Insulation R-Value Calculator
Calculate the total R-value of a wall or ceiling assembly from individual insulation layers.
How to use this tool
- Enter the R-value for each insulation layer (set unused layers to 0).
- Include sheathing and drywall R-values for a complete assembly calculation.
- The result shows total R-value and U-factor for the wall or ceiling assembly.
Calculate total wall or ceiling R-value by adding insulation layers together.
Formula
Total R-value = Rlayer1 + Rlayer2 + Rlayer3 + Rsheathing + Rdrywall
U-factor = 1 ÷ Total R-value (units: BTU / (hr·ft²·°F))
R-values of parallel layers in series add directly.
How it works
This calculator finds the total thermal resistance (R-value) of a wall or ceiling assembly by summing the R-values of up to five individual layers in series — for example, batt insulation, rigid foam, sheathing, and drywall. Thermal resistance is additive when layers are stacked in series because heat must pass through each one sequentially. The U-factor (thermal transmittance) is the reciprocal of the total R-value and indicates how much heat flows through one square foot of the assembly per hour for each degree Fahrenheit of temperature difference. The calculation assumes no thermal bridging through studs or framing, which in practice reduces the effective whole-wall R-value by 15–25%.
Worked example
Worked example
- Layer 1 (batt insulation): R-13.
- Layer 2 (rigid foam): R-5.
- Layer 3: R-0 (none).
- Sheathing: R-0.45; Drywall: R-0.45.
- Total R = 13 + 5 + 0 + 0.45 + 0.45 = R-18.9.
- U-factor = 1 ÷ 18.9 ≈ 0.0529 BTU/(hr·ft²·°F).
The wall assembly has a total R-value of 18.9 and a U-factor of approximately 0.0529 BTU/(hr·ft²·°F).
Key terms
- R-value
- Thermal resistance; measures how well a material resists heat flow. Higher R-values mean better insulation. Units are ft²·°F·hr/BTU in the US.
- U-factor (U-value)
- Thermal transmittance; the rate of heat flow through one square foot of an assembly per 1°F temperature difference. U-factor = 1 ÷ R-value.
- Series layers
- Insulation layers stacked so heat must pass through each one in sequence; total R-value equals the sum of each layer's R-value.
- Thermal bridging
- Heat conduction through high-conductivity materials (like wood or steel studs) that bypass insulation layers, reducing the effective whole-wall R-value compared to the cavity R-value.
- Assembly R-value
- The combined thermal resistance of all layers in a wall or ceiling, including insulation, sheathing, air films, and finish materials; used for energy code compliance.
Frequently asked questions
- What does R-value mean?
- R-value measures thermal resistance — how well an insulation layer resists heat flow. Higher R-values mean better insulation. Values are additive when layers are stacked.
- What is U-factor?
- U-factor (or U-value) is the inverse of total R-value (U = 1/R). It measures heat transfer rate — a lower U-factor means better insulating performance.