Insulation Thickness and R‑Value Calculator
Estimate insulation performance by converting thickness to R-value or finding the thickness needed for a target rating. Choose common materials or enter custom conductivity, switch units, calculate U-value and heat loss, then export results instantly.
Developed by: Nohman Habib
Example Data
| Scenario | Material | Thickness | Estimated R / RSI | Notes |
|---|---|---|---|---|
| Wall cavity upgrade | Fiberglass Batt | 3.5 in | R ≈ 11.2 (RSI ≈ 1.97) | Typical 2×4 cavity depth. |
| Rigid foam sheathing | XPS Foam Board | 25 mm | RSI ≈ 0.87 (R ≈ 4.95) | Improves continuous insulation. |
| High-performance layer | Polyiso Foam Board | 2 in | R ≈ 12.0 (RSI ≈ 2.11) | Check cold-weather derating where applicable. |
Formula
- Imperial R-value: R = thickness(in) × (R per inch)
- Metric RSI: RSI = thickness(m) ÷ k (W/m·K)
- Conversion: R = RSI × 5.678263 and RSI = R ÷ 5.678263
- U-value: U = 1 ÷ R (Imperial) and USI = 1 ÷ RSI (Metric)
- Heat loss estimate: Q = U × Area × ΔT (Btu/hr) or Q = USI × Area × ΔT (W)
How to Use This Calculator
- Select whether you want to compute R-value from thickness or thickness from a target R/RSI.
- Choose Imperial or Metric units based on your project measurements.
- Select a material preset, or choose Custom and enter your own R per inch or k-value.
- Enter thickness or target R/RSI. Optionally add area and ΔT to estimate heat loss.
- Press Calculate to view R/RSI, U-value, and any heat-loss estimate, then export as CSV or PDF.
FAQs
1) What is the difference between R and RSI?
R is the Imperial thermal resistance unit, while RSI is the Metric equivalent. They measure the same concept and convert using R = RSI × 5.678263.
2) What does k-value mean?
k-value is thermal conductivity in W/m·K. Lower k means better insulation. RSI is calculated as thickness in meters divided by k.
3) Why can real-world insulation perform worse than the calculated R?
Installation gaps, compression, moisture, air leakage, and thermal bridging through framing can reduce effective performance compared to ideal lab ratings.
4) Can I calculate multi-layer assemblies with this tool?
This calculator focuses on a single insulation layer with optional heat-loss estimates. For full assemblies, calculate each layer’s R and add them for total resistance.
5) What thickness should I enter for loose-fill insulation?
Enter the installed depth after settling. Loose-fill R per inch depends on density and installation method, so use manufacturer specs when possible.
6) Is the heat-loss result an annual energy cost?
No. The heat-loss estimate is instantaneous rate (Btu/hr or W) at the provided ΔT. To estimate cost, you need hours of operation and energy pricing.