Insulation Calculator
Find how many insulation packages you need, or the total R-value of layered insulation.
🌡️ What is the Insulation Calculator?
The insulation calculator answers two different, equally common questions about thermal insulation in one tool. The first mode, Packages Needed, converts an area to insulate and a package coverage figure into the number of batts, rolls, or bags to buy. The second mode, Total R-Value, sums the R-values of stacked insulation layers to find the combined thermal resistance of a wall, ceiling, or floor assembly.
Homeowners insulating an attic or crawl space need the first mode to know how many packages to order from the coverage printed on the wrapper. Builders and DIYers comparing options, like adding rigid foam over existing batts, or checking whether a wall assembly meets a code-required R-value, need the second mode. Both are common enough that treating them as two modes of one calculator saves switching between tools.
A common point of confusion is whether R-values add or need some other combination rule. They add directly. R-value is a measure of thermal resistance, and resistances of layers a heat flow passes through in series sum, exactly like electrical resistors in series. An R-13 batt behind an R-5 rigid foam board gives R-18 total, not some averaged or diminished figure.
This tool is useful because it handles the practical rounding that buying insulation requires (you cannot buy 27.5 packages, so it rounds up after waste) and the layered-R-value arithmetic that assemblies require, with the full working shown so you can verify every step and see the standard used at each stage.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 - Attic floor, 1000 sq ft, R-13 batts at 40 sq ft per package
Example 2 - Small shed wall, 500 sq ft, R-19 batts at 50 sq ft per package
Example 3 - Layered wall assembly, R-13 batt + R-5 rigid foam + R-2 air gap
❓ Frequently Asked Questions
🔗 Related Calculators
How do you calculate how many packages of insulation you need?
Divide the area to insulate by the coverage per package printed on the wrapper, add a waste allowance, and round up. 1000 square feet of attic at 40 square feet per package of R-13 batts is 25 packages, 27.5 with 10 percent waste, rounded up to 28 packages.
How do R-values of layered insulation add together?
R-values in series add directly, the same way electrical resistors in series add. An R-13 batt plus an R-5 rigid foam board plus an R-2 air gap gives a total of R-13 + R-5 + R-2 = R-20. This is because R-value is thermal resistance, and resistances in a heat-flow path in series sum.
What is R-value?
R-value is a material's resistance to conductive heat flow, measured in square foot degree Fahrenheit hours per BTU (ft²·°F·h/BTU) in the US. A higher R-value means better insulating performance. Doubling the thickness of the same material roughly doubles its R-value.
How much does one package of insulation batts cover?
Coverage varies by R-value, thickness, and manufacturer, but common fiberglass batt packages cover 32 to 88 square feet. Always check the coverage printed on the package label rather than assuming a standard figure, since higher R-value batts are thicker and cover less area per package.
How much waste should I add when buying insulation?
10 percent is typical for a straightforward attic floor or open wall cavity job. Add more, 15 percent or higher, for spaces with many obstructions like pipes, wiring, recessed lights, or irregular joist spacing that increase cutting waste.
What R-value do I need for my attic or walls?
Recommended R-values depend on climate zone; the US Department of Energy generally recommends R-30 to R-60 for attics and R-13 to R-21 for walls in most regions, with colder climates needing higher values. Check your local building code or the DOE's zone map for the exact recommendation where you live.
Can I combine two layers of insulation to reach a target R-value?
Yes. Because R-values in series add directly, you can stack an existing layer with a new one to reach a target. If your attic already has R-19 and you want R-49, add roughly R-30 more, for example two layers of unfaced R-15 batts (R-30 total added), giving R-19 + R-30 = R-49.
Does compressing a batt reduce its R-value?
Yes. Compressing a batt into a space thinner than its rated thickness reduces its R-value because R-value depends on the thickness of trapped air pockets in the material. A batt rated R-19 compressed into a 2x4 wall cavity performs closer to R-13 to R-15, so always match the batt thickness to the cavity depth.
What is the difference between R-value and U-value?
R-value measures thermal resistance, higher is better; U-value measures thermal transmittance (the inverse of total R-value), lower is better. U-value is R-value inverted, U = 1/R, and is more common in window and door performance ratings than in bulk insulation.
How many packages of R-13 insulation do I need for 1000 square feet?
If one package of R-13 batts covers about 40 square feet, 1000 square feet needs 25 packages before waste. Adding a 10 percent waste allowance brings that to 27.5, rounded up to 28 packages. Always check the coverage per package printed on the actual product label, since it varies by manufacturer, and recompute from that figure for an exact count.
Is it better to use one thick layer or two thinner layers of insulation?
Total R-value is the same either way, since R-values in series simply add, so R-19 in one layer performs the same as two R-9.5 layers stacked. Two thinner layers can be useful for staggering seams to reduce air gaps, particularly when adding insulation over an existing layer running perpendicular to it.