R-Value Calculator

This R-value calculator adds up the thermal resistance of a wall, ceiling, floor or roof assembly built from layers — drywall, insulation, sheathing, rigid foam, brick or concrete. Pick a material and thickness for up to five layers, add any extra R-value from a product label, and choose whether to include the still-air films on each surface. It returns the total R-value, the U-factor (its reciprocal), the metric RSI and U-value, and the heat flow through a given area for a temperature difference.

It is useful for comparing insulation options, checking an assembly against an energy-code R-value requirement, or estimating how much adding exterior foam improves an existing wall. Material values are typical per-inch figures for common products; use the label value of the exact product when you have it.

Layers in series simply add up. The result is the center-of-cavity (clear-field) value: studs, joists and other framing conduct more heat than insulation, so the whole-wall R-value of a framed wall is usually 10–25% lower than the cavity figure. Continuous exterior foam is the usual way to reduce that thermal bridging.

Thermal resistances of layers that heat must pass through one after another add directly, so the total R-value is the sum of each layer’s R per inch times its thickness, plus any fixed label values and the surface air films (R-0.68 inside, R-0.17 outside are standard design values). The U-factor is the reciprocal of the total R-value and gives heat flow in BTU per hour per square foot per °F.

Metric RSI is R × 0.1761 (m²·K/W), and the metric U-value is 1 ÷ RSI in W/m²·K. Steady-state heat flow is area × temperature difference × U. This is a one-dimensional, clear-field calculation: it ignores framing, gaps, compression and moisture, which all lower real-world performance.