How much insulation to hit your target R-value.
Enter the area, what R-value is already in place, and your target. See the thickness of material to add, how many rolls or bags to buy, and the total R-value you will reach. Updates live as you type.
The space
Fill length and width and the area fills in for you, or type the area directly.
Attics usually want R-30 to R-60, walls R-13 to R-21, by climate zone. R-38 is a common attic target.
Coverage depends on the fill depth printed on the package. Set it to match the product you plan to buy.
What this means
Thickness needed by material
Inches to add for your target R-value
Compare the material options
| Material | R per inch | Inches to add | Total thickness |
|---|
Insulation and R-value, explained
What R-value means and how to reach it
R-value measures how well a material resists heat flow, so a higher number means better insulation. Every product has an R-value per inch: blown cellulose is about R-3.5, fiberglass batts about R-3.2, and closed cell spray foam about R-6.5. To find the thickness you need, divide the R-value you want to add by the R per inch of your chosen material.
If insulation is already in place, you only add the difference. Reaching R-38 in an attic that already holds R-11 means adding R-27, not R-38, because R-values of stacked layers add together.
Target R-values by climate zone
How much you need depends on where you live. Energy codes divide the country into climate zones, and colder zones call for more. Attic targets commonly range from about R-30 in the mildest southern zones to R-49 or R-60 in the coldest northern ones. Wall assemblies usually land between R-13 and R-21. This calculator lets you set any target, so you can match your local code or push past it for lower energy bills.
Buying the right amount without waste
Once you know the thickness, count the packages by area. Divide the square footage you are covering by the coverage printed on each bag or roll, then round up. Buy one extra package to allow for trimming, odd corners and settling. Add new insulation on top of sound existing material rather than tearing it out, and never compress batts to force them into a tight space, because compression squeezes out the air pockets that give insulation its R-value.
Common questions
How do I convert R-value to inches of insulation?
Divide the R-value you need by the material R per inch. To add R-30 with blown cellulose at R-3.5 per inch, you need about 30 divided by 3.5, which is roughly 8.6 inches.
Does adding insulation over old insulation work?
Yes. As long as the existing insulation is dry and not compressed or moldy, new insulation laid on top adds its R-value to what is already there. Stacking layers is a normal and effective way to top up an attic.
Why should I not compress insulation to fit?
Insulation works by trapping still air in its fibers or cells. Compressing it removes those air pockets, so a batt crammed into too thin a cavity delivers a lower R-value than its label says. Pick a product sized for the space instead.
What R-value should my attic be?
It depends on your climate zone, but most attics land between R-30 and R-60. Warmer southern zones often target around R-30 to R-38, while colder northern zones call for R-49 to R-60. Check your local building code for the exact minimum.
Estimates for planning only. R-values are nominal, per inch figures for common products, and real performance varies with density, gaps, moisture and installation. Never compress batts to fit, since compression lowers the R-value. Verify package coverage and local code before you buy.