Insulation R-Value Calculator

What is an R-value calculator?

An R-value calculator totals the thermal resistance of an insulation assembly — each layer’s R-value per inch multiplied by its thickness — and derives the U-factor and a rough heat-loss figure. Higher R means better insulation.

How this calculator works

• List each insulation layer as “R-value per inch, thickness in inches” on its own line.
• Each layer contributes R-per-inch × thickness; the layers add up.
• U-factor = 1 ÷ total R, the inverse measure used in energy codes.
• Enter area and a temperature difference for a rough BTU-per-hour heat-loss figure.

How to use the result

1. Compare your total R against your local building code minimums for walls, ceiling, and floor.
2. Use the heat-loss number to gauge the benefit of adding another layer — and the energy bill it saves.
3. Check the label on insulation: batt packages state R-value per inch (fiberglass ~3.5, foam board ~5–6, spray foam ~6–7).
4. Remember that R-values assume proper installation — gaps and compression cut effective performance.

What R-value do I need?

US code minimums vary by climate zone, but common targets: attic R-38–R-60, walls R-13–R-21, floors R-19–R-25. A standard 3.5-inch wall cavity filled with fiberglass batts is about R-13; adding a foam layer raises it from there.

Common mistakes

• Confusing R-value with U-factor. R measures resistance (bigger is better); U measures heat transfer (smaller is better). U = 1/R.
• Adding R-values incorrectly. Series layers add, but parallel paths (studs) don’t — real assemblies have framing that conducts heat.
• Compressing batts. Squeezing insulation into a cavity reduces its R-value rather than increasing it.

Insulation is an energy-efficiency cost — the same payback thinking behind the payback period calculator, and the same area math as the paint calculator’s wall coverage.