Insulation R-Value Calculator
Estimate combined insulation R-value, framing-adjusted effective R-value, U-factor, package quantity, heat flow and material cost.
Default project preview
20.61 R
New insulation value
19.25
U-factor
0.0485
Estimated heat flow
1,455 BTU/h
Coverage order
1,100 ft²
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Assembly area
Enter this project assumption in ft². The current imperial default is 1000 ft².
Existing insulation R-value
Enter this project assumption in R. The current imperial default is 5 R. Minimum: 0.
New insulation value per thickness unit
Enter this project assumption in R per in. The current imperial default is 3.5 R per in.
New insulation thickness
Enter this project assumption in in. The current imperial default is 5.5 in.
New insulation layers
Enter this project assumption in a unitless value. The current imperial default is 1. Minimum: 1.
Thermal-bridging reduction
Enter this project assumption in %. The current imperial default is 15 %. Minimum: 0. Maximum: 60.
Indoor-outdoor difference
Enter this project assumption in °F. The current imperial default is 30 °F.
Coverage per package
Enter this project assumption in ft². The current imperial default is 40 ft².
Coverage allowance
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 50.
Price per package
Enter this project assumption in a unitless value. The current imperial default is 55. Minimum: 0.
How to read the outputs
New insulation value
1 layer. Default preview: 19.25.
U-factor
inverse of effective R-value. Default preview: 0.0485.
Estimated heat flow
entered area and temperature difference. Default preview: 1,455 BTU/h.
Coverage order
allowance included. Default preview: 1,100 ft².
Packages
rounded full packages. Default preview: 28.
Material cost
insulation packages. Default preview: $1,540.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Nominal R-value = existing R + insulation value per thickness × thickness × layers. Effective R applies the entered thermal-bridging reduction. U-factor = 1 ÷ effective R.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Estimate total thermal resistance from layer R-values or thickness and declared resistance per unit thickness.
Calculation steps
- 1Enter the actual project dimensions or operating values.
- 2Apply the calculator formula using consistent units.
- 3Review the unrounded result and the editable allowance.
- 4Round only to the product, stock length or equipment increment actually purchased.
Worked numerical example
Adding R-13 cavity insulation and R-5 continuous insulation gives a nominal R-18 before framing-path effects.
Purchasing and rounding
Use declared R-values for the exact insulation product and installed thickness. When comparing purchases, keep cavity insulation and continuous insulation as separate layers; compressing batts or substituting a different thickness can change the achieved resistance even when the package label looks similar.
Interpret the result
The summed R-value is a one-dimensional layer calculation. Whole-assembly performance is usually lower where framing, fasteners, gaps or thermal bridges bypass the insulation, so use assembly data when code compliance or energy modeling depends on the complete wall, roof or floor.
Page-specific assumptions
- Inputs describe one consistent project condition.
- Entered yields and efficiencies match the selected product or equipment.
- Site-specific code and engineering requirements are checked separately.
Common mistakes
- Mixing imperial and metric inputs.
- Using nominal dimensions where actual coverage controls.
- Rounding intermediate values too early.
- Treating a quantity estimate as design approval.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Insulation R-Value Calculator questions
Can R-values be added directly?+
Layer R-values can usually be added in a simple one-dimensional path, but framing and other bridges reduce whole-assembly performance.
Why is effective R-value lower than nominal?+
Studs, joists, fasteners and gaps conduct more heat than the insulation, reducing average assembly resistance.
Does more insulation always produce the same savings?+
No. Each added increment provides diminishing heat-flow reduction, and air sealing can be equally important.