2021 IECC Sections R501-R505
Every lesson up to this point assumed new construction. Most of GCHS's own work is additions, remodels, and repairs to houses that are decades old. R501 through R505 tell an inspector or designer how much of the energy code applies to work on an existing house, and the answer depends entirely on what kind of work it is. Getting this right cuts both ways: applying full new-construction requirements to a simple repair asks for more than the code requires, while missing a change in space conditioning lets a converted garage go into service without the insulation and air sealing it needs.
R501.1.1 sets the governing principle for this whole chapter: the code is not used to force removal or alteration of an existing building lawfully in place, and unaltered portions of the existing building are not required to comply. R501.2 then sends additions, alterations, repairs, and changes of occupancy to R502 through R505. Replacing a furnace does not obligate an owner to re-insulate the attic. Re-siding a house does not, by itself, require new windows. The chapter's five sections then work out exactly what "complies" means for each kind of work, because an addition, an alteration, a repair, and a change of occupancy are held to different standards. The 2021 code also says that changing unconditioned space into conditioned space is handled under the addition section, R502 (R501.2).
R502.1 gives a designer a choice for a new addition. The addition can comply alone, as a stand-alone building meeting current values and mandatory provisions on its own, with no credit from and no obligation on the existing house. Or the existing building and the addition can comply together as a single building, most often through a UA calculation or an energy model, letting a particularly efficient addition offset a weaker existing envelope. The third route is to show that the building with the addition does not use more energy than the existing building did. The combined routes take more analysis, not less; a designer reaches for them when the stand-alone addition genuinely cannot carry itself.
When an addition follows the prescriptive route (R502.3), its new envelope assemblies meet R402.1, R402.2, R402.3.1 through R402.3.5, and R402.4, with one exception: new envelope assemblies in an addition are exempt from the R402.4.1.2 blower door test. New ducts, service hot water systems, and lighting in the addition meet R403, R403.5, and R404.1, except that ducts extended from an existing system to an addition are excused from R403 (R502.3.2).
R503.1 sets the alteration rule: the work being done must meet the requirements for new construction, and the altered building may not conform less to the code, or use more energy, than it did before. R503.1.1 building envelope carries a specific list of exceptions, available where the building's energy use is not increased, that excuse full compliance as impractical or destructive relative to the value of the work: storm windows over existing fenestration, surface-applied window film on existing single-pane glazing, roof recover, and construction where an existing wall, floor, or roof cavity is simply not exposed. The mirror image of that last exception is the rule an inspector checks most in a rehab gut job: where the work does expose an existing cavity, that cavity must be filled with insulation before it is closed back up. Likewise, a roof with no insulation in its cavity whose sheathing or insulation is exposed during reroofing must be insulated above or below the sheathing. New lighting in an alteration meets R404.1, except that replacing fewer than 10 percent of the luminaires in a space, with no increase in installed lighting power, is excused (R503.1.4).
R503.1.1.1 singles out replacement fenestration for its own rule, because a window replacement is close to new construction in a way that re-siding an intact wall is not: it is a new product, chosen from a catalog of rated products, installed without having to open up anything else. Where some or all of an existing fenestration unit is replaced with a new fenestration product, sash and glazing included, the replacement unit must meet the current U-factor and SHGC for the climate zone. On the Gulf Coast that means U-0.40 and SHGC 0.25 or lower, the same Zone 2 prescriptive values from Lesson 3. Where several new units are installed, an area-weighted average U-factor, SHGC, or both may be used, the same calculation covered in Lesson 3's worked example. The narrower repair, glass-only replacement within an existing sash and frame, is discussed below.
The strongest trigger in this chapter has nothing to do with square footage or cost: it is turning unconditioned space into conditioned space. An attached garage tied into the house's HVAC system, an attic finished into a conditioned home office, a porch enclosed and heated and cooled, all move the thermal envelope boundary. R502.2 says any unconditioned or low-energy space altered to become conditioned space must be brought into full compliance with the code: walls, ceiling, floor, air sealing, and any new fenestration all have to meet current requirements. R502.2 offers three alternatives to meeting every requirement one by one. The performance path may be used with an annual energy cost up to 110 percent of the normally allowed cost. The total UA of the existing building, addition, and alterations together may not exceed the existing building's total UA. Or the annual energy cost or energy use of the whole project may not exceed that of the existing building, provided the addition and alterations comply with R405 in its entirety.
In the field
Garage conversions are the single most common change-in-conditioning project GCHS reviews, and the best time to catch one is at plan review, not at final inspection. By the time drywall is up, the floor, the new infill walls where the overhead door used to be, the ceiling below the attic, and the air sealing at the shared wall to the house are all buried. A reviewer who flags the conditioning change on the plans gets a conversation about insulation and testing; an inspector who finds it after the fact is looking at a partial teardown. The habit to build is simple: any time a floor plan shows a garage, porch, or attic becoming a bedroom, office, or bonus room, ask what is conditioning it before approving anything else on the sheet.
R504 treats a repair as narrower than an alteration: the code defines a repair as the reconstruction or renewal of part of an existing building to maintain it or to correct damage, without an obligation to bring the repaired assembly up to current requirements. R504.2 names three cases: glass-only replacements in an existing sash and frame, roof repairs, and replacing only the bulb, ballast, or both within existing luminaires without increasing installed lighting power. A cracked window pane replaced in its existing sash and frame is a repair, not a fenestration replacement, because no new sash or frame was installed. Patching an accidentally torn air barrier during unrelated work is a repair for the same reason: it restores what was there. What a repair may not do is use its own narrower obligation as cover to strip out energy features that were required when they were installed (R501.3 says Chapter 5 gives no basis for removing energy conservation systems); an exposed insulated cavity that gets closed back up empty during a repair is not a repair anymore, it is an uncorrected alteration.
R505.1 sets a single test for when a change of use pulls a space into full compliance: any space that is converted to a dwelling unit, or a portion of one, from another use or occupancy must comply with the code. Converting an unconditioned storage building into a dwelling triggers compliance, and where the performance option is used the annual energy cost is allowed to run up to 110 percent of the normal limit. Converting a heated workshop into unheated cold storage creates no dwelling space, so R505 has nothing to say about it. A duplex merged into a single-family home is not a space converted from another use, so it is handled under the ordinary alteration rules instead. R505.1.1 adds that any unconditioned or low-energy space altered to become conditioned space complies with R502.
R501.6 relaxes the energy provisions for a historic building: they are not mandatory, provision by provision, where a report submitted to the code official, and signed by the owner, a registered design professional, or a representative of the State Historic Preservation Office or the local historic preservation authority, demonstrates that compliance with that provision would threaten, degrade, or destroy the historic form, fabric, or function of the building. This is not a blanket exemption for anything labeled historic; it requires a specific, documented showing for the specific provision at issue.
A Zone 2 house gets 20 square feet of replacement fenestration rated at U-0.32 and 10 square feet of a smaller replacement clerestory unit rated at U-0.55, chosen for a specific daylighting effect. The area-weighted average U-factor is (20 times 0.32, plus 10 times 0.55) divided by 30 total square feet: (6.4 plus 5.5) divided by 30, or 11.9 divided by 30, which comes to about 0.397. Because 0.397 is at or below the Zone 2 maximum of 0.40, the combination passes even though the smaller unit alone would not have. The same averaging works the other direction: pairing one badly underperforming unit with several very efficient ones can still fail if the weaker unit carries too much of the total area.
A useful ladder for field use: a repair generally triggers nothing beyond restoring what was there; an alteration requires the altered portion to comply, subject to the exceptions above; and a change in space conditioning, or a space converted to a dwelling unit from another use, requires full compliance for the affected space. Most disputes here come from misclassifying which rung a project sits on, not from misreading the numbers once the classification is right.
1. Under R501.1.1, when an alteration is made to an existing building, the code generally requires that:
2. Under R502.1, an addition to an existing building is deemed to comply where:
3. Which of the following is a recognized R503.1.1 exception that relieves an alteration from meeting current envelope requirements?
4. During a remodel, a contractor opens an existing exterior wall and the stud cavities are exposed. Under R503.1.1, those cavities:
5. An existing window unit is replaced entirely, frame, sash, and glazing, in a Climate Zone 2 house. Under R503.1.1.1, the new unit must:
6. A broken pane of glass is replaced in an existing wood sash, with the frame and sash left in place. This work is best treated as:
7. An attached, unconditioned garage is converted into a bedroom and connected to the house's HVAC system. The converted space must:
8. An unconditioned attached garage is converted into a bedroom. Which alternative to meeting every new-construction requirement one by one does R502.2 allow?
9. Under R504, which statement about repairs and existing energy features is most accurate?
10. Under R505.1, the trigger for the change-of-occupancy provisions is:
11. A heated workshop is converted to unheated cold storage. Under R505.1:
12. A designated historic building is being altered. Under R501, the energy provisions:
13. A Zone 2 addition installs 20 square feet of U-0.32 fenestration and 10 square feet of U-0.55 fenestration. What is the area-weighted average U-factor, and does it meet the Zone 2 maximum of 0.40?
14. Which is the correct order of increasing energy-code obligation for existing-building work?