11.1 Simulated Performance Alternative (R405)
Key Takeaways
- R405.1 limits the annual performance analysis to heating, cooling, mechanical ventilation, and service water-heating energy only.
- R405.2 requires Table R405.2 mandatories, a thermal envelope at or above 2009 IECC Table 402.1.1 or 402.1.3 efficiency and SHGC, and proposed annual energy cost no higher than the standard reference design.
- R401.2.5 then requires an R408.2 package omitted from the proposed design or proposed annual energy cost at or below 95 percent of the reference; source-energy substitution uses multipliers of 3.16 for electricity and 1.1 for other fuels.
- The proposed compliance report is due with the permit application; a site-specific confirmed report that declares the performance path on the energy-report and plan title pages is due before the certificate of occupancy.
- Table R405.4.2(1) sets reference ACH50 at 5.0 in Climate Zones 0–2 and 3.0 in Climate Zones 3–8, caps reference vertical fenestration at 15 percent of CFA when proposed glazing is 15 percent or more, equalizes reference orientation N/E/S/W, and locks thermostats at 75°F cooling and 72°F heating; the proposed home uses measured air exchange.
11.1 Simulated Performance Alternative (R405)
Quick Answer: A 2021 IECC R405 house complies only when the proposed design meets Table R405.2 mandatory sections, the thermal envelope is at least as efficient as 2009 IECC Table 402.1.1 or 402.1.3 (including SHGC), and annual energy cost is no higher than the standard reference design—then the R401.2.5 overlay still applies (an R408.2 package kept out of the proposed model, or proposed cost ≤ 95 percent of reference). The modeled energy is heating, cooling, mechanical ventilation, and service water heating only. A proposed report is due with the permit; a site-specific confirmed report is due before the certificate of occupancy.
Independent OpenExamPrep material for ICC Exam 79 treats R405 as a plans-exam and closeout problem. You are not re-running the annual simulation. You are checking whether the software was allowed to invent the reference home, whether the 2009 envelope floor is shown, whether the energy-cost (or source-energy) comparison is complete, and whether the as-built house still matches the confirmed certificate. Chapter 3 covered which primary option R401.2.2 is. This section is the report you actually open.
What R405.1 puts inside the annual comparison
R405.1 limits total-building-performance analysis to heating, cooling, mechanical ventilation, and service water-heating energy only. Plug loads, interior lighting kilowatt-hours, and process equipment do not enter the R405 cost or source-energy budget. That limited budget is why a designer can trade envelope or equipment against the reference home. It is not a waiver of lighting, air sealing, or controls. Table R405.2 still lists those provisions as mandatory.
If a report tries to improve the annual-cost ratio by omitting R404.1 high-efficacy lighting or by skipping R402.4.1.2 envelope testing, stop the review. Those items never belonged in the energy-cost trade and still have to be built.
Three R405.2 tests
R405.2 requires the proposed design to meet all of the following:
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Table R405.2 mandatory sections. The table is the punch list that survives tradeoffs. It includes R401.2.5 additional energy efficiency; the R401.3 certificate; vapor retarder; eave baffle; access-hatch insulation retention; crawl-space wall insulation installation; air-barrier installation (R402.4.1.1); envelope testing (R402.4.1.2); fenestration U-factor and SHGC caps in R402.5; HVAC controls; ducts under R403.3 including R403.3.1 but except R403.3.2, R403.3.3, and R403.3.6; mechanical piping insulation; heated-water circulation and temperature-maintenance systems; drain-water heat recovery units; mechanical ventilation; equipment sizing and efficiency rating; systems serving multiple dwelling units; snow-melt; pools and spas; portable spas; residential pools and permanent spas; lighting equipment; and interior lighting controls. A reference to a listed section includes its relative subsections except where the table carves them out.
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2009 envelope backstop. The building thermal envelope shall be greater than or equal to the levels of efficiency and solar heat gain coefficients in Table R402.1.1 or R402.1.3 of the 2009 IECC. This floor exists so a 2021 heat pump or a compact floor plan cannot buy an envelope weaker than the 2009 prescriptive house. Watch table numbering: 2009 used Table 402.1.1 as the R-value table and 402.1.3 as the U-factor table, the reverse of 2021's R402.1.3 / R402.1.2 pair. A certificate that claims "2021 Table R402.1.3 is the R405 backstop" has the wrong edition.
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Annual energy cost no higher than the standard reference design. Energy prices shall come from a source approved by the code official. R405.2 names the U.S. Energy Information Administration State Energy Data System Prices and Expenditures reports as an example. The code official may require time-of-use pricing.
Exception: source energy expressed in Btu or Btu per square foot of conditioned floor area may be substituted for energy cost. The source-energy multiplier is 3.16 for electricity and 1.1 for fuels other than electricity. Do not accept a homemade 3.0 electricity factor.
The R401.2.5 overlay on a performance house
Table R405.2 already lists R401.2.5. For buildings using R401.2.2, that overlay is one of two choices:
- One R408.2 additional-efficiency package is installed without including those measures in the R405 proposed design; or
- The proposed design's annual energy cost is less than or equal to 95 percent of the standard reference design's annual energy cost.
The option selected is identified on the R401.3 certificate. A run that shows proposed cost equal to 100 percent of reference, and that also models the R408 HVAC upgrade inside the proposed home, has used the same equipment twice and has not met the overlay.
Proposed report versus confirmed report
R405.3.1 requires documentation that the software's methods and accuracy conform to R405. R405.3.2 then splits the compliance report into two deliverables.
Permit application (proposed design). The report shall include the building street address or other site identification; the name of the individual who performed the analysis; the name and version of the compliance software; documentation of all inputs entered for the reference design and/or the rated home; and a certificate that the proposed design complies with R405.2. That certificate documents component-level insulation R-values or U-factors, duct-system and building-envelope air-leakage testing assumptions, and the type and rated efficiencies of proposed heating, cooling, mechanical ventilation, and service water-heating equipment. If on-site renewable energy is proposed, the certificate reports type and production size. Where a site-specific report is not generated, the proposed design shall be based on the worst-case orientation and configuration.
Certificate of occupancy (confirmed report). The confirmed report is site-specific. It shall include the street address or other identification; a declaration of the total building performance path on the title page of the energy report and on the title page of the building plans; a statement, bearing the analyst's name, that the as-built building complies with R405.2; software name and version; a site-specific energy analysis that still complies with R405.2; and a final confirmed certificate based on inspection. The confirmed certificate reports energy features actually in the home: component-level R-values or U-factors, results from required duct and envelope leakage tests, and the type and rated efficiencies of installed heating, cooling, mechanical ventilation, and service water-heating equipment. Installed on-site renewables, if any, are listed by type and production size.
A generic "model home, orientation varies" packet that never states worst-case orientation is not a complete permit report. An inspector who issues a certificate of occupancy on the original permit run, with leakage still shown as assumptions, has skipped R405.3.2.2.
Inspector-relevant rows in Table R405.4.2(1)
R405.4.1 requires the standard reference design and the proposed design to be configured and analyzed with identical methods except where Table R405.4.2(1) changes a row. Do not reprint the entire table on a field checklist. The rows that produce Exam 79 misses are:
| Component | Standard reference design | Proposed design |
|---|---|---|
| Above-grade walls | Mass only if the proposed wall is mass; otherwise wood frame. Gross area same as proposed. U-factor from Table R402.1.2. Solar absorptance 0.75; emittance 0.90 | As proposed |
| Vertical fenestration | Area equals proposed glazing if proposed glazing is less than 15 percent of conditioned floor area; area is 15 percent of CFA if proposed glazing is 15 percent or more. Orientation equally north, east, south, and west. U-factor and SHGC from Table R402.1.2 (use SHGC 0.40 where that table has no SHGC). No exterior shading | As proposed |
| Air exchange | 5.0 ACH50 in Climate Zones 0 through 2; 3.0 ACH50 in Climate Zones 3 through 8, at 50 Pa | The measured air exchange rate |
| Thermostat | Manual; cooling setpoint 75°F; heating setpoint 72°F | Same as the standard reference design |
Skylights and thermally isolated sunrooms are none in the reference and as-proposed in the rated home. Reference opaque doors are 40 square feet, north-facing, at the fenestration U-factor from Table R402.1.2. Mechanical ventilation is added on top of leakage; the reference ventilation type matches the proposed home but energy recovery is not assumed for the reference fan energy. Heating, cooling, and service water heating generally follow the proposed fuel and type, with federal-minimum stand-ins if a system is omitted, and capacities follow R403.7.
R405.5.1 is the software lock. Calculation procedures shall generate the standard reference design from proposed-design inputs only. The user shall not be allowed to edit reference-wall U-factors, reference ACH50, or reference window area by hand. A printout that shows a Climate Zone 2 reference home at 3.0 ACH50, or reference glazing at 22 percent of CFA, is not a legal R405 model. The tool must also size reference HVAC as a single zone per R403.7, account for indoor and outdoor temperatures and part-load ratios, and print an inspection checklist of the proposed features that produced compliance (R-value, U-factor, SHGC, HSPF, AFUE, SEER, EF).
When a required input is not specified in R402 through R405, R405.5.3 sends the user to an approved source—not to a salesperson's default.
Scenario: proposed report silent on the 2009 backstop
A Climate Zone 5 production house submits an R405 report with proposed annual energy cost at 94 percent of the reference, so the 95 percent overlay looks satisfied, and with a high-SEER heat pump. Envelope takeoffs list 2×4 walls at R-13, clear double-pane glass, and a ceiling well below 2009 Table 402.1.1. The certificate never states that the envelope meets 2009 Table 402.1.1 or 402.1.3. The file fails R405.2 item 2 even though the cost ratio was discussed. Treat the missing backstop as a plans-exam rejection. Require a revised certificate that lists component efficiencies against the 2009 tables, then confirm those components on the site-specific closeout report before occupancy.
This chapter teaches 2021 IECC residential provisions for Exam 79 study. It is independent OpenExamPrep material, not a claim of ICC endorsement. Confirm the code year on the live ICC Exam Catalog; do not treat a state energy-subcode overlay as the national 2021 text.
Which end uses does the 2021 IECC R405.1 total-building-performance analysis include?
A performance-path report shows proposed annual energy cost equal to 100 percent of the standard reference design and models an R408.2 HVAC package inside the proposed design. Which R401.2.5 problem is present?
A Climate Zone 5 R405 submittal shows proposed annual energy cost at 94 percent of the reference and a high-SEER heat pump, but the certificate never compares the envelope to 2009 IECC Table 402.1.1 or 402.1.3 and the walls are 2×4 R-13 with clear glass. What fails?
On a confirmed R405 report, which statement matches Table R405.4.2(1) and R405.5.1?