5.3 Floors, Basement Walls & Slab-on-Grade Insulation (R402.2.7–R402.2.9)
Key Takeaways
- Floor cavity insulation must use one of three 2021 methods: permanent contact with the underside of the subfloor (or fill the cavity), cavity insulation on the top of underside sheathing with full-depth perimeter coverage and air-sealed framing, or cavity plus underside continuous insulation whose combined R-value equals the required floor R-value.
- The 2018 table footnote that allowed leftover shallow floor cavities to be filled with a mere R-19 is not in the 2021 IECC; floors must meet Table R402.1.3, using continuous insulation when the cavity cannot hold the required R-value.
- Unconditioned basement walls skip Table R402.1.3 wall insulation only if all six R402.2.8 conditions are met, including an insulated floor and stair stringer, no uninsulated ducts or DHW or hydronic surfaces in the basement, and no HVAC supply or return serving the basement.
- Where basement walls are insulated, insulation is installed from the top of the basement wall down to 10 feet below grade or to the basement floor, whichever is less.
- Slab-on-grade floors with the surface less than 12 inches below grade follow Table R402.1.3; heated slabs add R-5 under the full slab area in addition to the required edge R-value, and heated-slab edge insulation need not extend below the slab.
Floors, Basement Walls, and Slab-on-Grade Insulation
Quick Answer: 2021 IECC floor insulation must stay in permanent contact with the subfloor (or fill the joist cavity), or sit on underside sheathing with full-depth sealed perimeter framing, or combine cavity plus underside continuous insulation to the full Table R402.1.3 floor R-value. Basement walls are insulated to that table unless the basement is unconditioned and every R402.2.8 condition is met. Slab-on-grade floors with the surface less than 12 inches below grade take the table’s slab R-value and depth; a heated slab adds R-5 under the full area on top of the edge requirement.
Floors over garages, basements, and crawl spaces, plus basement walls and slabs, are where envelope hours are won or lost on Exam 79. The 2021 edition also closed a well-known 2018 loophole on shallow floor joists. Candidates who still recite “fill the cavity, R-19 minimum” are answering a retired code.
Floors (R402.2.7)
Floor cavity insulation shall comply with one of the following:
Method 1 — contact with the subfloor, or fill the cavity. Insulation shall be installed to maintain permanent contact with the underside of the subfloor decking in accordance with manufacturer instructions to maintain the required R-value, or readily fill the available cavity space. Batts stapled up with a sagging air gap under the plywood fail permanent contact. Netting or stays that hold the batt against the subfloor meet the contact rule when they match the manufacturer’s instructions. Spray foam that fills the bay also satisfies the fill-the-cavity branch of Method 1, but the installed R-value still has to meet Table R402.1.3 unless Method 3 supplies the missing R with continuous insulation.
Method 2 — insulation on the sheathing that closes the cavity from below. Floor framing cavity insulation may be in contact with the top side of sheathing that separates the cavity from the unconditioned space below. Insulation shall extend from the bottom to the top of all perimeter floor framing members, and those framing members shall be air sealed. This is the “insulate down, sheath the bottom, seal the band” method used over a garage or unconditioned basement when the designer wants a sealed floor air barrier at the sheathing line rather than at the subfloor.
Method 3 — cavity plus underside continuous insulation. A combination of cavity and continuous insulation shall be installed so the cavity insulation contacts the top side of the continuous insulation on the underside of the floor framing. The combined R-value of the cavity and continuous insulation shall equal the required R-value for floors. Insulation shall extend from the bottom to the top of all perimeter floor framing members, and the framing members shall be air sealed.
The retired 2018 leftover-cavity footnote
The 2018 IECC R-value table included a floor footnote that allowed insulation sufficient to fill the framing cavity, R-19 minimum, when the joists were too shallow for the listed floor R-value. That footnote is gone in 2021. Climate Zone 5 and Marine 4 floors are R-30. Climate Zones 7 and 8 are R-38. A 2×8 floor (nominal 7-1/4-inch cavity) that cannot hold R-30 of cavity-only insulation does not get to stop at R-19. The 2021 answer is Method 3: for example R-19 in the cavity plus R-11ci (or another pair that sums to R-30) on the underside, or a deeper joist, or the U-factor path with a complying floor U-factor from Table R402.1.2 (U-0.033 in Climate Zones 5 and 6, U-0.028 in Climate Zones 7 and 8).
| Climate zone | Table R402.1.3 floor R-value |
|---|---|
| 0–2 | R-13 |
| 3 and 4 except Marine | R-19 |
| 5, Marine 4, and 6 | R-30 |
| 7–8 | R-38 |
Basement walls (R402.2.8)
Basement walls shall be insulated in accordance with Table R402.1.3.
| Climate zone | Basement wall R-value |
|---|---|
| 0–2 | 0 |
| 3 | 5ci or 13 |
| 4 except Marine | 10ci or 13 |
| 5 through 8 | 15ci or 19 or 13+5ci |
Footnote f of Table R402.1.3: basement wall insulation is not required in Warm Humid locations as defined by Figure R301.1 and Table R301.1. That is a mapped humidity exception, not a contractor preference.
Unconditioned-basement exception — all six conditions
Basement walls associated with unconditioned basements need not take the table wall R-value where all of the following are met:
- The floor overhead, including the underside stairway stringer leading to the basement, is insulated in accordance with R402.1.3 and applicable provisions of R402.2 and R402.2.7.
- There are no uninsulated duct, domestic hot water, or hydronic heating surfaces exposed to the basement.
- There are no HVAC supply or return diffusers serving the basement.
- The walls surrounding the stairway and adjacent to conditioned space are insulated in accordance with R402.1.3 and applicable R402.2 provisions.
- The door(s) from conditioned space to the basement are insulated in accordance with R402.1.3 and applicable R402.2 provisions, and weatherstripped in accordance with R402.4.
- The building thermal envelope separating the basement from adjacent conditioned spaces complies with R402.4 (air barrier and leakage).
One miss among the six returns the project to insulating the basement walls. A supply boot “just for the laundry,” an uninsulated DHW tank sitting in the open, or a stair stringer with no insulation under the treads is enough.
Installation depth (R402.2.8.1)
Where basement walls are insulated, the insulation shall be installed from the top of the basement wall down to 10 feet (3048 mm) below grade or to the basement floor, whichever is less. An 8-foot basement wall is insulated full height. A deep walk-out or daylight wall that extends 12 feet below grade is insulated 10 feet down, not necessarily to the footer. Insulation that starts a foot below the sill plate, leaving the rim and the upper concrete exposed, does not start at the top of the wall.
Field scenario: unconditioned basement laundry
A Climate Zone 5 two-story lists the basement as unconditioned. The floor above has R-30 batts in permanent contact with the subfloor. Stair walls are R-13+10ci, the basement door is an insulated, weatherstripped unit, and the air barrier at the band joist is sealed. The plans show a washer, dryer, and a 50-gallon electric water heater in the basement, plus a 6-inch supply diffuser “to take the chill off the laundry.”
That diffuser is an HVAC supply serving the basement, which breaks condition 3. The electric water heater is a domestic hot-water surface exposed to the basement; if it is uninsulated beyond its factory jacket in a way the code official reads as an uninsulated DHW surface, condition 2 is also in play. Either defect knocks out the unconditioned-basement wall exception. The inspector’s path is to require Table R402.1.3 basement wall insulation (15ci or 19 or 13+5ci in Climate Zone 5) from the top of the wall down to the floor (the floor is less than 10 feet), or to remove the supply and fully insulate the DHW and hydronic surfaces, restore the six-condition exception, and keep the basement outside the thermal envelope.
A “make-up air” story does not create a seventh exception. If the basement is served by the HVAC system, it is being treated as conditioned space for R402.2.8 purposes.
Slab-on-grade floors (R402.2.9)
Slab-on-grade floors with a floor surface less than 12 inches (305 mm) below grade shall be insulated in accordance with Table R402.1.3.
Exception: Slab-edge insulation is not required in jurisdictions designated by the code official as having a very heavy termite infestation. That is an adopted-jurisdiction designation, not a builder’s soil-report claim at one lot.
| Climate zone | Slab R-value and depth |
|---|---|
| 0–2 | 0 |
| 3 | 10ci, 2 feet |
| 4 except Marine through 8 | 10ci, 4 feet |
A slab 18 inches below grade is outside R402.2.9’s “less than 12 inches below grade” trigger. A slab 8 inches below grade in Climate Zone 5 needs R-10ci for 4 feet of developed distance unless the termite exception applies.
Installation (R402.2.9.1)
Where installed, insulation shall extend downward from the top of the slab on the outside or inside of the foundation wall. Insulation located below grade shall be extended the distance in Table R402.1.3 (or the proposed-design distance, as applicable) by any combination of:
- vertical insulation,
- insulation extending under the slab, or
- insulation extending out from the building.
Insulation extending away from the building shall be protected by pavement or by not less than 10 inches (254 mm) of soil. The top edge of insulation installed between the exterior wall and the edge of the interior slab may be cut at a 45-degree angle away from the exterior wall.
A Climate Zone 4 detail with 2 feet of vertical R-10ci and 2 feet of horizontal R-10ci under the slab meets a 4-foot table depth. Two feet of vertical only does not.
Heated slabs (Table R402.1.3 footnote d)
R-5 insulation shall be provided under the full slab area of a heated slab in addition to the required slab-edge insulation R-value. The slab-edge insulation for heated slabs shall not be required to extend below the slab. Hydronic tubing or electric resistance in the slab is the usual trigger. In Climate Zone 5 that means R-10ci at the edge (depth not required below the slab for the heated-slab edge) plus R-5 under the entire slab. Skipping the under-slab R-5 because the edge is already R-10 is a footnote d fail.
Inspection sequence
- Floors: probe for contact or fill; if Method 2 or 3, look for underside sheathing or ci, full-depth perimeter insulation, and sealed band joists. Reject “R-19 in a 2×8, Climate Zone 5” as a 2018 leftover.
- Basements: decide conditioned versus unconditioned using all six exception items, then measure insulation from the top of the wall to the lesser of 10 feet below grade or the floor.
- Slabs: confirm the surface is less than 12 inches below grade, measure vertical plus under-slab plus outward distance, check 10-inch soil or pavement over outward insulation, and add under-slab R-5 when the slab is heated.
Independent OpenExamPrep commentary for Exam 79 stays with 2021 first-printing residential provisions. Do not bring 2018 floor footnotes into a 2021 answer.
A Climate Zone 5 floor over a garage is framed with 2×8 joists that cannot hold R-30 of cavity insulation. Which 2021 IECC approach satisfies Section R402.2.7?
A Climate Zone 4 basement is declared unconditioned. The floor overhead is insulated, stair walls and the basement door meet Table R402.1.3, and the band joist is air sealed. A washer-dryer laundry with a supply diffuser and an uninsulated hydronic manifold sits in the basement. Do the basement walls require Table R402.1.3 insulation?
A conditioned basement wall extends 12 feet from the top of the wall to the basement floor, and the finish grade is near the top of the wall. How far down must the insulation extend under Section R402.2.8.1?
A Climate Zone 5 house has a hydronic heated slab-on-grade with the top of the slab 4 inches above exterior grade. Besides the Table R402.1.3 edge requirement, what extra insulation does footnote d require?