Insulation R-Value Calculator
A wall, roof, or floor is built from layers, and their insulation values add up. This calculator sums the R-values of up to four layers in series to give the assembly's total R-value, then computes the U-factor (its reciprocal) and the heat loss through a given area at a chosen temperature difference. The math is exact heat-transfer theory; the R-value of each specific material comes from the manufacturer or your building code tables, which you enter. The result tells you how well the assembly resists heat flow and how much heat it loses.
R-value formula
Total R-value = R1 + R2 + R3 + R4 (layers in series)
U-factor = 1 / total R-value
Heat loss = U-factor * area * temperature difference
Heat loss per sq ft = U-factor * temperature difference
R-values of layers in series add directly. The U-factor is the reciprocal of the total R-value. Heat loss is the U-factor times area times the temperature difference across the assembly.
Insulation context
- R-value measures resistance to heat flow; higher is better.
- Layers in series (one behind another) have additive R-values.
- The U-factor is the reciprocal of R-value and measures heat transmission.
- Material R-values come from manufacturers or building-code tables.
- Air films and framing also contribute; include them as layers where relevant.
Insulation R-value: frequently asked questions
How do I calculate the total R-value of an assembly?
For layers in series, such as the sheathing, insulation, and drywall of a wall, add their R-values together. The total R-value is the sum of the individual layer R-values. A higher total R-value means better resistance to heat flow.
What is the U-factor?
The U-factor (overall heat transfer coefficient) is the reciprocal of the total R-value. It is the rate of heat flow per unit area per degree of temperature difference. Lower U-factors indicate better insulation, the inverse of R-value.
How is heat loss through the assembly found?
Heat loss equals the U-factor times the area times the temperature difference. With U in BTU per hour per square foot per degree Fahrenheit, area in square feet, and the difference in degrees Fahrenheit, the result is BTU per hour.
Should I use R-value or RSI?
R-value here uses US customary units (hr times sq ft times degrees F per BTU). The metric equivalent, RSI, uses SI units. This calculator works in US units; convert your layer values to the same system before adding them.
Are these formulas exact?
Yes. Adding series R-values, taking the reciprocal for U, and computing heat loss as U times area times temperature difference are exact relationships from heat-transfer theory. The R-values of specific products come from the manufacturer or code tables that you enter.
Official sources
- U.S. Department of Energy: Insulation and R-value guidance.
- U.S. Federal Trade Commission: R-Value Rule (16 CFR Part 460).
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.