Insulation Calculator
Work out how much insulation you need — R-value to add, layer thickness, number of rolls, batts or bags, and the material cost, with waste included.
Updated October 2026
R-value measures resistance to heat flow — higher is better, and it adds up, so topping up R-15 to R-49 means adding R-34, not R-49. The thickness shown is how deep the new layer must be for that added R with your chosen material. For blown-in insulation the coverage per bag dropsas the target depth rises, so read the coverage straight off the bag's chart at your R-value rather than assuming a fixed number. Figures are an estimate; they exclude labor, vapor barriers and air sealing.
For your project
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Insulation is sold by the package, but the number you actually need depends on the area, how much R-value you're adding and the product you choose. This calculator turns those into a shopping list — how many rolls, batts or bags to buy, the thickness of the new layer, and the material cost.
It handles the detail people get wrong: R-value adds up, so topping up existing insulation only needs the difference, not the full target. Enter your area, the R-value you're aiming for and what's already there, and you get the added R, the depth to install, the packages to buy with a waste allowance, and the total cost.
How to use it
- 1Enter the area you're insulating in square feet (for an attic, that's the floor area).
- 2Set your target R-value and the R-value already in place (0 if it's bare).
- 3Enter the R-value per inch of your chosen material and the coverage printed on each package.
- 4Add the price per package and a waste allowance, then read the packages, thickness and cost.
Frequently asked questions
What R-value do I need?+
The US Department of Energy recommends roughly R-30 to R-60 for attics depending on your climate zone — R-49 to R-60 across most of the US, less in the warm south. Walls are typically R-13 to R-21 and floors R-25 to R-30. Check your zone's recommendation and set that as the target.
Do I add R-value or replace it?+
You add it. R-values of layers stack, so if you already have R-15 and want R-49, you only need to add R-34 of new insulation on top. This calculator does that subtraction for you and sizes the new layer accordingly — you don't tear out good existing insulation.
How thick does the insulation need to be?+
Divide the R-value you're adding by the material's R-value per inch. Fiberglass batt is about R-3.1 per inch, blown fiberglass about R-2.5, cellulose about R-3.5, rigid foam board around R-5, and closed-cell spray foam about R-6.5. So R-34 of cellulose is roughly 10 inches deep.
Why does coverage per bag change for blown-in insulation?+
A bag of loose-fill contains a fixed amount of material, so the deeper you blow it, the less floor area one bag covers. The bag's label has a coverage chart listing square feet per bag at each R-value — use the figure for your target R, not a single fixed number, or you'll come up short.
Does this include labor?+
No — it estimates material (packages and cost) only. It doesn't include labor, a vapor barrier, baffles, or the air sealing that should come before insulating. Sealing gaps and leaks first is what lets the insulation reach its rated performance.
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