Columbus and Central Ohio Zone 5A insulation targets under the Ohio code
Franklin, Delaware, Licking, Fairfield, Union, Madison, and Pickaway counties are all Climate Zone 5A in the county table of the Residential Code of Ohio, the code the City of Columbus lists for one-, two-, and three-family buildings along with the 2018 IECC. Hamilton, Clermont, and Brown counties along the Ohio River are Zone 4A. Crossing that line changes more than most people expect and less than some marketing claims: attic ceilings are R-49 in both zones, while floors, masonry walls, windows, and unvented roofs all change.
The table lists the prescriptive values for new construction and additions from Table 1102.1.2, the existing-building alternative from Table 1107.4.1 where it differs, and the related Zone 5 rules from the chapters on roofs, walls, and foam plastic. Use it as a reference alongside the building department that permits your job. Alterations are generally held to new-construction values only for the parts being altered.
| Component | Zone 5A value, new construction | Existing-home alternative, Zone 4 contrast, and related rules |
|---|---|---|
| Attic ceiling | R-49 | Existing-building table: R-38. R-38 also satisfies R-49 wherever its full height extends over the wall top plate at the eaves. Same R-49 as Zone 4. Energy Star voluntary: R-60 for an uninsulated attic. |
| Vaulted ceiling with no attic space | R-30 permitted where R-49 does not fit | Limited to 500 square feet or 20 percent of the insulated ceiling area, whichever is less (section 1102.2.2). |
| Wood-frame exterior wall | R-20 cavity, or R-13 cavity plus R-5 continuous | Existing-building table keeps the same Zone 5 value; Zone 4 existing homes list R-13. Class I or II vapor retarder on the interior side (section 702.7), with Class III allowed behind vented cladding. |
| Mass wall (solid brick, block) | R-13 outside, or R-17 when more than half is inside | Zone 4 lists R-8 or R-13. Historic buildings may set energy provisions aside under section 1107.6 when compliance would damage historic fabric. |
| Floor over unconditioned space | R-30, or insulation filling the cavity at R-19 minimum | Zone 4: R-19. Floor insulation must stay in permanent contact with the underside of the subfloor (section 1102.2.8). Covers floors over garages and vented crawls. |
| Basement wall | R-10 continuous or R-13 cavity | Same Ohio value as Zone 4. Required from the top of the wall down to 10 feet below grade or the basement floor (section 1102.2.9). Energy Star voluntary for Zone 5: R-15 continuous or R-19 batt. |
| Crawl space wall, unvented crawl | R-10 continuous or R-13 cavity | Runs down to grade and then 24 inches farther; exposed earth covered with a Class I vapor retarder, joints lapped 6 inches and sealed (section 1102.2.11). |
| Slab on grade | R-10, 2 feet deep | Heated slabs add R-5 under the full slab area. Same as Zone 4. |
| Windows | U-0.30 maximum, no SHGC limit | Zone 4: U-0.32 with SHGC 0.40. Existing-building table: U-0.35. |
| Air leakage, new homes | 5 ACH50 maximum, tested | RESNET/ICC 380, ASTM E779, or ASTM E1827, with a signed report to the building official (section 1102.4.1.2). Whole-house mechanical ventilation is mandatory (section 1103.6). |
| Unvented roof, foam under batts or loose fill | R-20 minimum foam layer against the deck | Zone 4: R-15. In Zone 5 the air-impermeable insulation must be a Class II vapor retarder or carry a Class II coating (section 806.5). |
| Rim joist foam left uncovered | Up to 3-1/4 inches, 0.5 to 2.0 lb per cubic foot | Flame spread index 25 or less and smoke-developed index 450 or less (section 316.5.11). Thicker foam or finished space needs a thermal barrier. |
| Columbus design data | 5,660 heating degree days, 5 degree winter design temperature | From Table 301.2(1). The same table lists Cincinnati at 4,410 heating degree days and 6 degrees. |
Three practical notes for Central Ohio projects. First, retrofits: section 1109 holds altered assemblies to the new-construction table, but filling existing cavities that are exposed during the work, or work that leaves cavities unexposed, is an accepted exception as long as energy use does not rise, and the existing-building table offers an alternative with an R-38 attic. Second, a space that changes from unconditioned to conditioned, such as a finished attic, a converted garage, or a heated outbuilding on a residential lot, must be brought into full compliance with the energy chapter. Third, code values are floors. Energy Star recommends up to R-60 for an uninsulated Zone 5 attic and R-15 continuous or R-19 cavity for basement and crawl walls, and a house tested well under 5 ACH50 needs deliberate fresh air.
For how these values play out on real houses, see what Zone 5A means for a Columbus home, the county reference, and the attic service page.
Data sources
- Residential Code of Ohio, Chapter 11 Energy Efficiency (OAC 4101:8-11-01)
- Residential Code of Ohio, Chapter 8 Roof-Ceiling Construction (OAC 4101:8-8-01)
- Residential Code of Ohio, Chapter 3 Building Planning (OAC 4101:8-3-01)
- City of Columbus Building and Zoning Services: codes in effect
- Energy Star recommended home insulation R-values
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