Getting Started
This calculator determines thermal transmittance (U-factor) for cold-formed steel (CFS) building envelope assemblies per AISI S250-21, North American Standard for Thermal Transmittance of Building Envelopes with Cold-Formed Steel Framing, 2021 Edition.
Select the appropriate tab above for your assembly type. Results update automatically as you adjust inputs via the number fields, sliders, or dropdowns. Use "Export PDF Report" below any results panel to generate a printable report with full traceability. Reference tables and equations are accessible via the Reference Data button in the right panel.
CFS Wall — Overall Thermal Zone (OTZ) Method (Section B3.1)
The OTZ method is a modified parallel-path approach that accounts for lateral heat flow around steel studs. Instead of using a single framing-fraction based on stud thickness, it defines an Overall Thermal Zone (OTZ) — an effective width of wall area thermally influenced by each stud — which is wider than the stud itself. The assembly is then split into two parallel heat flow paths: one through the CFS framing zone and one through the unaffected cavity.
Required Inputs:
- Steel Framing Properties — designation thickness (33–68 mils), framing spacing (6"/12"/16"/24" o.c.), flange width (CFSflange), web depth within insulation (CFSdepth), and cavity air space depth (CFSwebairspace).
- Insulation R-Values — cavity insulation R-value (Rins) and exterior continuous insulation R-value (Rshe).
- Other Assembly Components — R-values for exterior siding (Sideext), exterior sheathing (Sheathext), and interior sheathing (Sheathint).
Calculation Steps:
| Step | Operation | Reference |
|---|---|---|
| 1 | Look up the thermal conductivity (k) of the steel framing from Table B3.1.3-1 based on designation thickness. For non-standard thicknesses between 33 and 68 mils, linearly interpolate between the bounding standard values. | Table B3.1.3-1 |
| 2 | Determine the total cavity R-value (Rcav). If the cavity contains both insulation and an air space, Rcav = Rins + Rair (0.91 h·ft²·°F/Btu). If insulation only, Rcav = Rins. If air space only, Rcav = Rair. If neither, Rcav = 0. | Section B3.1.6 |
| 3 | Look up the six OTZ regression coefficients (C₀ through C₅) from Table B3.1.1-1 based on framing spacing and designation thickness. For non-standard thicknesses, linearly interpolate all six coefficients between bounding thicknesses. | Table B3.1.1-1 |
| 4 | Calculate the Overall Thermal Zone (OTZ) width using the second-order polynomial:OTZ = C₀ + C₁·Rcav + C₂·Rshe + C₃·Rcav² + C₄·Rshe² + C₅·Rcav·Rshe |
Eq. B3.1.1-1 |
| 5 | Calculate the C-shape framing factor (FFcs):FFcs = (CFSt / 1000) / CFSflangewhere CFSt is the designation thickness in mils and CFSflange is the flange width in inches. |
Eq. B3.1.2-1 |
| 6 | Calculate the thermal resistance of the steel web (Rs-wall):Rs-wall = CFSdepth / k |
Eq. B3.1.3-1 |
| 7 | Calculate the parallel-path U-factor (U₁) and R-value (R₁) for the insulated cavity portion using a weighted parallel path between insulation and the steel web:U₁ = (1 − FFcs)/Rins + FFcs/Rs-wallR₁ = 1 / U₁ |
Eq. B3.1.3-2 Eq. B3.1.3-3 |
| 8 | If a cavity air space exists, calculate U₂ and R₂ for the air space zone using the same parallel-path approach with Rair (0.91) and the steel web through the air space depth:U₂ = (1 − FFcs)/Rair + FFcs/(CFSwebairspace/k)R₂ = 1 / U₂If no air space, R₂ = 0. |
Eq. B3.1.4-1 Eq. B3.1.4-2 |
| 9 | Sum the combined cavity resistance:R₃ = R₁ + R₂ |
Eq. B3.1.5-1 |
| 10 | Calculate Rsps, the total series resistance through the C-shape (framing) path by summing all layer R-values in series:Rsps = Ro + Sideext + Rshe + Sheathext + R₃ + Sheathint + Riwhere Ro = 0.17 (exterior air film) and Ri = 0.68 (interior air film). |
Eq. B3.1.6-1 |
| 11 | Calculate Rspc, the total series resistance through the cavity (non-framing) path:Rspc = Ro + Sideext + Rshe + Sheathext + Rins + Rair + Sheathint + Ri |
Eq. B3.1.6-2 |
| 12 | Calculate the OTZ framing factor (FFotz) — the fraction of wall area influenced by the stud:FFotz = OTZ / FSwhere FS is the framing spacing on-center. |
Eq. B3.1.7-1 |
| 13 | Calculate the overall U-factor (Uo) using a parallel-path weighted average of the two paths:Uo = (1 − FFotz)/Rspc + FFotz/RspsThe overall R-value is simply 1/Uo. |
Eq. B3.1.8-1 |
For non-standard designation thicknesses between 33 and 68 mils, select "Custom (interpolated)..." from the thickness dropdown. Both the thermal conductivity (k) and all six OTZ coefficients will be linearly interpolated between the bounding standard values per Section B3.1.
Standard C-shape studs and track only, per AISI S240 Section A5.6. Spacing must be 6", 12", 16", or 24" o.c. Designation thickness must be 33, 43, 54, or 68 mils (or interpolated between these). No web holes are assumed. For non-standard shapes or configurations, use Section B3.2 (ASTM C1363 testing required).
Roof/Ceiling Joist and Rafter (Section B4.1)
The joist/rafter method uses a correction factor (Fc) approach. Rather than calculating the steel thermal bridging explicitly, a tabulated correction factor is applied to the nominal cavity insulation R-value. This factor, always ≤ 1.0, reduces the effective R-value to account for heat loss through the steel joist flanges and web. The correction factors were derived from detailed finite-element analysis and are tabulated by joist depth, spacing, and insulation R-value.
Required Inputs:
- Joist Depth — 3.5"–4", 6", 8", 10", or 12" (per Table B4.1-1).
- Joist Spacing — 16" or 24" on-center.
- Cavity Insulation R-Value (Rins) — R-30, R-38, or R-49 (standard), or any value between R-30 and R-49 using interpolation.
- Additional Assembly R-Value (Rs-roof) — the sum of all other assembly layer R-values excluding the steel framing and cavity insulation (air films, sheathing, ceiling finish, etc.).
Calculation Steps:
| Step | Operation | Reference |
|---|---|---|
| 1 | Look up the correction factor (Fc) from Table B4.1-1 based on joist depth, joist spacing, and insulation R-value.
For standard R-values (R-30, R-38, R-49), read the value directly. For intermediate R-values, linearly interpolate between the two bounding table entries for the same depth and spacing per Note 2. For example, for 12" depth at 24" spacing with R-35: interpolate between Fc at R-30 (0.35) and Fc at R-38 (0.61). |
Table B4.1-1 Note 2 |
| 2 | Calculate the roof U-factor (Ur) using the correction factor and inputs:Ur = 1 / (Rs-roof + Fc · Rins)The product Fc·Rins represents the effective cavity insulation R-value after accounting for steel thermal bridging. Rs-roof is added in series to represent all other assembly layers. The overall R-value is simply 1/Ur. |
Eq. B4.1-1 |
Rs-roof should include air films (Ro + Ri = 0.17 + 0.68 = 0.85), ceiling finish (e.g., 1/2" gypsum ≈ 0.56), and any other sheathing or membrane layers. It does not include the cavity insulation, which is handled separately through Fc·Rins.
Standard C-shape ceiling joists and roof rafters only. Spacing must be 16" or 24" o.c. Insulation must be between R-30 and R-49. Maximum base steel thickness of 0.064 inches (Table B4.1-1 Note 1). Joist depths: 3.5"–4", 6", 8", 10", or 12".
Roof/Ceiling Truss (Section B4.2)
The truss method uses simplified closed-form equations to determine U-factor. Because trusses have wider spacing (≥ 24" o.c.) and the steel bottom chord members represent a smaller fraction of the ceiling area compared to joists, the thermal bridging effect is less severe. The standard provides three equations depending on whether rigid foam insulation is installed between the gypsum ceiling and the bottom chord.
Required Inputs:
- Bottom Chord Insulation R-Value (Rins) — the R-value of insulation placed at the bottom chord level of the truss.
- Rigid Foam Below Truss — None, R-3, or R-5 rigid foam board between the gypsum ceiling and bottom chord. Selects which equation to use.
- Truss Spacing — must be ≥ 24 inches on-center per Section B4.2.
Calculation Steps:
| Step | Operation | Reference | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Select the appropriate equation based on rigid foam configuration:
|
Eq. B4.2-1 Eq. B4.2-2 Eq. B4.2-3 |
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| 2 | Calculate Ut by substituting the Rins value into the selected equation. The overall R-value is simply 1/Ut. Note: truss spacing does not appear in the equations but must be ≥ 24" o.c. for the equations to be valid. This is a hard applicability constraint, not a calculation variable. |
Section B4.2 |
The constants inside the denominator (0.330, 4.994, 7.082) are not simple R-value additions — they are regression-derived values that capture the combined effect of air films, gypsum board, and (where applicable) rigid foam, as well as the interaction with steel bridging. Do not add additional air film or gypsum R-values to the result.
CFS truss assemblies only with spacing ≥ 24 inches on-center. Maximum 3 web member penetrations through cavity insulation per 4-foot length of truss. The calculator enforces the 24-inch minimum as a hard stop — values below this will not calculate.
Unit System
Use the Imperial / Metric toggle in the header to switch units. Per Section A3 of the standard, all calculations are performed internally in U.S. customary (Imperial) units. When metric mode is active, inputs are converted to Imperial before calculation and results are converted back for display. Conversion factors used: R-value × 0.17611 = RSI (m²·K/W); U-factor × 5.67826 = U-SI (W/m²·K); inches × 25.4 = mm.
Verification & Reports
Results update in real time as inputs change. Expand "View Calculation Steps" below any results panel to see every intermediate value and equation reference for auditing. Click "Export PDF Report" to generate a printable report including the assembly diagram, all inputs, results, and the full calculation step trace.