2021 IECC Section R402.2
Lesson 2 covered how much insulation Table R402.1.3 requires at each envelope component. This lesson covers how that insulation actually has to be installed for it to deliver the R-value on the label, component by component: ceilings under vented and unvented attics, the eave detail that protects both ventilation and insulation at once, attic access openings, mass walls, floors over unconditioned space, foundation and slab-edge insulation, and the documentation that lets an inspector verify what was installed after the drywall goes up.
Where a ceiling separates conditioned space from a vented attic, the required R-value from Table R402.1.3 applies to the flat, full-depth field of the attic, R-49 across most of Climate Zone 2. The code allows one narrow substitution tied to the eaves: R-38 installed over 100 percent of the ceiling area satisfies an R-49 requirement, but only where the full, uncompressed R-38 depth extends over the wall top plate at the eaves (typically with a raised-heel truss); in the same way, where a colder zone requires R-60, R-49 installed over the whole ceiling satisfies it when full R-49 depth reaches the top plate. That substitution depends on the eave detail, does not apply under the total UA alternative, and is not a general license to underinsulate part of the attic, and an inspector who finds R-49 across most of the attic but only R-19 at the last two feet before the eave, with no raised-heel truss or baffle detail to justify it, has found a real deficiency rather than an acceptable average.
Air-permeable insulation, blown or batt fiberglass and cellulose, has to be protected at the eave (R402.2.3) by a baffle installed adjacent to the soffit and eave vents that extends over the top of the insulation. The baffle keeps a net free vent opening at least as large as the vent, can be any solid material, sits at the outer edge of the top plate to leave the most room for insulation over it, and runs continuously where soffit venting is not continuous. The baffle does two jobs at once: it keeps the insulation from spilling into and blocking the soffit vent opening, and it stops incoming ventilation air from wind-washing across the insulation surface and stripping away its effective R-value right where the attic is already shallowest. An inspector who finds blown insulation covering the soffit vents has almost always found a missing or ineffective baffle as well, and both attic ventilation and insulation performance are compromised at the same time.
Access hatches and doors between conditioned space and an unconditioned attic or crawl space, whether a scuttle hole or a hatch, must be insulated to the same R-value the table requires for the ceiling or wall they sit in, and weatherstripped (R402.2.4 and R402.2.4.1). Where loose-fill insulation is used, a dam or retainer has to keep it from spilling into the living space or from higher to lower sections of the attic. Horizontal pull-down stair hatches in Zones 0 through 4 get a narrow exception: an average U-factor of 0.10 or an average R-10, at least 75 percent of the panel at R-13 or better, a framed opening no larger than 13.5 square feet, and a weatherstripped edge, and that exception is not available under the total UA alternative. An uninsulated, unsealed pull-down stair is otherwise a large permanent hole in an already well-insulated ceiling. A conditioned, unvented attic changes where that boundary sits entirely: the thermal and air barrier move from the ceiling plane up to the roof deck, the attic volume becomes part of the conditioned space, and any ducts run through that attic are now inside the envelope rather than outside it. The most important thing an inspector checks in that configuration is that the insulation and air barrier are actually continuous at the roof deck, rather than mixed with leftover ceiling-plane air sealing or ridge and soffit vents that were never closed off. A knee wall between a conditioned second-floor room and an unconditioned attic space is exterior wall in every sense that matters, conditioned on one side and not on the other. Table R402.4.1.1 says knee walls shall be sealed, and wall insulation has to sit in substantial contact and continuous alignment with the air barrier, so the knee wall needs both insulation and a continuous air barrier on the attic-facing side; insulation alone with no backing air barrier is one of the most common energy defects found in story-and-a-half houses.
Concrete, masonry, and other mass wall assemblies get their own row in Table R402.1.3, written as a fraction such as R-8/13 (Zone 3) rather than a single number. The lower number applies to insulation placed on the exterior of the mass or integral to it; the higher number applies where more than half the insulation ends up on the interior side of the mass wall (Table R402.1.3 note h). R402.2.5 counts as mass wall above-grade concrete block, concrete, insulated concrete forms, masonry cavity, brick but not brick veneer, adobe, and similar walls, or any wall with a heat capacity of at least 6 Btu/ft2-F. Interior insulation cuts the thermal mass off from the conditioned space and largely eliminates the benefit the mass otherwise provides in moderating temperature swings, so the code requires more R-value to compensate when insulation is placed that way.
Floor cavity insulation over an unconditioned crawl space or garage has to be installed under R402.2.7 so that it maintains permanent contact with the underside of the subfloor decking above it; the code's two alternatives put the insulation against the top side of the sheathing below, or combine cavity and continuous insulation, with the perimeter framing insulated and air sealed either way. An air gap between the top of the batt and the subfloor lets air circulate across the top of the insulation and bypass it almost entirely, which is why a batt that has sagged, been compressed by ductwork, or was never strapped up tight in the first place is one of the most common floor-insulation defects a GCHS inspector finds in older houses being brought up for rehab.
Climate Zone 2 requires no insulation at basement walls, slabs, or crawl space walls under the prescriptive table; that changes at Zone 3, where slab-edge insulation of R-10 continuous is required to a depth of 2 feet, and the depth requirement grows to 4 feet in Zone 4 and colder. Where rigid exterior insulation is used on a foundation wall, it needs a rigid, opaque, weather-resistant protective covering that extends at least 6 inches below grade (R303.2.1), because exposed foam degrades quickly in sunlight and is easily damaged by string trimmers, mowers, and ordinary foot traffic right at the grade line. One assembly is explicitly exempt from foundation insulation regardless of climate zone (R402.2.11): the horizontal ledge of a foundation that supports a masonry veneer, since insulating that bearing surface would interfere with both structural support and drainage for no meaningful thermal benefit. A thermally isolated sunroom or heated garage gets relaxed minimums under R402.2.12: a ceiling of R-19 in Zones 0 through 4 and walls of R-13 (and, under R402.3.5, fenestration up to U-0.45 and skylights up to U-0.70 in Zones 2 through 8). The relief applies only where the space is thermally isolated, which the code defines as physically and conditioning separated from the house, with its own heating and cooling zone or its own equipment; a sunroom sharing the main HVAC system with no separating doors or walls is simply a room of the house and has to meet the ordinary Table R402.1.3 values instead.
In the field
On a GCHS attic top-up job, the crew almost always finds the same three problems together: insulation compressed or missing where old ductwork crosses the joists, no baffle at one or more eaves so blown insulation has drifted into the soffit, and a pull-down attic stair with no insulation or weatherstripping at all. None of the three is expensive to fix compared with the ceiling field itself, but all three have to be caught before the new insulation goes down, since covering a wind-washed eave or a bypassing attic stair with fresh blown insulation on top just hides the defect instead of curing it.
A nominal R-value on a bag or a roll assumes the insulation is installed in substantial contact with a continuous air barrier, filling the cavity completely, with no gaps, voids, compression, or misalignment around wiring, plumbing, or electrical boxes. Even a small uninsulated gap disproportionately increases heat flow through an assembly, because heat takes the easiest path available rather than an average of the whole cavity, which is why inspectors evaluate installation grade and not just whether insulation is present. R303.1.1 requires the manufacturer to mark the R-value on each piece of insulation 12 inches or more wide, or the installer to provide a certification of the type, manufacturer, and R-value installed in each envelope element. For blown-in fiberglass and cellulose the certification adds the initial and settled thickness, settled R-value, density, coverage area, and number of bags, and for attic insulation R303.1.1.1 adds depth markers, at least one for every 300 square feet, facing the attic access. The installer signs, dates, and posts the certification in a conspicuous location on the job site, and R303.1.2 has the installer leave it in a conspicuous place in the building where the material carries no readable mark. Insulation with no visible marking and no certificate gives an inspector nothing to verify against, and the code puts the burden of documenting it on the installer rather than accepting a verbal assurance.
A worked example on the eave substitution: a Zone 2 attic requires R-49 across the flat field. At the eave, truss heel height limits the vertical clearance once the baffle is in place. Where the full, uncompressed R-38 depth extends over the top plate at that location, the code deems that detail to satisfy the R-49 requirement at the eave, provided the baffle keeps the fill from being compressed by the roof sheathing above it and the flat field beyond the eave still reaches the full R-49.
1. Under R402.2.1, full uncompressed R-38 insulation extending over the wall top plate at the eaves is deemed to satisfy a ceiling requirement of:
2. In a vented attic insulated with blown fiberglass, what must be installed at the eaves?
3. An inspector finds blown insulation covering the soffit vents in a vented attic. This is a defect because it:
4. A scuttle-hole attic hatch in a Zone 2 ceiling that requires R-49 must be:
5. In an unvented, conditioned attic, what is the most important thing the energy inspector must verify?
6. A knee wall separates a conditioned second-floor room from an unconditioned attic. That knee wall must have:
7. A mass wall entry in Table R402.1.3 is written as a fraction such as R-8/13. The higher number applies where:
8. Floor insulation over an unconditioned crawl space must be installed so that it:
9. Exterior insulation on a foundation wall must be protected by a rigid, opaque, weather-resistant covering extending at least how far below grade?
10. Under the 2021 IECC, the prescriptive slab-edge insulation requirement for Climate Zone 2 is:
11. Insulation is NOT required on which part of a foundation, regardless of climate zone?
12. An inspector is reviewing a thermally isolated sunroom on a Zone 2 house. Its ceiling and wall insulation should be checked against:
13. A sunroom shares the main house's HVAC system and has no separating doors or walls. Its envelope must:
14. When installed insulation carries no legible R-value marking, R303.1.1 requires which party to provide a certification of what was installed?