7.1 Table R402.4.1.1: Ceilings, Walls, Rim Joists & Floors

7.1 Table R402.4.1.1: Ceilings, Walls, Rim Joists & Floors

Quick Answer: Table R402.4.1.1 is a two-column checklist. A continuous air barrier is required; breaks and joints shall be sealed; air-permeable insulation shall not be used as a sealing material. Dropped ceilings and soffits must keep the air barrier and the insulation in the same plane, with gaps sealed. Walls seal the foundation-to-sill, the top plate, and knee walls, and they fill corners and headers with a material of at least R-3 per inch. Rim joists need an exterior air barrier, sealed rim-to-sill and rim-to-subfloor junctions, and insulation in permanent contact with the exterior rim board. Footnote b states that full enclosure is not required at rim joists or in unconditioned/ventilated attics.

Exam 79's air-leakage-and-moisture task is only 4 percent of the outline, but Table R402.4.1.1 is item-dense. Plans examiners look for an air-barrier note on the wall and roof sections. Field inspectors walk the same assemblies with a flashlight at the sill, the rim, and every dropped soffit. Independent OpenExamPrep material for ICC Exam 79 covers the 2021 IECC residential provisions (first printing). A public code viewer may sit under a state title; do not apply that state's energy-subcode amendments on the national exam.

R402.4.1.1 Installation

Section R402.4 requires the building thermal envelope to limit air leakage. The thermal envelope shall comply with R402.4.1.1 through R402.4.1.3. Sealing methods between dissimilar materials shall allow for differential expansion and contraction. Rigid foam butted to concrete without a flexible sealant is a materials fail even if both pieces look "tight."

Components indicated in Table R402.4.1.1 shall be installed in accordance with the manufacturer's instructions and the criteria in that table, as applicable to the method of construction. Where required by the code official, an approved third party shall inspect all components and verify compliance. Footnote a: inspection of log walls shall be in accordance with ICC 400.

The table title in 2021 is Air Barrier, Air Sealing and Insulation Installation. Every row has an air barrier cell, an insulation installation cell, or both. A house can have the right R-value from Table R402.1.3 and still fail this table.

General requirements

Fiberglass batts, blown cellulose, and mineral wool are insulation. They are not caulk. Stuffing a leftover batt into the gap between a rim board and a sill plate does not seal that joint. Closed-cell spray foam can serve as a sealing material when it is used that way and installed per its listing. A kraft-faced batt cannot.

Ceiling and attic

Air-barrier criteria

• The air barrier in any dropped ceiling or soffit shall occupy the same plane as the insulation, and any gaps in the air barrier shall be sealed.
• Access openings, drop-down stairs, or knee-wall doors to unconditioned attic spaces shall be sealed.

Insulation-installation criterion

• The insulation in any dropped ceiling or soffit shall occupy the same plane as the air barrier.

Those two sentences are a pair. If drywall drops for a kitchen soffit while the blown cellulose stays on the attic floor above, the air barrier and the insulation occupy different planes. Both columns fail.

Field scenario: dropped kitchen soffit

A Climate Zone 5 kitchen has a 12-inch dropped soffit for recessed lights and a range hood. The contractor blows the ceiling R-value across the attic floor and leaves the soffit cavity open to the attic. The interior gypsum follows the soffit. The insulation does not. Attic air then washes the back of the kitchen drywall, and the unsealed soffit cheeks become bypasses.

Two repairs satisfy the table:

1. Carry a sealed air barrier (gypsum, rigid board, or an evaluated membrane) up the soffit cheeks to the attic-floor plane so it meets the insulation, and seal every gap.
2. Insulate the soffit itself and keep the air barrier at the soffit — same plane — so that location's thermal envelope is the soffit, not the attic floor above it.

Access hatches, pull-down stairs, and knee-wall doors into an unconditioned attic are openings in the same ceiling air barrier. They shall be sealed. Hatch R-value and weatherstripping details live in R402.2.4; this table is the air-seal requirement for those openings.

Walls

The wall air-barrier rows are three separate sentences:

1. The junction of the foundation and sill plate shall be sealed.
2. The junction of the top plate and the top of exterior walls shall be sealed.
3. Knee walls shall be sealed.

A foam sill gasket that is missing at a garage-to-house mudsill, a top plate that was never sealed to the sheathing or to the ceiling air barrier, and an attic knee wall with only faced batts and no air barrier are three different fails.

Insulation-installation criteria

• Cavities within corners and headers of frame walls shall be insulated by completely filling the cavity with a material having a thermal resistance of not less than R-3 per inch.
• Exterior thermal envelope insulation for framed walls shall be installed in substantial contact with the air barrier and shall follow that barrier without gaps.

An empty 2×6 corner, a double header with an open void, or leftover batt scraps that do not fill the header fail the R-3 per inch complete-fill rule. Rigid foam, dense-packed cellulose, or spray foam that actually fills the cavity at that R-value or better is the intended product class. "Substantial contact" means the cavity insulation is not hanging off the interior gypsum while the exterior air barrier sits two inches away across an empty gap.

Windows, skylights, and doors

The space between framing and skylights, and the jambs of windows and doors, shall be sealed. The insulation-installation cell is an em dash. Low-expanding foam or backer rod and sealant at the rough opening is the usual method. High-expansion foam that bows a vinyl jamb is a manufacturer's-instructions fail and a leak path once the frame distorts. Fenestration product leakage ratings (0.3 / 0.5 cfm/ft²) are R402.4.3 in Chapter 6; this row is the rough-opening seal.

Rim joists

Footnote b applies here: air barrier and insulation full enclosure is not required in unconditioned/ventilated attic spaces and at rim joists.

Footnote b is the exam trap. It does not delete the exterior air barrier, the two junction seals, or the insulation-in-contact rule. It says you do not have to wrap the rim in a six-sided insulated, air-sealed box the way a floor cavity over a garage may be enclosed. A vented attic similarly does not need insulation fully enclosed by an air barrier on all sides.

Floors, including cantilevers and floors above garages

Air barrier: the air barrier shall be installed at any exposed edge of insulation.

Insulation installation — one of these:

1. Floor framing cavity insulation shall maintain permanent contact with the underside of subfloor decking, or
2. Floor framing cavity insulation shall be in contact with the top side of sheathing, or continuous insulation, installed on the underside of floor framing and extending from the bottom to the top of all perimeter floor framing members.

A cantilever with batts that sag onto a soffit panel, leaving a void under the subfloor, fails option 1 unless the builder is using option 2 and that underside sheathing or continuous insulation reaches every perimeter member. The exposed edge still needs an air barrier. The same pair of rules applies above an unconditioned garage.

Basement, crawl space, and slab foundations

The table writes "Class I" on the crawl-earth row and "Class 1" on the below-grade-wall row; both mean Class I. Polyethylene on a below-grade concrete wall can be a vapor retarder under IRC R702.7 and still cannot be counted as that wall's air barrier. In an unvented crawl, the Class I covering on exposed earth is both vapor retarder and air barrier, with the R402.2.10 overlap, tape, and 6-inch stem-wall turn-up rules from Chapter 5.

Inspection sequence

1. Confirm a continuous air barrier on the drawings and that air-permeable insulation is not specified as the sealant.
2. At rough-in, check the sill-plate seal, the top-plate seal to the exterior wall, the knee-wall air barrier, and both rim-board junctions.
3. Look into corners and headers for a complete fill at R-3 per inch or better, in substantial contact with the air barrier.
4. Stand under cantilevers and garage ceilings and confirm either subfloor contact or underside sheathing/ci that reaches every perimeter member, plus an air barrier at exposed edges.
5. In the crawl or basement, confirm the earth Class I covering where required, sealed concrete penetrations, and that below-grade polyethylene is not being sold as the wall air barrier.

Independent OpenExamPrep study material for Exam 79 treats Table R402.4.1.1 as a field checklist, not a suggestion. Shafts, tubs, boxes, boots, and sprinklers are the next section.