Table C402.1.3 of the International Energy Conservation Code sets the minimum insulation R-value requirements for every opaque component of a commercial building’s exterior envelope — roofs, above-grade walls, below-grade walls, floors, and slab-on-grade perimeters — organized by climate zone and construction type. It does not contain U-factors, C-factors, or F-factors; those sit in Table C402.1.4 as a separate compliance path. If you are using the R-value method, this one table tells you the minimum insulation that must go into each assembly.1International Code Council. 2018 International Energy Conservation Code – C402.1.3 Insulation Component R-Value-Based Method
What the Table Covers
The full title is “Opaque Thermal Envelope Insulation Component Minimum Requirements, R-Value Method.” R-value measures resistance to heat flow, and a higher number means better insulation. Each row of the table corresponds to a different assembly type, and the values apply only to opaque construction. Windows, glass doors, and skylights are governed by separate fenestration requirements and do not appear here.
The assembly categories include:
- Roofs, split into insulation entirely above deck, metal building roofs, and attic-and-other configurations.
- Above-grade walls, broken out by mass walls, metal building walls, metal-framed walls, and wood-framed walls, because each construction type conducts heat differently.
- Below-grade walls, where heat loss into the surrounding soil follows a different pattern than exposed walls.
- Floors, with separate entries for mass floors and joist-framed floors.
- Slab-on-grade foundations, measured at the perimeter where heat escapes along the slab edge.
Every component has to independently meet or exceed its minimum. Extra insulation in the roof does not offset a wall that falls short. Trading surplus performance in one assembly against a shortfall in another is only available through the component performance alternative in Section C402.1.5, not through Table C402.1.3 itself.
How to Read the Entries
The notation catches people out the first time. Entries use the format “cavity R-value + continuous R-value ci.” So “R-13 + R-3.8ci” means R-13 cavity insulation between the studs plus R-3.8 of continuous insulation installed over the framing. “R-0 + R-12ci” means no cavity insulation is required but R-12 continuous insulation is. A standalone “R-20” means R-20 cavity insulation with no continuous layer called out. “NR” means no requirement for that cell.
Cavity and continuous insulation are not interchangeable for compliance. You cannot add R-values across the two categories and call the total compliant. Cavity values count only toward the cavity requirement, and continuous values count only toward the continuous requirement.2International Code Council. 2021 International Energy Conservation Code – Chapter 4 CE Commercial Energy Efficiency Within either category, multiple layers can be summed.
Finding the Right Climate Zone
The columns correspond to IECC climate zones, running from Zone 0 (hottest tropical) to Zone 8 (subarctic). The IECC assigns every U.S. county to a zone in Table C301.1. For locations not listed, the zone is determined using heating and cooling degree days against the thresholds in Table C301.3.3International Code Council. 2021 International Energy Conservation Code – Chapter 3 CE General Requirements
Pick the zone before you touch the table. A project in Zone 2 along the southern coast carries far lower minimums than one in Zone 7 up north, and reading the wrong column either wastes money on unneeded insulation or produces a design that fails plan review.
Representative R-Values Across Zones
Specific numbers shift between editions of the IECC, and states frequently adopt different editions with local amendments. The examples below come from the 2018 IECC and show how requirements scale with climate.
Roofs
Insulation entirely above the deck runs from R-20ci in Zone 1 up to R-35ci in Zones 7 and 8. Metal building roofs call for combinations such as R-19 + R-11 liner system in moderate zones, climbing to R-30 + R-11 in the coldest. Attic-and-other roof assemblies require R-38 through Zone 5 and R-49 in Zones 6 through 8.
Above-Grade Walls
Mass walls, such as concrete block or poured concrete, start at R-5.7ci in Zones 1 and 2 and climb to R-25ci in Zone 8, aided by their thermal mass. Wood-framed walls in Zones 1 through 5 can comply with either R-13 + R-3.8ci or R-20 cavity-only. Zone 8 pushes that up to R-13 + R-15.6ci or R-20 + R-10ci. Metal-framed walls consistently require higher continuous insulation because steel studs conduct heat much more readily than wood.
Floors and Below-Grade Walls
Joist-framed floors over unconditioned space are marked NR in Zones 1 and 2, then jump to R-30 for Zones 3 through 8. Below-grade walls have no requirement through Zone 5, then start at R-7.5ci in Zone 6 and reach R-12.5ci in Zone 8.
When the R-Value Method Isn’t the Right Fit
Section C402.1 gives three compliance paths, and the R-value table is only one of them. The U-factor method in Table C402.1.4 evaluates the whole assembly — framing, sheathing, finishes, and insulation together — rather than the insulation alone.4International Code Council. 2021 International Energy Conservation Code – C402.1.4 Assembly U-Factor, C-Factor or F-Factor-Based Method That path takes more calculation but gives credit for high-performance framing and advanced sheathing, and it is often more natural for slab-on-grade perimeters, which the code addresses through F-factors rather than R-values.
When some assemblies fall short of the table but others exceed it, the component performance alternative in Section C402.1.5 lets the surplus offset the shortfall. The trade-off is calculated using Equation 4-2, which compares the proposed envelope performance against the code-required values and confirms the whole envelope performs at least as well overall. Fenestration must still meet its own solar heat gain coefficient requirements even under this alternative.5International Code Council. 2021 International Energy Conservation Code – C402.1.5 Component Performance Alternative A whole-building energy modeling path is available under Section C407 for designs that none of the prescriptive tables fit.
A project only needs to satisfy one of these paths. If the R-value method works for every assembly, there is no reason to run the U-factor numbers.
Thermal Bridging and Edition Differences
The R-values in Table C402.1.3 describe “clear field” performance, meaning the insulated area between framing members. Steel studs, shelf angles, balcony connections, and other structural elements that penetrate the insulation layer create thermal bridges that the table historically has not captured.
The 2024 IECC adds requirements aimed at these bridges. Its definition of continuous insulation allows point penetrations from fasteners but excludes linear thermal bridges like framing members or Z-furring that pass through the insulation layer. Under the 2024 edition, meeting the R-value in the table is not enough on its own if slab edges, parapets, window-to-wall interfaces, and similar details are not detailed to limit bridging.
Which edition applies depends on what the state or local jurisdiction has adopted. Many areas still enforce the 2018 or 2021 editions, sometimes with local amendments that alter specific values. Confirm the applicable edition and any local modifications before pulling numbers from any version of the table.
Documenting Compliance
Whatever path a project follows, the thermal performance of each opaque assembly has to be shown in the construction drawings submitted for permit review. Building officials check these during plan review before construction, and shortfalls come back as revisions.
The Department of Energy’s COMcheck software is the most widely used tool for producing the paperwork. It supports the IECC, ASHRAE Standard 90.1, and various state energy codes. You enter the envelope components, and the software checks them against the applicable requirements and generates a compliance certificate.6Building Energy Codes Program. COMcheck It is available as both a web application and a downloadable desktop version. Many jurisdictions expect a COMcheck report in the permit package, and including one up front tends to move plan review along faster.