# Insulation R-Value Calculator

> Add up the thermal resistance of up to three layers from thickness and conductivity (or R per inch), plus surface films, and get the R-value and U-value.

Category: Construction · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/insulation-r-value-calculator

Each layer’s R-value is its thickness divided by its thermal conductivity. Layers in series add up, and the U-value is 1 ÷ total R.

Also called: U-value calculator, Thermal resistance calculator.

## Inputs
- Units (options: Metric, Imperial; default metric)
- Layer 1 thickness (default 100)
- Layer 1 conductivity or R per inch (default 0.035)
- Layer 2 thickness (default 12.5; optional)
- Layer 2 conductivity or R per inch (default 0.25; optional)
- Layer 3 thickness (default ; optional)
- Layer 3 conductivity or R per inch (default ; optional)
- Surface resistances (inside + outside) (unit m²·K/W; default 0.17; optional)

## Results
- Total R-value (SI)
- Total R-value (US)
- U-value
- U-factor (US)

## Formula

```
R_layer = t ÷ λ (or t × R per inch);  R_total = Σ R_layer + R_si + R_se;  U = 1 ÷ R_total;  R_US = R_SI × 5.678263
```

Where:
- t: layer thickness
- λ: thermal conductivity, W/m·K
- R_si, R_se: inside and outside surface resistances

## Worked example
1. Insulation: 0.100 ÷ 0.035 = 2.857 m²·K/W.
2. Plasterboard: 0.0125 ÷ 0.25 = 0.050; plus surface films 0.17 → total 3.077 m²·K/W.
3. U = 1 ÷ 3.077 = 0.325 W/m²·K; US R = 3.077 × 5.678 = R-17.5.

**Result:** 100 mm of 0.035 insulation behind plasterboard gives R 3.08 m²·K/W (about R-17.5 US) and a U-value of 0.325.

## What the result means

Higher R (lower U) means less heat flow. Compare the U-value with the target in your energy code, which may count framing and thermal bridges.

## Assumptions and limits
- Uniform, continuous layers (one-dimensional heat flow).
- Studs, fixings and gaps form thermal bridges that lower the real R-value; full calculations (e.g. ISO 6946 combined method) account for them.

## Questions

### Why are US and metric R-values different numbers?

They use different units. One SI unit (m²·K/W) equals 5.678 US units (ft²·°F·h/Btu), so R-19 US is about R 3.35 SI.

### Do air films matter?

Slightly: about 0.17 m²·K/W on a wall, which matters more for thin, poorly insulated elements such as single glazing.

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_Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials._

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/insulation-r-value-calculator