Insulation Takeoff: R-Value, Thickness, and the ASHRAE 90.1 Climate Zone Mistake

Cavity insulation and continuous insulation are different line items

ASHRAE 90.1-2022 uses a specific notation for wall assemblies: R-13 + R-7.5 c.i. means R-13 cavity insulation plus R-7.5 continuous (unbroken by framing) insulation. The "+ c.i." is not a suggestion. It's a code-minimum assembly requirement that demands a separate layer of rigid insulation on the exterior face of the sheathing, continuous across studs, plates, and typically detailed around window openings with taped joints.

A cavity-only R-20 wall and a cavity-plus-c.i. wall that totals R-20 on paper have dramatically different effective thermal performance because of thermal bridging through the steel framing. Per ASHRAE 90.1-2022 Appendix A Table A9.2, a 16-inch-on-center cold-formed steel stud wall with R-13 batt in the cavity delivers an actual U-factor of about 0.124 (effective R-8.1) — not R-13. The parallel-path method and the isothermal-planes method both converge on a 30–40% R-value degradation from thermal bridging. That is why continuous insulation exists in the code at all.

Reading ASHRAE 90.1-2022 Tables 5.5-1 through 5.5-8 correctly

90.1-2022 organizes prescriptive envelope requirements into eight tables, one per climate zone (Zones 1 through 8, with 3C and 5C separated). Each table gives minimum R-value or maximum U-factor for roof, wall, floor, slab, below-grade wall, and opaque door assemblies, broken out by construction class:

• Mass walls (concrete, CMU, precast)
• Metal building walls
• Steel-framed walls
• Wood-framed walls
• Below-grade walls

Reading the wrong construction class is the single most common bid error I see on envelope takeoffs. A steel-framed wall requirement in Climate Zone 5 is R-13 + R-7.5 c.i. A wood-framed wall in the same zone is R-13 + R-3.8 c.i. A mass wall is just R-11.4 c.i. Those are three different product orders, three different labor rates, and three different accessory scopes (furring, Z-girts, thermal clip systems).

R-value targets by climate zone (steel-framed walls, 90.1-2022)

Numbers are prescriptive minimums per ASHRAE 90.1-2022 Tables 5.5-1 through 5.5-8, steel-framed wall class. Owner-driven specs, LEED, and stretch-code jurisdictions (New York City Local Law 97, Boston's BERDO) routinely add 20–30% over these minimums.

Polyiso LTTR: the aging trap

Polyisocyanurate is the dominant rigid insulation in commercial roofing and continuous-insulation applications because of its high initial R-value per inch (R-6.0 to R-6.5 per inch fresh off the assembly line). That number is not the number you get for the life of the building. Polyiso ages — the blowing agent diffuses out of the cell structure — and the R-value stabilizes at something lower.

The industry-standard aged value is LTTR (Long-Term Thermal Resistance) per CAN/ULC S770 or the PIMA QualityMark program. Typical LTTR values run R-5.5 to R-5.7 per inch at 5 years. That is a roughly 15% drop from the fresh-product R-value. ASHRAE 90.1-2022 Section 5.8.1.8 requires the LTTR value (not the initial value) be used for code compliance calculations on polyiso products.

Practical implication for takeoff: a spec that calls for "R-30 continuous polyiso" needs to be built out with 5.5 inches of nominal product in a tapered insulation system, not the 4.8 inches you'd get using the fresh R-6.25 value. The difference is a half-inch of thickness across the entire roof. On a 60,000 sf flat roof, that's roughly $18,000–$24,000 of additional polyiso board at current pricing ($1.40–$1.60/bf).

Mineral wool vs polyiso: when the swap pencils

Mineral wool board (Roxul ComfortBoard, Owens Corning Thermafiber) runs R-4.2 per inch, doesn't age, doesn't burn (ASTM E84 Class A flame spread < 25), and is vapor-open. It's a harder sell on fresh-product pricing because you need more thickness for the same rated R-value. But on a continuous-insulation face-sealed assembly where fire rating matters (chase walls, high-rise podiums, Type I-A construction per IBC 2021 Chapter 6), mineral wool frequently wins once you factor the cost of gypsum layers, intumescent treatments, and fire-retardant coatings polyiso needs to achieve the same assembly rating.

Vapor retarder placement by climate zone

IBC 2021 Section 1404.3 and IRC R702.7 govern vapor retarder classification and placement. The placement rule is climate-driven and is where takeoffs lose money when estimators substitute product types:

The trap: a "kraft-faced batt" is Class II (roughly 0.4 perm). Polyethylene sheet is Class I (0.05 perm). Latex paint on drywall is Class III (3–10 perm). The spec in Section 07 27 00 usually calls out the perm rating explicitly; the schedule and the wall detail may not match. In a cold climate (Zone 6+), an interior polyethylene sheet is fine on paper — but layered with exterior continuous insulation it can create a double-vapor-barrier sandwich that traps moisture in the stud cavity. Building science will catch this before the AHJ does, and the redesign gets priced against the contractor.

Thermal bridging per Appendix A

ASHRAE 90.1-2022 Appendix A provides the calculation methods for effective U-factor of framed wall assemblies accounting for thermal bridging. For takeoff purposes, you don't need to run the Appendix A calc yourself, but you need to know what the engineer did when they set the prescriptive spec. The 90.1-2022 default uses the isothermal-planes method for steel-framed walls, which assumes perfect coupling between the inside face of the sheathing and the outside face of the interior finish — a reasonable approximation when continuous insulation is present, and an aggressive overestimate when it isn't.

Spray foam closed-cell (R-6.5 per inch) and mineral wool exterior c.i. (R-4.2 per inch) both dramatically reduce the thermal bridging penalty. When the envelope engineer has specified closed-cell SPF for the cavity and XPS or polyiso for the exterior c.i., the takeoff needs to price the specific product types called out — they are not interchangeable with batts on cost or thickness.

"The envelope line on a commercial bid is where estimators lose the bid without ever knowing why. The engineer wrote R-13 + R-7.5 c.i. The estimator priced R-20 batt. The submittal reviewer caught it. The GC got the difference as a $58,000 change order at month four. That job didn't have profit in it anymore."

Angela Sims, Envelope Consultant — Simpson Gumpertz & Heger, Boston, MA

The fire-rated insulation scope for chase walls and shafts

IBC 2021 Chapter 7 (Fire and Smoke Protection Features) requires 1-hour to 2-hour rated assemblies around shafts, stair enclosures, and corridor walls in most occupancies. The insulation in those assemblies must carry the rating. Mineral wool achieves this natively (non-combustible per ASTM E136, melting point >2,150°F). Fiberglass batts rated for the assembly must be specified with the UL listing number tied to the assembly type (UL U419, U465, etc.). Polyiso cannot be used in a rated wall assembly without an additional layer of Type X gypsum each side at minimum — a scope that is almost always overlooked on bid day.

For takeoff, pull out the Section 078400 (firestopping) and the UL assembly references on the A-sheets before pricing any insulation in a rated assembly. Get the assembly number right, get the insulation product line right, and confirm the gypsum count on each side of the partition. On a mid-rise building, the rated-assembly insulation scope is typically 12–20% of total insulation cost.

Bottom line

Insulation takeoff is not a sf-times-unit-price exercise. It's a climate zone, an assembly class, a product chemistry, a vapor retarder class, and a fire rating — each of which has a different price tag. Read the ASHRAE 90.1 table row that matches the assembly class, use LTTR for polyiso, confirm vapor retarder placement against the climate zone, and cross-check rated assemblies against the UL listing. Every one of those steps has paid for itself on bids I've run.