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Insulation Calculator: R-Value, Depth and Bags
Find how many inches of insulation you need to reach a target R-value, and how many bags of blown-in cellulose or fiberglass, packages of batts or board feet of spray foam that takes for your attic, walls or floor.
How the calculation works
Every insulation material has an R-value per inch: how much it resists heat flow for each inch of thickness. To reach a target, subtract the R-value you already have, then divide by the material's R per inch:
inches to add = (target R − existing R) ÷ R per inch
For loose fill, multiply the area by that depth to get cubic feet, then by the installed density to get pounds and bags. Batts are sold already rated (R-13, R-19, R-30 and so on), so you only need the area. Spray foam is priced by the board foot: one square foot, one inch thick.
Worked example: topping up an attic
A 1,000 sq ft attic has about 3.5 in of old fiberglass, roughly R-11. To reach R-49 you need R-38 more. Blown cellulose gives about R-3.6 per inch, so that is 10.6 in. The volume is 1,000 × 10.6 ÷ 12 = 880 cu ft; at about 1.5 lb per cu ft settled, that is around 1,320 lb, or 53 bags of 25 lb. Many home centers lend the blowing machine free with a minimum bag purchase.
R-value per inch
| Material | R per inch |
|---|---|
| Fiberglass batts | 2.9 to 3.8 |
| Blown-in fiberglass | 2.2 to 2.9 |
| Blown-in cellulose | 3.2 to 3.8 |
| Mineral wool batts | 3.3 to 4.2 |
| Open-cell spray foam | 3.5 to 3.8 |
| Closed-cell spray foam | 6.0 to 7.0 |
| Rigid foam board (XPS / polyiso) | 5.0 / 6.0 to 6.5 |
How much insulation do you need?
The US Department of Energy recommends levels by climate zone. Roughly:
- Attics: R-30 in the warmest zones, R-49 in most of the country, and up to R-60 in the coldest northern states.
- 2 × 4 walls: R-13 to R-15 in the cavity. 2 × 6 walls: R-19 to R-21.
- Floors over unheated crawl spaces or garages: R-13 in the south up to R-30 in cold climates.
Before you insulate
- Seal air leaks first: gaps around chimneys, plumbing stacks, light fixtures and the attic hatch let warm air bypass even thick insulation.
- Keep blown insulation away from recessed lights not rated IC and at least 3 in from chimneys and flues. Use baffles so soffit vents stay open.
- Wear a dust mask or respirator, goggles and long sleeves when working with fiberglass or cellulose.
Sizing a new AC after insulating? Try the BTU calculator.
Frequently asked questions
How many bags of blown insulation do I need for 1,000 sq ft?
Going from bare to R-38 with cellulose takes about 10.5 in, roughly 1,300 lb or 53 bags of 25 lb. Blown fiberglass needs about 15 in for R-38, around 25 bags of 28 lb. Always check the bag's coverage chart.
How thick is R-49 insulation?
About 13.5 in of blown cellulose, 17 to 20 in of blown fiberglass, or 15 in of fiberglass batts.
Can I add new insulation on top of old?
Yes, as long as the old insulation is dry and not moldy. Subtract its R-value (roughly R-3 per inch for fiberglass) from your target and add only the difference. Use unfaced batts or loose fill on top.