Insulation Calculator
Use this insulation calculator to estimate total thermal resistance from layered materials and convert it to U-value or RSI. Enter the R-value and thickness for each layer you want to include, then the calculator adds them up to show how the assembly performs. It is useful when you want to compare wall or roof build-ups before construction starts.
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Quick Answer:
Insulation layers add together as total R-value. Once you know the total R, you can convert it to U-value with U = 1 / R or to RSI with RSI = R / 5.6786. Higher R-values mean better insulation, while lower U-values mean less heat passes through the assembly.
What This Insulation Calculator Helps You Do
Use this insulation calculator to estimate total thermal resistance from layered materials and convert it to U-value or RSI. Enter the R-value and thickness for each layer you want to include, then the calculator adds them up to show how the assembly performs. It is useful when you want to compare wall or roof build-ups before construction starts.
How to Calculate Insulation Calculator
- Add the layer values - Enter the R-value and thickness for each material layer you want to include in the assembly.
- Keep the units consistent - Use the same thickness unit for every layer so the total R-value stays meaningful.
- Choose the output mode - Pick total R-value, U-value, or RSI depending on the style of result you want to compare.
- Review the meaning - Use the result to compare wall or roof insulation options before ordering materials.
Insulation Calculator Formula
Total R = sum(layer R-value x layer thickness) | U-value = 1 / Total R | RSI = Total R / 5.6786
| Symbol | Definition | Unit |
|---|---|---|
| drywallR | Drywall R-value per inch | R/in |
| drywallThickness | Drywall thickness | in |
| fiberglassR | Fiberglass batt R-value per inch | R/in |
| fiberglassThickness | Fiberglass thickness | in |
| cementBoardR | Cement board R-value per inch | R/in |
| cementBoardThickness | Cement board thickness | in |
| airGapR | Air gap R-value per inch | R/in |
| airGapThickness | Air gap thickness | in |
| concreteR | Concrete R-value per inch | R/in |
| concreteThickness | Concrete thickness | in |
| gravelR | Gravel R-value per inch | R/in |
| gravelThickness | Gravel thickness | in |
| brickR | Brick R-value per inch | R/in |
| brickThickness | Brick thickness | in |
Worked Examples
USA - Layered wall assembly
- drywallR: 0.05
- drywallThickness: 0.75
- fiberglassR: 3.4
- fiberglassThickness: 3.0
- cementBoardR: 0.05
- cementBoardThickness: 0.75
- airGapR: 1.43
- airGapThickness: 1.0
- concreteR: 0.08
- concreteThickness: 3.0
- gravelR: 0.6
- gravelThickness: 1.0
- brickR: 0.2
- brickThickness: 2.0
Result: Total R-value = 12.94
This is a layered assembly check for a mixed-material wall. The estimate is 12.94 .
UK - Compare U-value
- drywallR: 0.04
- drywallThickness: 0.5
- fiberglassR: 3.0
- fiberglassThickness: 4.0
- cementBoardR: 0.05
- cementBoardThickness: 0.75
- airGapR: 1.2
- airGapThickness: 1.0
- concreteR: 0.09
- concreteThickness: 2.5
- gravelR: 0.5
- gravelThickness: 1.0
- brickR: 0.18
- brickThickness: 2.0
Result: U-value = 0.07
Lower U-values mean a better insulated assembly. The estimate is 0.07 .
EU - RSI conversion
- drywallR: 0.06
- drywallThickness: 0.75
- fiberglassR: 3.2
- fiberglassThickness: 3.0
- cementBoardR: 0.05
- cementBoardThickness: 0.75
- airGapR: 1.4
- airGapThickness: 1.0
- concreteR: 0.08
- concreteThickness: 3.0
- gravelR: 0.6
- gravelThickness: 1.0
- brickR: 0.2
- brickThickness: 2.0
Result: RSI = 2.17
RSI is the metric form of thermal resistance. The estimate is 2.17 .
GCC - Thicker insulation stack
- drywallR: 0.05
- drywallThickness: 0.75
- fiberglassR: 4.0
- fiberglassThickness: 4.0
- cementBoardR: 0.05
- cementBoardThickness: 0.75
- airGapR: 1.5
- airGapThickness: 1.0
- concreteR: 0.08
- concreteThickness: 3.0
- gravelR: 0.6
- gravelThickness: 1.0
- brickR: 0.2
- brickThickness: 2.0
Result: Total R-value = 18.82
More fiberglass thickness quickly raises the total thermal resistance. The estimate is 18.82 .
How to Interpret Your Results
| Range | Meaning | Action |
|---|---|---|
| Under R-10 | Low thermal resistance | Check whether the assembly needs more insulation. |
| R-10 to R-20 | Moderate insulation | Common for basic wall and roof assemblies. |
| R-20 to R-40 | Good insulation | Often suitable for energy-conscious construction. |
| Above R-40 | High insulation | Use the result to compare against local energy targets. |
Frequently Asked Questions
It estimates the main planning quantity for insulation work using the formula shown on the page. That gives you a practical number before you order materials, compare suppliers, or talk to a contractor. This calculator adds the thermal resistance of each layer and converts the total to U-value or RSI.
Enter the values that match the unit labels beside the fields. If the page expects feet, inches, gallons, pounds, or watts, keep everything in that unit family so the result stays reliable.
The calculator multiplies or divides the main quantity by the values you enter, so every measurement feeds directly into the final answer. A small change in depth, area, density, or factor can make a large difference on a bigger project.
Yes, as long as the units stay consistent within the calculation. If the page expects feet, inches, gallons, or pounds, convert first so the final result is accurate and easy to interpret.
Treat the result as a planning estimate. Use the main output for sizing or ordering, then review the detail rows for waste, weight, cost, or conversion notes before you finalize the purchase.
Yes if the job involves cut losses, uneven ground, spill risk, or irregular shapes. A small allowance is usually safer than ordering exactly to the bare math, especially for insulation projects that are hard to top up later.
It is exact for the numbers you enter, but real-world projects can still vary because of compaction, tolerances, site conditions, and product differences. Use the result as a solid working estimate, not a final structural or procurement check.
Yes. That is one of its main uses. The result helps you estimate how much to buy, what it may weigh, and what the budget might look like before you place an order or request a quote.
Disclaimer: This insulation calculator provides a planning estimate only. Actual wall or roof performance depends on thermal bridges, air gaps, installation quality, and the full building assembly.
Sources
Last reviewed: March 2026