What is the difference between R and RSI?

R is Imperial (ft²·°F·h/BTU), RSI is Metric (m²·K/W). They convert by R = 5.678263 × RSI.

Should I include surface films?

Yes for steady-state comparisons. ISO 6946 recommends Rsi = 0.13 and Rse = 0.04 m²K/W for internal and external surfaces, respectively.

How do framing factors affect R-value?

Framing creates thermal bridges. The effective U-value is the area-weighted average of the cavity and stud paths.

Are material R-values per inch exact?

They are typical ranges. Consult product datasheets for certified values and test standards like ASTM C518.

Does moisture or compression change R-value?

Yes. Moisture and compression can reduce thermal resistance. Always follow manufacturer installation guides.

Is this calculator suitable for code compliance?

It’s a design aid based on ISO 6946 and ASHRAE fundamentals. For compliance, verify assemblies per your authority having jurisdiction and energy code.

Insulation R-Value Calculator

This professional-grade calculator helps builders, energy consultants, architects, and homeowners compute the total thermal resistance (R or RSI) of layered wall, roof, and floor assemblies. It supports surface film coefficients and an optional framing factor to deliver an ISO 6946–style effective R-value. Mobile-first, accessible, and optimized for performance.

Calculator

Units

Imperial (R)

Metric (RSI)

Surface films

Include interior film Include exterior film

Assembly layers (series)

Framed assembly (optional)

Framing factor (%)

Cavity depth

Cavity insulation

Stud material

Results

Total R (series, no framing) R 0.00

U-factor (no framing) BTU/(h·ft²·°F) 0.000

Note: U = 1/R (Imperial) or 1/RSI (Metric). Results update as you type.

Data Source and Methodology

Authoritative sources: ISO 6946:2017 “Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods” and ASHRAE Handbook—Fundamentals (latest edition), Heat Transfer chapter. ISO 6946 provides standard surface resistances and the parallel-path method; ASHRAE provides material properties and thermal analysis fundamentals. Direct references:

ISO 6946:2017 — iso.org/standard/65708.html — Surface resistances and series/parallel calculations.
ASHRAE Handbook—Fundamentals (2021/2025), Ch. on Heat Transfer and Building Envelope — ashrae.org
ASTM C168-22 “Standard Terminology Relating to Thermal Insulation” — astm.org/c0168-22.html

All calculations are strictly based on the formulas and data provided by this source.

The Formula Explained

Series (no framing):

R_total = R_{si} + \sum_{i=1}^{n} R_i + R_{se}

U = \dfrac{1}{R_{total}}

Layer resistance:

R_i = \left(\dfrac{R}{\text{inch}}\right)_i \times \text{thickness}_i \quad \text{(Imperial)}

R_i = \left(\dfrac{RSI}{\text{mm}}\right)_i \times \text{thickness}_i \quad \text{(Metric)}

Parallel-path effective U (framing):

U_{eff} = f \cdot U_{stud} + (1-f) \cdot U_{cavity}

R_{eff} = \dfrac{1}{U_{eff}}

Unit conversion:

R = 5.678263 \times RSI \quad ; \quad RSI = \dfrac{R}{5.678263}

R_{si} \approx 0.13\ \text{m}^2\cdot\text{K/W} \quad ; \quad R_{se} \approx 0.04\ \text{m}^2\cdot\text{K/W}

Glossary of Variables

Units: Choose Imperial (R in ft²·°F·h/BTU) or Metric (RSI in m²·K/W).
Surface films (Rsi, Rse): Standard interior/exterior surface resistances applied in series per ISO 6946.
Layer material: Typical materials with representative R-per-inch; “Custom” lets you define your own value.
Thickness: Material thickness: inches (Imperial) or millimeters (Metric).
Framing factor (f): Percentage of the assembly area occupied by framing members (studs, plates, etc.).
Cavity depth: Stud depth or insulation thickness in the cavity path.
Total R (series): Sum of all resistances in series (including optional surface films).
U-factor: Overall heat transfer coefficient: U = 1/R (Imperial) or 1/RSI (Metric).
Effective R (with framing): Parallel-path result combining cavity and stud paths by area weighting.

Worked Example

How It Works: A Step-by-Step Example

Suppose you have a 2×4 wood-stud wall with fiberglass batt insulation (3.5 in), 1/2 in gypsum interior, and 1 in XPS exterior, including surface films. Choose Imperial (R). Layers:

Interior film: R ≈ 0.13 × 5.678 ≈ 0.74
1/2 in gypsum (R/in ≈ 0.45): R ≈ 0.45 × 0.5 = 0.225
3.5 in fiberglass (R/in ≈ 3.2): R ≈ 3.2 × 3.5 = 11.2
1 in XPS (R/in ≈ 5.0): R = 5.0 × 1 = 5.0
Exterior film: R ≈ 0.04 × 5.678 ≈ 0.23

Total series R ≈ 0.74 + 0.225 + 11.2 + 5.0 + 0.23 = 17.395. U ≈ 1 / 17.395 = 0.0575 BTU/(h·ft²·°F).

With framing factor f = 23% and wood studs (R/in ≈ 1.25): R_cavity_path ≈ 17.395; R_stud_path replaces the 11.2 batt with 1.25 × 3.5 = 4.375, so R_stud ≈ 10.57. U_cavity ≈ 1/17.395 = 0.0575; U_stud ≈ 1/10.57 = 0.0946. U_eff = 0.23 × 0.0946 + 0.77 × 0.0575 = 0.0662; R_eff ≈ 1/0.0662 = 15.1.

Frequently Asked Questions (FAQ)

Do these R-per-inch values match every product?

No. They are representative. Always verify with manufacturer datasheets (ASTM C518/C177 testing).

What framing factor should I use?

Common assumptions: 23% for 16 in o.c. wood studs, 20% for 24 in o.c. Use project-specific takeoffs when possible.

How accurate is the steel stud model?

Parallel-path with R-per-inch for steel is a simplification. Steel studs cause significant thermal bridging; consult advanced 2D/3D simulations (ISO 10211) for critical designs.

Can I model continuous exterior insulation?

Yes. Add it as a layer. Continuous insulation reduces thermal bridging and boosts effective R significantly.

Why include surface films?

They represent boundary layer resistances in steady-state. ISO 6946 provides standard values for consistent comparisons.

How do I convert between R and RSI?

The tool switches units automatically: R = 5.678263 × RSI, and U uses the reciprocal in the chosen system.

Is this calculator a substitute for energy modeling?

No. It’s a fast, transparent estimator for assemblies. For compliance or complex details, use approved simulation tools.

Tool developed by Ugo Candido. Content verified by the CalcDomain Building Science Editorial Board.
Last reviewed for accuracy on: September 15, 2025.