Attic Insulation Calculator
Find out how many inches of insulation an attic needs to reach a target R-value, based on what is already up there and the material you plan to add. For insulation contractors and the homeowners they quote.
Free, no sign-up. Runs in your browser; nothing you type is stored unless you choose to email yourself the results.
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Three steps, about a minute
Step 1
Pick a Target R-value from the list, using the recommendation for the home's climate zone.
Step 2
Choose the Material you plan to install, from fiberglass batts and blown-in products to spray foam.
Step 3
Measure Existing insulation depth, set Existing R per inch, then read Inches of new material.
How the attic insulation calculator works
Current R-value = existing depth in inches x existing R per inch. R-value to add = target minus current, never below zero. Inches of new material = R-value to add divided by the new material's R per inch. Take the defaults: 4 inches of old insulation at R-3 per inch gives R-12. Reaching R-49 means adding R-37, which takes about 10.6 inches of blown cellulose at roughly R-3.5 per inch. The R-per-inch figures in the list are typical values, and real products vary, so check the bag or spec sheet. Old insulation that is compressed, wet or damaged performs below its rating. Recommended attic levels depend on climate zone, and your local energy code sets the minimum, so confirm the target before quoting.
| Target R-value | R-49 |
|---|---|
| Material | Blown cellulose (~R-3.5/in) |
| Existing insulation depth | 4 in |
| Existing R per inch | 3 |
| Current R-value | R-12 |
|---|---|
| R-value to add | R-37 |
| Inches of new material | 10.6 in |
R-per-inch values are typical ranges for each material. Recommended attic R-values depend on your climate zone.
Using the numbers like a pro: tips for insulation contractors
Quote blown insulation from the manufacturer's bag chart, not just the R-per-inch math. The chart lists bags per 1,000 sq ft and the minimum settled thickness for each R-value.
Measure existing depth in several spots and photograph it next to a ruler. Old insulation is often uneven, and the photos show the homeowner exactly why a top-off is needed.
Put air sealing, baffles and attic hatch insulation on the quote as line items. They protect the new insulation's performance and give you a clear reason to price above a bare per-bag number.
Put a calculator like this on your own website
Homeowners searching for attic R-values are often close to booking. A calculator on your site shows how many inches they are missing, then offers an attic check or written quote in exchange for a name, address and phone number. The result gives them a reason to act now instead of next season. We set up tools like this for insulation contractors, matched to the materials they install.
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Attic Insulation Calculator questions
What R-value should attic insulation be?
It depends on the climate zone. ENERGY STAR publishes recommended attic levels by zone, generally between R-30 and R-60, with colder zones at the higher end. Your local energy code sets the minimum for new work and some renovations. Look up your zone first, then compare the recommendation with what is already in the attic.
How many inches of insulation should be in my attic?
Divide the R-value you want by the R-value per inch of the material. Blown cellulose at about R-3.5 per inch needs roughly 14 inches to reach R-49, while blown fiberglass at about R-2.5 per inch needs closer to 20 inches. Product ratings vary, so check the manufacturer's coverage chart for the exact settled depth.
Can you put new insulation over old insulation?
Usually yes, as long as the old insulation is dry, free of mold and pests, and not hiding roof leaks or major air leaks. Blown insulation can go right over existing material. If you add batts on top, use unfaced batts so you do not trap moisture with a second vapor retarder. Wet or contaminated material should come out first.
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