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This insulation calculator works out how much insulation your attic or floor needs. Choose your climate zone and enter how much insulation you already have, and it shows the recommended R-value, how much R-value to add, the depth in inches for your chosen material and the total volume to cover the area.
How to Use the Insulation Calculator
- Measure the area to insulate in square feet (length × width of the attic floor or crawlspace ceiling).
- Choose the location and your IECC climate zone, or enter your own target R-value.
- Measure the depth of any existing insulation with a ruler and enter it in inches.
- Choose the type of insulation you plan to add.
Recommended R-Values by Climate Zone
ENERGY STAR recommends these total levels when retrofitting wood-framed homes:
| Climate zone | Uninsulated attic | Attic with 3–4 in existing | Floor |
|---|---|---|---|
| 1 | R-30 | R-25 | R-13 |
| 2 | R-49 | R-38 | R-13 |
| 3 | R-49 | R-38 | R-19 |
| 4A and 4B | R-60 | R-49 | R-19 |
| 4C, 5 and 6 | R-60 | R-49 | R-30 |
| 7 and 8 | R-60 | R-49 | R-38 |
Zone 1 covers South Florida and Hawaii, while zones 7 and 8 cover northern Minnesota and Alaska. Most of the Midwest and Northeast is zone 5 or 6.
How the Calculation Works
R-value to add = Recommended R-value − (Existing depth × ≈R-3 per inch)
Depth to add = R-value to add ÷ R-value per inch of the new material
Older fiberglass or cellulose generally provides about R-2.5 to R-3.5 per inch, so the calculator uses R-3 per inch for existing insulation. If you know the actual R-value of your existing insulation, choose “Other” and enter the remaining R-value you want to add as the target.
R-Value per Inch by Insulation Type
| Insulation type | Approx. R-value per inch | Best for |
|---|---|---|
| Blown-in fiberglass | R-2.2 to R-2.7 | Open attic floors |
| Blown-in cellulose | R-3.2 to R-3.8 | Attic floors, closed walls |
| Fiberglass batts | R-2.9 to R-3.8 | Floors, walls, open framing |
| Mineral wool batts | about R-3.0 to R-3.3 | Fire and sound resistance |
| Open-cell spray foam | about R-3.5 to R-3.7 | Air sealing, irregular spaces |
| Closed-cell spray foam | about R-6 to R-7 | Rim joists, basements, tight spaces |
Example
A 1,000 sq ft attic in zone 5 with 4 inches of old insulation (about R-12) needs R-60 in total, so you should add about R-48. With blown-in fiberglass at R-2.5 per inch, that is about 19 inches or 1,600 cubic feet of insulation.
Tips Before You Insulate
- Seal air leaks around plumbing, wiring and light fixtures first; insulation does not stop air movement.
- Keep insulation away from recessed lights not rated IC and leave clearance around chimneys.
- Install baffles at the eaves so soffit vents stay open.
- Use the coverage chart printed on the bag to convert your target R-value into a number of bags.
Sources
Frequently Asked Questions
How much attic insulation do I need?
ENERGY STAR recommends R-49 to R-60 for attics in most of the US, which is about 20–24 inches of blown-in fiberglass or 14–17 inches of cellulose.
How many inches is R-38?
R-38 is about 15 inches of blown-in fiberglass, 11 inches of blown-in cellulose or 12 inches of fiberglass batts.
Can you add new insulation on top of old insulation?
Yes, as long as the old insulation is dry and free of mold. Do not use faced batts on top of existing insulation; use unfaced batts or loose-fill.
What R-value should floor insulation be?
ENERGY STAR recommends R-13 in zones 1–2, R-19 in zones 3–4A/4B, R-30 in zones 4C–6 and R-38 in zones 7–8.
Is blown-in or batt insulation better for an attic?
Blown-in insulation fills gaps around joists and obstructions better, while batts are easier for DIY in open, regular framing.
Read the guide: How Much Attic Insulation Do I Need? R-Value by Climate Zone
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