis covers heating, cooling, mechanical ventilation, and service water heating energy only (R405.1), so lighting and plug loads are not part of the score. The software builds two models of the same house: the "proposed design," using the actual plans, and the "standard reference design," an automatically generated code-minimum version of the same house built to Table R405.4.2(1) defaults. Both models are run with the same weather file, the same occupancy assumptions, and, importantly, the same heating, cooling, and water heating equipment (Table R405.4.2(1) says "as proposed"), so the comparison isolates the envelope and duct system rather than rewarding a house for simply installing better equipment. The one twist is that a house with electric heating and no heat pump is compared against a reference house that has a heat pump. If the proposed design's estimated annual energy cost is equal to or less than the standard reference design's, the house complies. This is what lets a designer trade a slightly weaker wall assembly for better windows, or a smaller duct system for tighter sealing, combinations the prescriptive table's fixed values never allow. The trade has a floor, though: R405.2 also requires the proposed building thermal envelope to be at least as efficient as the levels in the 2009 IECC tables, and the area-weighted average SHGC used in a trade-off may not exceed 0.40 in Climate Zones 0 through 3 (R402.5).

Mandatory provisions that still apply under R405

Modeling flexibility does not reach every part of the code. R405.2 carries forward a list of sections that a house must still satisfy regardless of how well it scores: the building thermal envelope, air leakage testing, and duct leakage testing are all checked as physical, built conditions in the field, not simulated. On every path, envelope air leakage may not exceed 5.0 air changes per hour at 50 pascals (R402.4.1.2). A permanent certificate documenting the house's insulation, fenestration, air leakage result, and duct leakage result still has to be posted, the same certificate required on every compliance path since Lesson 1 (R401.3). An inspector who sees a passing energy model report on the counter still has to see a passing blower door number and a passing duct leakage number on the same job, because the model assumed both would be met.

In the field

A production builder GCHS worked with submitted a REM/Rate report showing an easy pass on a custom infill house with an oversized glazing package facing west. The plans examiner still pulled the mandatory-item checklist separately: was the air leakage test actually scheduled, was duct leakage testing specified for the split system serving a bonus room over the garage, and was the permanent certificate template updated to match the modeled insulation values rather than a generic one left over from a different plan. The model can prove the trade-off works on paper; it cannot walk the job and confirm the sealing got done.

R406: the Energy Rating Index compliance path

R406 uses the same category of energy model as R405 but reports the result differently. An Energy Rating Index score describes how a specific house's calculated energy use compares to two fixed points: a score of 100 represents the energy performance of a 2006 IECC reference version of that same house, and a score of 0 represents a net-zero-energy house. The code itself defines the ERI reference design as a version of the rated house that meets the 2006 IECC (R202), and R406.4 sends the scoring method to the RESNET 301 rating standard. A lower score means less calculated energy use. This is the same index a HERS rater's software produces for an ENERGY STAR certification or a mortgage lender's rating, which is why GCHS's rehab-adjacent partners sometimes already have an ERI number in hand from a rater before a plans examiner ever asks for one.

Key numbers
  • Energy Rating Index scale: 0 = net-zero energy home, 100 = roughly a home built to the 2006 IECC (the ERI reference design in R202); lower is better; scoring follows the RESNET 301 rating standard (R406.4)
  • Maximum ERI score, Climate Zone 2: 52 under the base path (Table R406.5)
  • Maximum ERI score, Climate Zone 3: 51 (Table R406.5), included here for contrast with Zone 2
  • Maximum ERI score, Climate Zone 2, after the R401.2.5 additional efficiency reduction of 5 percent: 49
  • Envelope backstop without on-site renewable energy: proposed building thermal envelope UA no greater than the prescriptive UA (Table R402.1.2) multiplied by 1.15 (R406.3)
  • Items listed in Table R406.2 still apply on the ERI path, including the certificate (R401.3), air sealing installation and testing (R402.4.1.1, R402.4.1.2), ducts (R403.3), ventilation (R403.6), and lighting (R404.1, R404.2)
  • With on-site renewable energy in the analysis, the envelope must at least equal the 2015 IECC levels (R406.3.2), and renewable credit may not exceed 5 percent of total energy use (R406.4)
  • R405 backstop: the proposed envelope must be at least as efficient as the 2009 IECC tables (R405.2)

The ERI target table and the additional efficiency credit

Table R406.5 sets the maximum ERI score by climate zone. Zone 2, GCHS's primary service area, has a maximum score of 52; Zone 3 is slightly stricter at 51. The 2021 edition then adds its additional efficiency requirement (R401.2.5): a house on the ERI path must score at least 5 percent below the table value, which in Zone 2 means an ERI of 49 or lower rather than 52. A plans examiner checking a 2021 ERI submittal should compare the score against that reduced number, not the bare table value.

The envelope backstop

An ERI score by itself does not protect against a house that scores well only because of an expensive mechanical system or a renewable energy system layered onto a weak envelope. R406.3.1 requires the proposed house's building thermal envelope UA, the same total heat-loss calculation used in the total UA alternative, to be no greater than 1.15 times the UA the house would have using the prescriptive Table R402.1.2 U-factors. It also caps the area-weighted SHGC at 0.30 in Climate Zones 0 through 3. In practice this means a design can be a little looser than the prescriptive table in the envelope, but not by much. Where on-site renewable energy is part of the analysis, R406.3.2 uses a different floor instead: the envelope must at least equal the levels in the 2015 IECC tables. Either way, R406.4 limits the ERI credit from on-site renewable energy to 5 percent of total energy use, so a large photovoltaic array cannot rescue a badly undersized envelope.

Worked example: checking the envelope backstop

A Zone 2 plan's prescriptive-table UA (Table R402.1.2 U-factors times the same assembly areas, as in the total UA alternative in earlier lessons), comes out to 220 for this house's size and shape. The backstop limit is 220 times 1.15, or 253. The proposed design's actual modeled UA is 240. Because 240 is less than 253, the design passes the backstop even though its envelope alone is somewhat looser than the strict prescriptive table would allow, which is exactly the kind of case the ERI path exists for: a real envelope trade-off, checked against a real limit, not an unlimited trade against equipment or renewables.

Documentation and compliance software

Both paths depend on software that runs an approved simulation engine, commonly REScheck for simpler prescriptive-style comparisons and rating software such as REM/Rate or Ekotrope for the fuller R405 and R406 models; these are named here only as examples GCHS staff are likely to see on a plan set, not as products the code requires or endorses. What the code does require is a compliance report or certificate attached to the plans identifying the software, the version, the inputs used, and the result, along with the mandatory-item documentation described above. A plans examiner's job is not to re-run the model, but to confirm the report matches the plans in front of them, the mandatory provisions are separately called out, and the permanent certificate reflects what was actually modeled and, later, what was actually tested.

Practice questions

1. Under R405, a house complies when the proposed design's estimated annual energy cost is:

  1. At least 10 percent below the standard reference design's estimated annual energy cost, with no exceptions
  2. Equal to the prescriptive table's assumed cost
  3. Equal to or less than the standard reference design's estimated annual energy cost
  4. Not applicable, because R405 has no numeric test

2. In the R405 comparison, mechanical equipment efficiency in the standard reference design is generally:

  1. Left blank, since equipment is not modeled
  2. Set to match the proposed design's equipment, so the comparison isolates envelope and duct performance
  3. Always set to the least efficient equipment allowed by federal law, so any real equipment earns a credit
  4. Always set to the most efficient equipment listed for the climate zone, so the house must beat it

3. A house complies using R405 with a strong energy model score. Which of the following is still required as a separate, physically verified item?

  1. Air leakage testing and duct leakage testing where required
  2. Nothing further, since the model already accounts for both air leakage and duct leakage
  3. A second, independent energy model
  4. A blower door test, but only if the modeling software specifically requests one for its report

4. On the Energy Rating Index scale used in R406, a score of 0 represents:

  1. A house that fails to comply
  2. The 2006 IECC reference home
  3. A house with no insulation
  4. A net-zero-energy home

5. Under Table R406.5, the maximum Energy Rating Index score for a house in Climate Zone 2 on the base ERI path is:

  1. 45
  2. 52
  3. 60
  4. 65

6. A Zone 2 house is being documented on the ERI path under the 2021 IECC, including the R401.2.5 additional efficiency requirement. Its maximum allowed ERI score is:

  1. 49, a lower (stricter) target than the base 52
  2. 52, unchanged
  3. 55, a higher (looser) target than the base 52
  4. There is no additional efficiency step on the ERI path

7. The purpose of the R406.3 envelope backstop is best described as:

  1. Requiring every house to meet the prescriptive table exactly, with no ERI path allowed
  2. Limiting how much renewable energy a house may install
  3. Preventing a weak building envelope from being masked by efficient equipment or renewables, by capping the proposed UA relative to the prescriptive UA
  4. Setting a minimum blower door result

8. Under R406.3, without on-site renewable energy, the proposed building thermal envelope UA must be no greater than the prescriptive UA multiplied by:

  1. 0.85
  2. 1.00
  3. 1.30
  4. 1.15

9. A Zone 2 house has a prescriptive-table UA of 220. Its modeled proposed UA is 240. Does the design meet the R406.3 envelope backstop?

  1. No, because any proposed UA above the prescriptive UA of 220 fails the backstop outright
  2. Yes, because 240 is less than 220 multiplied by 1.15 (253)
  3. No, because 240 exceeds the prescriptive UA of 220 by more than 5 percent
  4. Cannot be determined without knowing the house's ERI score and its renewable energy credit

10. An inspector reviews a plan set that names REM/Rate as the modeling software used for an R406 submittal. This means:

  1. REM/Rate is one example of software commonly used to produce this kind of report; the code requires a compliant report, not a specific product
  2. The plan cannot be approved unless REScheck is used instead
  3. The house is automatically compliant regardless of its content
  4. The code requires REM/Rate specifically

11. A contractor argues that because the house passed its ERI score comfortably, the scheduled blower door test can be skipped. The correct response is:

  1. Agree, since a comfortably passing ERI score already proves the house is tight enough
  2. Agree, but only in Climate Zone 2, where the limit is a looser 5.0 ACH50 anyway
  3. Disagree; envelope air leakage testing is mandatory regardless of the ERI score
  4. Agree, if the heating and cooling equipment is high efficiency and the score has margin

12. Which document must still be posted in the house on an R405 or R406 compliance project, the same as on a prescriptive-path project?

  1. A printed copy of the energy model's input file and source code, kept with the electrical panel
  2. The permanent certificate of insulation, fenestration, air leakage, and duct leakage results
  3. A copy of Table R402.1.3
  4. Nothing; permanent certificates apply only to the prescriptive path and are not required with a modeled report

13. A designer wants to offset an oversized west-facing glazing package by modeling its orientation and shading. Which compliance path makes that possible?

  1. The prescriptive path, since it lists fixed values only
  2. No path allows any trade-off
  3. The total UA alternative only, since it weights every glazing area the same way
  4. The simulated performance (R405) or Energy Rating Index (R406) path, since both model the whole house rather than compare fixed component values

14. A Zone 2 ERI submittal under the 2021 IECC shows a score of 51. The examiner should find:

  1. Not compliant: the 5 percent additional efficiency reduction makes the maximum 49
  2. Compliant, since 51 is below the Table R406.5 value of 52 for Zone 2
  3. Compliant, as long as the envelope also meets the prescriptive table values in Table R402.1.2
  4. Compliant, because the Zone 3 maximum of 51 also applies to houses in the Zone 2 service area

Glossary

Standard reference design
An automatically generated, code-minimum version of the proposed house, built to Table R405.4.2(1) defaults, used as the comparison baseline in R405.
Energy Rating Index (ERI)
A single score from an energy model expressing a house's calculated energy use relative to a 2006 IECC reference home (100) and a net-zero-energy home (0); used in R406 and by HERS raters.
Envelope backstop
The R406.3.1 limit keeping a house's modeled building thermal envelope UA (without on-site renewables) from exceeding 1.15 times its prescriptive UA from Table R402.1.2, so equipment cannot mask a weak envelope; with renewables, R406.3.2 requires 2015 IECC envelope levels.
HERS rater
A certified professional who models and field-verifies a house's energy performance to produce a Home Energy Rating System score, which uses the same ERI scale referenced in R406.
Compliance report
The documentation attached to a plan set identifying the modeling software, inputs, and result used to demonstrate compliance under R405 or R406.
Additional efficiency package
The 2021 edition's extra efficiency step (R401.2.5): an R408.2 option on the prescriptive path, an option or a 5 percent lower energy cost on the performance path, and a score 5 percent below the target on the ERI path.
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