2021 IECC residential review
Eleven lessons have moved from scope and compliance paths through climate zones, fenestration, insulation installation, air barriers, ducts, mechanical systems, lighting, performance paths, existing-building rules, and rehab practice. This lesson pulls the numbers that show up most often on a GCHS energy final back into one place, organized as a review rather than new material, and closes with a practice exam drawn from the whole course. Every figure below repeats a number already stated and verified in Lessons 1 through 11; where the earlier lesson hedged a value or flagged it as something to confirm locally, this review keeps the same hedge rather than sharpening it into false precision.
R401.2 offers four compliance options: prescriptive (R401.2.1, R401 through R404), total building performance (R401.2.2, R405), Energy Rating Index (R401.2.3, R406), and a tropical climate region option (R401.2.4, R407) written for tropical regions, which does not apply on the Gulf Coast. That leaves three paths for a Gulf Coast house: the prescriptive path, inside which sits the total UA alternative; the simulated performance path (R405), which compares a modeled proposed design against a standard reference design; and the Energy Rating Index path (R406), which reports a single score. Lesson 1 introduced these, and every lesson since has referred back to whichever path was relevant to that topic. The 2021 edition's additional efficiency requirement, R401.2.5, applies to every one of those paths, not just the prescriptive one: a prescriptive-path house, including one using the total UA alternative, installs one of the five R408.2 options covered in Lesson 8; a performance-path house under R405 either installs an R408.2 option without counting it in the model, or shows an annual energy cost no more than 95 percent of the standard reference design's; and an ERI-path house under R406 must score at least 5 percent below its climate zone's Table R406.5 target, which for Zone 2 means an ERI of 49 rather than the bare table value of 52. Mandatory provisions, air leakage testing, duct leakage testing, installation quality, high-efficacy lighting, and the permanent certificate, apply the same way regardless of which path got the house there (Tables R405.2 and R406.2 list them for the performance paths).
Climate zone is assigned by county, not by a temperature reading, and GCHS's core service area, Harris, Galveston, and Brazoria counties, sits in Zone 2A, hot and moist (Table R301.1 also lists Fort Bend, Montgomery, and Walker counties as 2A), while counties farther north and west can fall into Zone 3A; the specific county assignment should always be verified rather than assumed. Table R402.1.3 sets the prescriptive minimums by zone and component. Zone 2 and Zone 3 share the same R-49 ceiling requirement, and the jump to a higher ceiling value, R-60, only appears at Zone 4 and colder. Where the two zones differ: Zone 2's vertical fenestration U-factor maximum is 0.40 against Zone 3's tighter 0.30, while SHGC holds at 0.25 in both zones because solar gain, not conduction, drives the Gulf Coast cooling load. Zone 2 requires no basement, crawl space, or slab-edge insulation at all; Zone 3 picks up a basement/crawl space wall value of R-5/13 and a slab-edge requirement of R-10 continuous to a 2-foot depth, a requirement that grows to a 4-foot depth at Zone 4 and colder. Wood-frame wall R-value is R-13 (or R-10 continuous insulation alone) in Zone 2 and R-20 (or R-13+5, or R-15 continuous insulation alone) in Zone 3, and floor R-value moves from R-13 to R-19 across the same boundary.
Meeting the R-value on the label is not the same as installing insulation correctly. The eave substitution, full uncompressed R-38 extending over the top plate, satisfies an R-49 ceiling requirement only at that specific detail, never as an average across the whole attic; a baffle at every eave protects the insulation from wind wash and keeps blown fill out of the soffit vent; attic access hatches need insulation equal to the surrounding surface plus weatherstripping; and R303 requires installed R-values to be field-identifiable, by marking or by a posted certificate, so an inspector has something to verify against. The air barrier is a separate, equally mandatory requirement from the insulation table: R402.4 requires a continuous air barrier aligned with the insulation, with Table R402.4.1.1 naming specific locations, including sealed rim joists, sealed electrical boxes on exterior walls, air sealing between the garage and conditioned space, and IC-rated sealed recessed luminaires (R402.4.5), while R402.4.2 gives new wood-burning fireplaces tight-fitting flue dampers or doors. The blower door test that verifies the air barrier is read in air changes per hour at 50 pascals: fan airflow in cubic feet per minute, times 60, divided by conditioned volume. Zone 2's limit is 5.0 ACH50; on the prescriptive path Zones 3 through 8 hold to a tighter 3.0 ACH50 (R402.4.1.3), because a colder climate pays a steeper energy penalty for the same leakage, while 5.0 ACH50 is the outer limit on any path (R402.4.1.2).
Ducts outside conditioned space, the common Gulf Coast attic case and equally a vented crawl space, need R-8 insulation for runs 3 inches or larger in diameter and R-6 for smaller ones (R403.3.1). Ducts entirely inside the building thermal envelope carry no minimum insulation R-value, but they are still tested in the 2021 edition, at a total leakage limit of 8.0 cfm per 100 square feet of conditioned floor area, looser than the 4.0 cfm per 100 square feet that applies to the postconstruction test (and to the rough-in test once the air handler is installed; without the air handler in place, rough-in tightens to 3.0). Beyond ducts, R403 also sets thermostat default setpoints of no higher than 70 degrees Fahrenheit for heating and no lower than 78 degrees Fahrenheit for cooling; a lockout on heat pump strip heat; mechanical system piping insulated to R-3 above 105 degrees Fahrenheit or below 55 degrees Fahrenheit; and equipment sized to ACCA Manual J and selected per Manual S rather than by rule of thumb or a like-for-like swap. Lighting closes the prescriptive path's mandatory items: 100 percent of permanently installed fixtures need high-efficacy sources (kitchen appliance lamps excepted), and most permanently installed interior fixtures need a dimmer, an occupant sensor, or an integral control, except in bathrooms, hallways, exterior locations, and required safety or security lighting.
R501 through R505 set a ladder of increasing obligation. A repair restores what was there, with no obligation to upgrade, but it may not strip out an existing energy feature; an alteration requires the altered portion to comply, subject to specific exceptions such as storm windows, roof recover, and unexposed cavities, with the mirror rule that an exposed cavity must be insulated before it is closed back up; and a change in space conditioning, unconditioned space becoming conditioned, is the strongest trigger, requiring full compliance for the affected space (R502.2). A space converted to a dwelling unit from another use must comply under R505.1. Replacement fenestration is one of the clearest R503 triggers of all: a window where some or all of the unit is replaced, sash and glazing included, must meet the current Table R402.1.3 U-factor and SHGC for the climate zone, not the performance of the unit it replaces, and multiple units in the same job may be averaged on an area-weighted basis. Ducts extended from an existing system to an addition are excused from R403, while a full duct system replacement is newly installed ducts under R403 and is tested like new construction. Several of GCHS's own rehab measures, ASHRAE 62.2 ventilation sizing and combustion appliance zone safety testing chief among them, sit on top of the IECC as program standards rather than inside it, and GCHS is careful to tell a homeowner or a funder which is which.
In the field
A useful way to walk any energy inspection sequence, new construction or rehab, is in the order the work actually disappears. First, before drywall closes the walls and ceiling: insulation grade, alignment with the air barrier, eave baffles, and the sealing details at fireplaces, recessed cans, and electrical boxes. Second, at mechanical rough-in: duct insulation by location, sealed joints at every takeoff and boot, and the rough-in duct leakage test if the air handler is far enough along to run it. Third, typically at the end of the job: the blower door test at 50 pascals, which the code allows at any time once all envelope penetrations are sealed, and the postconstruction duct leakage test. Last, before sign-off: the permanent certificate, posted where the space-conditioning equipment sits, showing the insulation values, fenestration ratings, and both test results actually achieved. Skipping the order, running the blower door before the duct system is sealed, or covering insulation before the eave detail is checked, is how a real deficiency gets buried instead of caught.
The practice exam linked below draws 60 questions at random from a bank covering all four domains this course touches: general requirements and compliance, building envelope, mechanical systems, and existing buildings, in roughly the same proportion those topics have carried across the eleven lessons. The exam is untimed in the sense that GCHS does not enforce a clock over your shoulder, but it is built around a 120-minute session, and staff should aim for at least 70 percent correct before treating a topic as mastered. This is a GCHS internal practice tool, not an official certification exam of any kind, and passing it does not itself qualify anyone for a code role; it is meant to surface which sections need a second read before a real inspection or plan review. Whatever the practice exam scores, the code actually enforced on a given job is whatever edition and amendments that jurisdiction has adopted, confirmed locally rather than assumed from this course.
1. Under the 2021 IECC, which requirement applies on every compliance path a Gulf Coast house can use: prescriptive (including its total UA alternative), simulated performance, and Energy Rating Index?
2. Under R401.2.5, which statement correctly matches a compliance path to how it satisfies the 2021 edition's additional efficiency requirement?
3. A Zone 2 house is documented on the Energy Rating Index path. Under Table R406.5 and R401.2.5, its maximum allowed ERI score is:
4. Comparing Table R402.1.3 for Zone 2 against Zone 3, which value stays exactly the same across both zones?
5. An inspector confirms a Zone 3 house has slab-edge insulation of R-10 continuous to a depth of 2 feet. Is this compliant with Table R402.1.3?
6. A rater tests a Zone 2 house with a conditioned volume of 16,000 cubic feet and a blower door reading of 1,200 CFM50. What is the ACH50, and does it meet the Zone 2 limit?
7. An energy inspector finds ductwork entirely inside the building thermal envelope on a rehab job. Under the 2021 IECC, is duct leakage testing still required, and at what limit?
8. A supply duct 4 inches in diameter runs through a vented Gulf Coast attic, outside the thermal envelope. Under R403.3.1, its minimum insulation value is:
9. A GCHS rehab extends ducts from an existing heating and cooling system to serve a new room addition. Under R503.1.2 and R502.3.2, the extended ducts are:
10. An inspector finds a Zone 2 attic with R-49 across the flat field, but only R-19 at the last two feet before the eave, with no raised-heel truss or baffle detail present. The correct finding is:
11. A rehab job replaces a home's leaking single-pane windows with new units. Under R503.1.1.1, the replacement fenestration must:
12. During a gut rehab, a contractor opens an existing exterior wall for new wiring, exposing the stud cavity. Under R503.1.1, that cavity:
13. GCHS converts an attached, unconditioned garage into a bedroom tied into the house's HVAC system. Under R501-R505, the converted space must:
14. GCHS air seals and adds attic insulation to a rehab house with a natural-draft gas water heater staying in place. A combustion appliance zone (CAZ) depressurization test is then required by the job's weatherization funding. Which statement best describes this test's status?
15. Under R403.1.1, the default thermostat setpoints for a house's primary heating and cooling system are:
16. An inspector reviews a refrigerant suction line running through a vented Gulf Coast attic with several feet of torn insulation jacket. Under R403.4, the correct finding is:
17. Under the 2021 IECC R404.1, what percentage of permanently installed lighting fixtures must contain only high-efficacy light sources, and which lighting is excepted?
18. A GCHS rehab crew is sequencing an energy inspection on a job with new attic insulation, sealed and insulated ducts, and a full envelope retrofit. Which order best matches the field sequence GCHS follows?
19. A house on the ERI path claims credit for a large photovoltaic array to offset a thin, undersized envelope. Which statement is correct under R406?
20. Which best summarizes why GCHS staff should verify the specific edition and any local amendments enforced by a given Houston-area city before citing a section number to a client?