How this calculator works
R-value is insulation's resistance to heat flow, and it adds up: two layers of R-19 give R-38. To get from the R-value you have to the one you want, you need to add the difference, in a thickness set by how many R-points each inch of your chosen material provides.
Worked example: a 40 × 30 ft attic (1,200 ft²) with R-11 now and a target of R-49 needs 38 more R-points. With blown fiberglass at R-2.5 per inch that is 15.2 inches, or 1,520 ft³ of insulation.
Assumptions and sources
- Loose-fill insulation settles and its R-value per inch varies with the product, so use the coverage chart on the bag for the exact bag count. Air-seal gaps and bypasses before insulating, keep insulation off the soffit vents (use baffles), and keep it clear of recessed lights unless they are IC-rated.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
Recommended attic insulation by US climate zone
From Energy Star guidance for existing homes with an uninsulated attic floor. Check your zone and local code.
| Climate zone | Typical regions | Recommended attic R-value |
|---|---|---|
| 1 | Southern Florida, Hawaii | R-30 to R-49 |
| 2 | Gulf Coast, southern Texas, Arizona | R-30 to R-60 |
| 3 | Southeast, southern California | R-30 to R-60 |
| 4 | Mid-Atlantic, Pacific Northwest, southern Midwest | R-38 to R-60 |
| 5 to 8 | Northern states, mountains, Alaska | R-49 to R-60 |
Tips before you buy
- Air-seal first. Sealing gaps around pipes, wires, recessed lights and the attic hatch often saves as much as the insulation itself.
- Keep a clear air path at the eaves with baffles, so that insulation does not block the soffit vents. See the attic ventilation calculator.
- Do not cover recessed lights, exhaust fans or flues unless they are rated for it, and keep insulation 3 inches from hot flues.
- Wear a mask, goggles, long sleeves and gloves. Blown-in insulation can be rented from home centers, and often comes free with a purchase of bags.
- Vermiculite insulation (loose, pebble-like) may contain asbestos. Do not disturb it: have it tested.
Quick answers by size
Tools & materials for this project
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Frequently asked questions
How much attic insulation do I need?
Most guidance recommends R-38 to R-60 for attics, depending on the climate. In practice that is 10 to 20 inches of loose-fill fiberglass or cellulose.
How many inches of insulation is R-49?
About 20 inches of blown fiberglass (R-2.5 per inch), or 14 inches of blown cellulose (R-3.5 per inch).
Can I add new insulation over old?
Usually yes, if the old insulation is dry and clean. Do not use batts with a paper or foil facing on top of existing insulation: use unfaced.
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Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.