5.2 Wood-Frame, Mass & Steel-Frame Walls (R402.2.5–R402.2.6)
Key Takeaways
- A mass wall is an above-grade wall of concrete block, concrete, ICF, masonry cavity, brick (not brick veneer), adobe, compressed earth block, rammed earth, solid timber, mass timber, or solid logs, or any wall with heat capacity of at least 6 Btu/ft²·°F.
- The second R-value in Table R402.1.3 for mass walls applies when more than half of the insulation is on the interior of the mass wall.
- Steel-frame ceilings, walls, and floors must meet Table R402.2.6 equivalent R-values or Table R402.1.2 U-factors, and any U-factor calculation for steel framing must use a series-parallel path method.
- Where wood-frame walls require R-13+10, Table R402.2.6 at 16 inches on center lists steel equivalents of R-0+20, R-13+15, R-15+14, R-19+13, and R-21+13.
- Wood-frame walls in Climate Zones 4 through 8 use Table R402.1.3 options of R-30, R-20+5ci, R-13+10ci, or R-0+20ci; cavity R-values and continuous R-values are not interchangeable under R402.1.4.
Wood-Frame, Mass, and Steel-Frame Walls
Quick Answer: Wood-frame walls use the Climate Zone row in Table R402.1.3 (for example R-13 or R-0+10ci in Climate Zones 0–2, and R-30 or R-20+5ci or R-13+10ci or R-0+20ci in Climate Zones 4–8). Mass walls are the heavy assemblies listed in R402.2.5, or any wall with heat capacity of 6 Btu/ft²·°F or more; the second table R-value applies when more than half the insulation is on the interior. Steel-frame walls, ceilings, and floors use Table R402.2.6 equivalent R-values or Table R402.1.2 U-factors calculated with a series-parallel path.
A wall that looks “insulated” can still fail Exam 79 if the candidate cannot tell wood, mass, and steel apart. Cold-formed steel conducts heat along every stud web. A concrete wall stores heat and needs a different R-value depending on which side holds the insulation. Wood remains the Table R402.1.3 baseline that steel equivalents are written against.
Wood-frame walls as the baseline
Section R402.2 does not rewrite wood-frame walls in a separate charging section. Wood-frame walls comply with Table R402.1.3 on the R-value path or Table R402.1.2 on the U-factor path. R402.1.4 then tells you how to add the numbers:
- Cavity insulation alone demonstrates the cavity R-value. Multiple cavity layers may be summed.
- Continuous insulation (ci) alone demonstrates the ci R-value. Multiple ci layers may be summed.
- Cavity R-value shall not be used to satisfy a ci requirement.
- Computed R-values shall not include other building materials or air films.
- Insulated siding used as ci is credited at the labeled R-value minus R-0.6.
| Climate zone | Wood-frame wall R-value options (Table R402.1.3) |
|---|---|
| 0–1 | R-13, or R-0+10ci |
| 2 | R-13, or R-0+10ci |
| 3 | R-20, or R-13+5ci, or R-0+15ci |
| 4 except Marine through 8 | R-30, or R-20+5ci, or R-13+10ci, or R-0+20ci |
A Climate Zone 5 2×4 wall with R-13 batts and no continuous insulation does not meet any of the CZ4–8 options. Adding R-10ci over the sheathing reaches the R-13+10ci option. Filling a 2×6 with R-20 and adding R-5ci reaches R-20+5ci. A layer of R-20ci over an empty cavity is the R-0+20ci option. Mixing those pieces incorrectly—calling OSB “R-5ci,” or adding cavity R-13 to sheathing and calling it R-20—is an R402.1.4 fail, not a close-enough field judgment.
Mass walls (R402.2.5)
Mass walls used as a component of the building thermal envelope shall be one of the following:
- Above-ground walls of concrete block, concrete, insulated concrete form (ICF), masonry cavity, brick but not brick veneer, adobe, compressed earth block, rammed earth, solid timber, mass timber, or solid logs.
- Any wall having a heat capacity greater than or equal to 6 Btu/ft²·°F (123 kJ/m²·K).
Brick veneer is called out so that a wood-frame or steel-frame wall with a 4-inch brick wythe on the outside stays a frame wall. The brick is cladding, not the mass wall. An 8-inch grouted masonry wall, a 6-inch ICF, a rammed-earth wall, and a mass-timber panel can be mass walls under item 1 without a heat-capacity worksheet. A specialty assembly that is not on the list can still be a mass wall if the designer documents heat capacity of at least 6 Btu/ft²·°F.
Which mass-wall R-value to use
Table R402.1.3 lists mass walls as two numbers separated by a slash. Footnote h (2021) states that the second R-value applies where more than half of the insulation is on the interior of the mass wall.
| Climate zone | Mass wall R-value (exterior-majority / interior-majority) |
|---|---|
| 0–1 | 3 / 4 |
| 2 | 4 / 6 |
| 3 | 8 / 13 |
| 4 except Marine | 8 / 13 |
| 5 and Marine 4 | 13 / 17 |
| 6 | 15 / 20 |
| 7–8 | 19 / 21 |
Interior insulation is the colder-climate penalty because it isolates the mass from the indoor air and reduces the diurnal-storage benefit the table already assumed. A Climate Zone 5 ICF with R-11 of foam split equally in both faces of the form is not “more than half interior,” so the first value (R-13) applies if the installed foam meets it. The same climate zone with R-10 of interior furring-and-batt and only R-4 of exterior foam is more than half interior, so the second value (R-17) applies.
On the U-factor path, Table R402.1.2 footnote b tightens mass-wall U-factors when more than half the insulation is interior, for example 0.065 in Climate Zone 5 and Marine 4 versus the table’s standard mass U-factor of 0.082.
Steel-frame ceilings, walls, and floors (R402.2.6)
Steel-frame ceilings, walls, and floors shall comply with the insulation requirements of Table R402.2.6 or the U-factor requirements of Table R402.1.2. The calculation of the U-factor for a steel-frame envelope assembly shall use a series-parallel path calculation method.
Steel webs punch through cavity insulation. A simple area-weighted parallel-path U-factor (cavity area at batt R-value, stud area at steel R-value) understates heat flow because heat also moves sideways into the web. Series-parallel (isothermal-planes / zone) methods account for that bridging. Exam items that show a steel-stud wall with “R-13, same as wood” are testing whether you know R402.2.6 exists.
Table R402.2.6 footnote a: the first number is cavity R-value; the second is continuous R-value. “R-30 + 3” means R-30 cavity plus R-3ci. Footnote b: insulation that exceeds the height of the framing shall cover the framing.
Real Table R402.2.6 rows to memorize
Steel-frame wall, 16 inches on center — wood requirement R-13+10 (the common Climate Zone 4–8 wood option):
- R-0 + 20
- R-13 + 15
- R-15 + 14
- R-19 + 13
- R-21 + 13
Steel-frame wall, 16 inches on center — wood requirement R-13 (Climate Zones 0–2 wood cavity option):
- R-13 + 4.2
- R-21 + 2.8
- R-0 + 9.3
- R-15 + 3.8
- R-21 + 3.1
Steel-frame wall, 24 inches on center — wood requirement R-13+10:
- R-0 + 20
- R-13 + 13
- R-15 + 12
- R-19 + 11
- R-21 + 11
Wider spacing slightly reduces the required continuous layer for several cavity options because there is less steel per square foot, but R-0+20 remains on both the 16-inch and 24-inch R-13+10 rows. Empty steel studs still need R-20ci when the wood analog is R-13+10.
Steel truss ceilings (wood requirement → steel equivalent):
- R-30 → R-38, or R-30 + 3, or R-26 + 5
- R-38 → R-49, or R-38 + 3
- R-49 → R-38 + 5
Steel joist floors:
- Wood R-13 → R-19 in 2×6, or R-19+6 in 2×8 or 2×10
- Wood R-19 → R-19+6 in 2×6, or R-19+12 in 2×8 or 2×10
Do not invent intermediate rows. If the wood requirement is R-20+5, use that Table R402.2.6 line (at 16 inches on center: R-13+12.7, R-15+12.3, R-19+11.6, R-21+11.3, or R-25+10.9), not a guess.
Field scenario: steel-stud addition
A Climate Zone 5 ranch adds a 14-foot-wide family-room wing framed with 3-5/8-inch cold-formed steel studs at 16 inches on center because the addition sits on a noncombustible podium. The existing house is wood 2×6 with R-20+5ci. The addition is on the prescriptive R-value path.
Table R402.1.3 wood-frame wall options for Climate Zone 5 include R-13+10ci. The contractor proposes R-13 kraft-faced batts in the steel cavities and R-10 foil-faced polyiso on the exterior, “same as wood R-13+10.” That pair is not on the 16-inch R-13+10 steel row. The table’s steel equivalents for that wood requirement start at R-13+15 (or R-0+20, R-15+14, R-19+13, R-21+13). R-13+10 in steel is a fail.
Acceptable field fixes include:
- Keep R-13 in the steel cavity and increase exterior ci to R-15.
- Leave the cavity empty and install R-20ci.
- Switch the addition to the U-factor path and submit a series-parallel U-factor for the actual steel gauge, spacing, and layers that is no greater than Table R402.1.2 wood-frame wall U-0.045 for Climate Zone 5.
A plans examiner who stamps “R-13+10 steel OK” has applied the wood table to steel. A field inspector who sees 1 inch of exterior foam (about R-5 to R-6) over R-13 steel batts in Climate Zone 5 has a documented R402.2.6 violation.
Inspection traps
- Treating brick veneer as a mass wall and using the 3 / 4 or 8 / 13 columns on a wood-stud backup wall.
- Using the first mass-wall R-value when the only insulation is interior furring.
- Installing wood-table cavity batts in steel studs with no extra continuous layer.
- Calculating a steel U-factor with a wood-frame parallel-path spreadsheet.
- Crediting painted drywall, house wrap, or uninsulated OSB toward the ci number.
Independent OpenExamPrep study material for Exam 79 treats Table R402.2.6 as a lookup, not a place to interpolate. If the exact wood R-value is not a table row, the steel assembly goes to a series-parallel U-factor against Table R402.1.2.
Which assembly qualifies as a mass wall under Section R402.2.5?
A Climate Zone 5 concrete basement walk-out wall is above grade for 5 feet. The designer places R-12 of insulation on the interior and R-4 continuous insulation on the exterior. Which Table R402.1.3 mass-wall R-value applies?
A Climate Zone 4 addition is framed with cold-formed steel studs at 16 inches on center. The matching wood-frame Table R402.1.3 option being used as the requirement is R-13+10. Which steel-frame insulation package appears on Table R402.2.6 for that row?