2021 IECC Chapter C5
Much of GCHS's work is on buildings that already stand: a fellowship hall that needs a roof, a clinic getting new windows, a vacant store becoming a food pantry. Chapter 5 of the 2021 IECC decides how much of the energy code that work pulls in. New work meets the code, old work you leave alone stays as it is, and the project may not make the building less efficient. The chart follows one kind of work at a time. It covers commercial buildings: one- and two-family dwellings, townhouses, and Group R-2, R-3, and R-4 buildings three stories or less above grade are residential buildings, and the residential provisions govern them. Local adoption governs the edition (Lesson 1).
The code cannot be used to force removal or alteration of a lawful existing building, except where Chapter 5 says so (C501.1.1). Section C501.2 sends additions, alterations, repairs, and changes of occupancy to Sections C502 through C505, alongside the existing-building rules of the other model codes. Owners must keep code-required energy systems maintained, and Chapter 5 is never a reason to remove them (C501.3). Everything hangs on three definitions in Chapter 2. An addition increases conditioned floor area, number of stories, or height. A repair renews or reconstructs part of a building to maintain it or correct damage. An alteration is any construction, retrofit, or renovation that is neither of those, including a change in the arrangement, type, or purpose of a system. Sort the job first.
An addition must meet the code as new construction without pulling in the untouched existing building (C502.1). It shows compliance in one of two ways: the addition alone complies, or the existing building and the addition comply together as a single building. New mechanical, water heating, and lighting systems in the addition follow C403, C404, and C405, with C408 commissioning (C502.3.3 to C502.3.6). New glass is checked against the area limits of C402.4.1 for the whole building, and where the total goes over, the addition uses the daylight-control path or a tradeoff path (C502.3.1). A closely related trigger is a change in space conditioning: altering a nonconditioned or low-energy space, such as an open porch, carport, or unheated storage room, so that it becomes conditioned is treated like an addition (C502.2). That work may use component performance (C402.1.5) with a proposed UA up to 110 percent of the target, or total building performance (C407) with annual energy cost up to 110 percent of the allowed cost.
Section C503.1 holds alterations to two tests. The new work meets the code as new construction, and the existing building may not end up less conforming than it was before. Six exceptions apply as long as the building's energy use does not increase: storm windows added over existing windows; surface-applied film on existing single-pane glass; existing ceiling, wall, or floor cavities exposed during construction and filled with insulation; construction where the existing roof, wall, or floor cavity is not exposed; roof recover; and skipping an air barrier for a roof recover or roof replacement when no other envelope work is done. The cavity exception is why insulation goes into a gut rehab: open a stud wall, fill the cavity, and the wall need not meet the full new-construction assembly. Other new envelope assemblies meet C402.1 through C402.5, and a building already over the 30 percent glass limit is exempt from C402.4.1 if the alteration adds no glass (C503.2).
Roof work is the most common trigger, and the words matter.
| Job | What it is | Energy code result |
|---|---|---|
| Roof recover | New covering over the old, no tear-off | Exception in C503.1 if energy use does not rise |
| Roof replacement | Old covering removed, damaged substrate repaired, new covering installed | If the existing insulation is entirely above the deck, comply with C402.1.3, C402.1.4, C402.1.5, or C407 (C503.2.1) |
| Roof repair | Renewing part of a roof for maintenance | A repair under C504.2, and no air barrier if the rest of the envelope is untouched |
Two limits apply to a replacement: the R-value of the roof insulation may never be reduced, and the roof's U-factor may never be increased (C503.2.1). A steep roof insulated at the attic floor is not caught, because its insulation is not entirely above the deck. Worked example. A community hall in Harris County (Climate Zone 2A) has a low-slope roof with R-12 of board insulation above the deck. Tearing the membrane off to the deck makes it a roof replacement. The R-value method in Table C402.1.3 asks for R-25ci above the deck in Zone 2, so the new roof must add 25 minus 12, or 13 more R, unless the designer uses the U-factor, component performance, or total building performance route (Lesson 3).
Where some or all of a fenestration unit is replaced, including sash and glazing, the new product must meet the Table C402.4 U-factor and SHGC (C503.2.2.1). In Zone 2 the vertical limits include fixed U-0.45, operable U-0.60, and entrance doors U-0.77. The exception lets the U-factors of replacement products be area-weighted, but only within one table category, so fixed and operable units are never averaged together. Glass-only replacement in an existing sash and frame is a repair, and replacing an existing exterior door does not require adding a vestibule, though an existing vestibule may not be removed (C504.2).
Worked example. A Group B clinic in Zone 2A replaces fixed units: 180 sq ft at U-0.40 and 80 sq ft at U-0.52. The weighted average is (180 x 0.40 + 80 x 0.52) / 260 = (72 + 41.6) / 260 = 0.437, which meets the fixed limit of 0.45 even though the second group alone does not. A simple average, 0.46, would wrongly fail the job.
New lighting systems in an alteration meet C405 and C408 (C503.5). The exception is replacing less than 10 percent of the luminaires in a space, provided installed interior lighting power does not increase. Replacing only the lamps, the ballasts, or both inside existing luminaires is a repair, again with no increase in installed power (C504.2). Worked example. A fellowship hall has 40 luminaires. Replacing 3 is 7.5 percent, so the exception holds. Replacing 4 is exactly 10 percent, which is not less than 10 percent, so the new lighting must meet the controls and power provisions (Lessons 9 and 10). The count is made per space.
New heating, cooling, and duct systems that are part of an alteration meet C403 and C408 (C503.3), and new cooling systems also meet the economizer provisions of C403.5 (C503.3.1). New service hot water systems meet C404 and C408 (C503.4). A replaced rooftop unit is new equipment that must meet the efficiency and control rules, while ductwork you did not touch stays as it is. Chapter 5 offers no blanket like-for-like exception, so check C403 for built-in exceptions before assuming a swap is free. C408 commissioning rides along (Lesson 12).
In the field
A church hall reroof is permitted as a roof recover. When the crew starts, the old membrane is soaked, so they tear it off to the deck. The job is now a roof replacement, and if the insulation sits above the deck, C503.2.1 applies. GCHS plan review asks for the scope in writing before permit. In the field, photograph the exposed deck and insulation, record the R-value that stays, and note every wall cavity opened during the job, because exposed cavities are filled (C503.1). A change order that adds windows or luminaires changes which sections apply, so it goes back through review.
Work on undamaged parts that is necessary to make a repair counts as part of the repair, not as an alteration, and routine maintenance and ordinary permit-exempt repairs sit outside the repair rules (C504.1). Section C504.2 lists glass-only replacement, roof repair, and lamp or ballast replacement among the repairs. Historic buildings get a separate door. A historic building is one listed or eligible for the National Register, designated under state or local law, or certified as contributing to a historic district (Chapter 2). The provisions are not mandatory if a report is filed with the code official, signed by a registered design professional or a representative of the State Historic Preservation Office or local historic authority, showing that compliance would threaten, degrade, or destroy the building's historic form, fabric, or function (C501.5). Finally, work that complies with ANSI/ASHRAE/IES 90.1 is excepted from Chapter 5 (C501.2), and a building built to that standard has its repairs held to the standard (C504.1).
A change of occupancy is a change of classification, of group, or of use that alters how this code applies (Chapter 2). Under C505.1, a space whose change would increase demand for fossil fuel or electrical energy must comply with the code. Where the use moves from one use listed in Table C405.3.2(1) or (2) to another, the installed lighting wattage must meet C405.3. A space in a building already over the C402.4.1 glass limit is exempt from that limit if glass area does not increase. The same 110 percent caps as in C502.2 apply to the tradeoff paths. Turning a vacant store into a food pantry with walk-in coolers and a commercial kitchen raises electrical demand and changes the lighting allowance, so it triggers both.
1. A church fellowship hall (Group A-3) in Harris County is enclosing a 600 square foot open covered porch as a heated and cooled classroom. Which part of 2021 IECC Chapter 5 governs the new space?
2. A roofer at a Group B clinic in Climate Zone 2A installs a new membrane directly over the existing roof covering without removing it. What does Section C503.1 say about this work?
3. A community hall in Zone 2A has a low-slope roof whose only insulation is board stock entirely above the deck. The crew strips the membrane to the deck, repairs damaged sheathing, and installs a new membrane. What applies under Section C503.2.1?
4. An inspector reviews replacement of complete fixed window units, frame, sash, and glass, at a Group B office in Climate Zone 2A. What must the new units meet under Section C503.2.2.1?
5. A glazier replaces cracked glass panes in the existing aluminum frames of a Group B clinic without changing the sash or frame. How does Section C504.2 treat this work?
6. At a Group B clinic in Zone 2A, replacement fixed windows total 150 sq ft at U-0.40 and 50 sq ft at U-0.55. Table C402.4 limits fixed vertical fenestration to U-0.45. Using the area-weighted average allowed by Section C503.2.2.1, what is the result?
7. A designer averages three fixed windows (U-0.42) and two operable windows (U-0.58) in one weighted calculation for a Zone 2A office and says the result meets the limits. What is the problem under Section C503.2.2.1?
8. A 60-luminaire open office in a Group B building has 5 luminaires replaced with LED units. Installed interior lighting power does not increase. What applies under Section C503.5?
9. An electrician replaces only the lamps and ballasts inside the existing luminaires of a community center gym. No fixtures are replaced, and installed lighting power does not increase. How does Section C504.2 classify this?
10. A rooftop unit serving a Group A-3 community room is replaced with a new unit that includes cooling. The existing ducts stay. Which Chapter 5 sections apply to the new equipment?
11. A 1,200 sq ft conditioned addition is built onto a 1970s Group B office whose envelope does not meet the 2021 IECC. Which statement is correct under Section C502.1?
12. A vacant Group B office suite becomes a food pantry, and the new use raises electrical demand. The designer uses component performance (Section C402.1.5) to comply with Section C505.1. What is the limit on the proposed UA?
13. A Group B building has 2,000 sq ft of above-grade wall and 700 sq ft of glass, which is 35 percent and above the 30 percent limit of C402.4.1. A tenant space in it changes use, and no glass is added. What does Section C505.1 say about the fenestration limit?
14. The owner of a 1920s community center listed on the National Register wants relief from the energy code for window work. What must be submitted to the code official under Section C501.5?