How to use the insulation calculator
- Enter the area to insulate and the R-value you want.
- Enter the R-value per inch of your material from its label, and choose whether it comes as loose fill or as batts and rolls.
- Enter the volume per bag or the coverage per package, a waste percent and, optionally, a price and the depth available.
- Read the thickness needed, the quantity to buy and whether the depth you have is enough.
Formula
Thickness = R-value wanted / R-value per inch
Volume of fill = area x thickness / 12 (cubic feet)
Bags = volume x (1 + waste) / volume per bag, rounded up; packages of batts = area x (1 + waste) / coverage, rounded up
R-value that fits = depth available x R-value per inch
R-value is a measure of resistance to heat flow, so a higher R-value insulates better. R-values of layers in a stack add up.
R-38 over 1,000 square feet with loose fill
At 3.5 per inch, R-38 needs 38 / 3.5 = 10.86 inches. Over 1,000 square feet that is 904.8 cubic feet, or 995.2 with 10% waste. With bags of 10 cubic feet that is 100 bags. The R-value per inch, bag volume and waste are example values.
With batts that cover 40 square feet a package, 1,000 square feet plus 10% is 1,100 square feet, or 28 packages. A joist space only 5.5 inches deep holds 5.5 x 3.5 = R-19.3 of this material, which is short of R-38, so the tool shows a warning.
Matching the material to the space
The depth you have limits the R-value you can reach with a given material, which is why the tool compares them. If the space is too shallow, you need a material with a higher R-value per inch, such as some foam boards, or you need to add a layer elsewhere. Add layers together with the R-value calculator.
Heating and cooling loads are related, and the BTU calculator can help you size equipment once the building is better insulated.
Why these inputs are yours to set
R-value per inch differs by material, brand and density, and loose fill may settle after it is installed. Read the label of the product you plan to buy, and use its coverage chart if it has one. The target R-value for your home depends on your climate and your local code.
Gaps, compressed batts, uninsulated framing and air leaks all lower what insulation delivers. For material on other surfaces, try the drywall calculator.
Assumptions and limits
- The target R-value, R-value per inch, package size, waste and price are your inputs, and the starting numbers are examples.
- Thickness is the same everywhere in the area, and settling is not modeled.
- The tool does not check local code, vapor barriers, ventilation or fire safety.
- Packages are rounded up to whole units.
Frequently asked questions
How thick does insulation need to be for a given R-value?
Divide the R-value you want by the R-value per inch of the material. R-38 at 3.5 per inch is about 10.86 inches.
How do I know how many bags of loose fill I need?
Multiply the area by the thickness in feet to get cubic feet, add waste, and divide by the volume in each bag. The bag label may also give a coverage at a specific depth.
What if the space is too shallow for the R-value I want?
Use a material with a higher R-value per inch or add insulation in another layer. The tool shows the R-value that fits in the depth you enter.
Is a higher R-value always better?
A higher R-value resists heat flow more, but the right amount depends on climate, cost and code, and other parts of the building matter too.