Insulation Calculator
R-value is the target. Thickness is what you install.
Insulation Calculator
Find the extra R-value, installed thickness, and number of packages a wall or ceiling needs.
Subtracted from the gross area
Use 0 for an empty cavity
Typical 3.1-4.3 per inch depending on density.
Printed on the bag or board label
Cuts and offcuts; 10% is typical
R-values are nominal and vary by product, density, and age. Coverage per package differs by brand and thickness, so replace the default with the figure on the label before buying.
Thicker than a 2x6 cavity (5.5 in). Use a deeper assembly or a higher-R material.
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What This Calculator Does
The Insulation Calculator converts a target R-value into the thickness of a chosen material, the number of packages to buy, and the material cost. It subtracts windows and doors from the gross area, adds a waste allowance, and flags when the required thickness will not fit a standard 2x4 or 2x6 cavity.
How It Works
Net area is gross area minus openings. The additional R-value is the target minus whatever R-value already exists, floored at zero. Thickness is that additional R divided by the material nominal R per inch. The area plus waste is divided by the coverage printed on one package and rounded up, because suppliers sell whole packages. Cost is packages multiplied by price per package.
Example
An 800 sq ft wall with 100 sq ft of windows and doors has 700 sq ft of net area. Going from R-13 to R-38 needs 25 more R. Fiberglass batt at 3.2 R per inch needs 25 / 3.2 = 7.8 inches. With 10% waste, 770 / 88 sq ft per package = 8.75, so buy 9 packages. At $55 each the material cost is $495, covering 792 sq ft with 92 sq ft left over.
Formula
gross area - openingsmax(0, target R - existing R)additional R / material R per inchceil(net area x (1 + waste %) / coverage per package)packages x price per packageReading the Results
Packages
Whole packages needed for the net area plus waste. Coverage varies by brand, thickness, and product form, so replace the default with the figure on the label.
Installed thickness
The depth of the additional material, not the total wall thickness. If it exceeds the cavity, the insulation will be compressed or will need a different assembly.
Total R-value
Existing R plus the added R. It is the target when the inputs are right, but real performance also depends on air sealing, moisture, and installation quality.
Sources and References
U.S. Department of Energy - Energy Saver: Insulation
R-value, climate zones, and where insulation matters most
A starting point for choosing a target R-value. Local code may set a higher minimum.
Federal Trade Commission - R-Value Rule, 16 CFR Part 460
Labeling and advertising requirements for insulation
Why the package label states R-value and coverage. Use the label, not a generic table, for the product you buy.
U.S. Department of Energy - Insulation Fact Sheet
Material R-values per inch and installation guidance
The basis for the nominal R-per-inch ranges shown for each material.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
What R-value should I aim for?
It depends on climate zone, the assembly, and local code. The Department of Energy publishes recommended levels by zone; a builder or energy auditor can confirm the requirement for your wall, floor, or ceiling. The calculator takes your target and works out what it takes to reach it.
Can I compress a thicker batt into a shallower cavity?
You can physically, but you should not. Compression squeezes the air pockets that do the insulating, so the installed R-value falls below the label. Choose a product that fits the cavity, or add rigid insulation to the assembly.
Why does the package count use a waste allowance?
Cuts around studs, outlets, windows, and corners create offcuts that cannot always be used. Ten percent is a common allowance for a simple wall; complex framing, diagonal runs, or many penetrations need more.
Do I need a vapour barrier as well?
That depends on climate, the wall assembly, and local code. Adding insulation can change where moisture condenses, so do not treat this calculator as a moisture design. Ask a local professional before closing a wall.
How do I know the existing R-value?
Use the label on the existing batts, the depth and type of the cavity fill, or an energy audit. If it is unknown, enter zero and treat the result as a full fill rather than a top-up; guessing high under-buys material.