Key takeaways
- R-value to add = target − what you have, and depth = R-value to add ÷ R per inch.
- The 2021 IRC ceiling figures are R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8.
- If you see pebble-like fill, stop: EPA says to assume vermiculite contains asbestos and leave it alone.
Enter your climate zone, what you already have, and what you'd add. Check your attic first: don't disturb any loose, pebble-like fill.
How the calculator works
- R-value to add: target − existing R-value
- Depth: R-value to add ÷ R per inch of the product
Worked example
Zone 5, with R-11 in place, adding blown cellulose at R-3.7 per inch:
- 60 − 11 = R-49 to add
- 49 ÷ 3.7 = about 13.2 inches
Targets by zone
| Climate zone | 2021 IRC ceiling figure | ENERGY STAR uninsulated attic |
|---|---|---|
| 1 | R-30 | R-30 |
| 2 | R-49 | R-49 |
| 3 | R-49 | R-49 |
| 4 | R-60 | R-60 |
| 5 and 6 | R-60 | R-60 |
| 7 and 8 | R-60 | R-60 |
The IRC table sets minimums for code work, while ENERGY STAR's are retrofit levels for existing wood-framed buildings. ENERGY STAR also gives a lower level for attics with 3 to 4 inches already, such as R-49 in zones 4 to 8. So you may not need to reach the top figure, and you can compare them with what the job would take. Section N1102.2.1 has an allowance for R-38 or R-49 at the eaves, which we haven't fully read, so ask your building department.
We haven't built a ZIP lookup. Find your zone on the IRC's map (Figure N1101.7) or ask your building department. ENERGY STAR's window zones are different.
R-value per inch
| Material | R per inch | Source |
|---|---|---|
| Blown cellulose | 3.7 | Secondary, DOE-attributed table |
| Blown fiberglass | 2.2 | Secondary, DOE-attributed table |
| Fiberglass batt | 2.5 to 4 | PNNL |
These vary by product, and PNNL says to use the maker's data. Blown products carry a coverage chart with the depth and bags you need, which replaces our estimate.
Stop if you see vermiculite
Pebble-like, gray-brown or silver-gold loose fill could be vermiculite. EPA says to assume it contains asbestos, and to:
- Leave it undisturbed
- Limit trips into the attic, and don't store things there or let children play there
- Not try to remove it yourself
- Hire a professional asbestos contractor before any work that would disturb it
Questions to settle first
We haven't sourced detailed guidance on these, so get professional advice:
- Old wiring: older wiring types and damaged wiring are a question for an electrician before you cover anything.
- Recessed lights and flues: clearances depend on the fixture and the code.
- Air sealing and ventilation: keep vent paths clear. See the attic ventilation calculator.
- Spray foam: this has its own product and safety requirements, and we don't cover it here.
Frequently asked questions
How much attic insulation do I need?
That depends on your zone and what you have. The 2021 IRC's ceiling figures range from R-30 to R-60.
How many inches is R-60?
About 16.2 inches of cellulose at R-3.7 per inch, or about 27.3 inches of fiberglass at R-2.2 per inch. Check the product's label.
Are there federal tax credits for insulation?
The IRS says its home improvement credit covered property placed in service by December 31, 2025, so check current rules before counting on any incentive.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- 2021 International Residential Code, Chapter 11, Table N1102.1.3
- ENERGY STAR: Insulation R-values
- PNNL Building America: Typical R-values
- EPA: Vermiculite insulation
- IRS: Energy efficient home improvement credit