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2021 IECC Climate Zone 1A Very Hot Humid

See Section R402 of the 2021 IECC for thermal envelope requirements. The table below summarizes the prescriptive R-value requirements for ceilings, walls, and foundations. Alternative compliance approaches are available, such as prescriptive U-factors (tabulated values) or a total UA calculation method. Specific additional requirements and alternative insulation options are detailed for each building component in Section R402.

Table 1.  2021 IECC Ceiling, Wall, and Foundation Prescriptive R-value Requirements
(extracted from 2021 IECC Table R402.1.3; clarifying notes and restrictions apply)

Ceiling R-value Wood Frame Wall R-Value Mass Wall R-Value Floor R-Value Basement Wall R-Value Slab R-Value and Depth Crawlspace Wall R-Value
R-30 R-13 cavity, or
R-0 cavity + R-10 continuous
R-3 on exterior, or
R-4 on interior
R-13 R-0 R-0 R-0

See Section 702.7 of the 2021 IRC for vapor retarder requirements for walls. The tables below summarize the requirements for Climate Zone 1A.

Table 2.  2021 IRC Vapor Retarder Materials and Classes
(extracted from 2021 IRC Table R702.7 (1))

Class Perm Rating Example Acceptable Materials
I 0 - 0.1 perms Sheet polyethylene; nonperforated aluminum foil
II 0.1 - 1 perms Kraft-faced fiberglass batts; vapor retarder paint (applied per manufacturer's instructions to achieve the required perm rating)
III 1 - 10 perms Latex or enamel paint (applied per manufacturer's instructions to achieve the required perm rating)

Table 3.  2021 IRC Options for Vapor Retarders on the Interior Side of a Wall
(extracted from 2021 IRC Table R702.7 (2); clarifying notes and restrictions apply)

Requirement Class I vapor retarder Class II vapor retarder Class III vapor retarder
Interior vapor retarder not required Not permitted Not Permitted Permitted

See Section R806 of the 2021 IRC for roof ventilation and condensation control requirements for roof and attic assemblies. Note that vapor diffusion ports may be required for condensation control in some assemblies. Similarly, air-impermeable insulation such as rigid foam board installed to the exterior of air-permeable insulation may be required in some assemblies to control condensation.