CalculateY
Quick Answer
To insulate 1,000 square feet of attic space to R-38 with fiberglass batts, approximately 12.1 inches of thickness and an estimated 87 bags or rolls are needed.
Attic: R-38 to R-60, Walls: R-13 to R-21, Floor: R-25 to R-30
Common Examples
| Input | Result |
|---|---|
| 1,000 sq ft, R-38, fiberglass | 12.1" thick, ~87 bags/rolls |
| 500 sq ft, R-13, cellulose | 3.5" thick, ~13 bags |
| 800 sq ft, R-19, mineral wool | 5.8" thick, ~33 bags/rolls |
| 600 sq ft, R-30, spray foam | 4.6" thick, ~20 units |
| 1,200 sq ft, R-49, fiberglass | 15.6" thick, ~134 bags/rolls |
How It Works
The Formula
Thickness (inches) = Desired R-Value / R-Value per Inch
Where the R-value per inch depends on the insulation material:
| Material | R-Value per Inch |
|---|---|
| Fiberglass Batts | 3.14 |
| Blown-In Cellulose | 3.70 |
| Closed-Cell Spray Foam | 6.50 |
| Mineral Wool | 3.30 |
Understanding R-Value
R-value measures thermal resistance, or how well a material resists heat flow. Higher R-values indicate better insulating performance. R-values are additive, so layers of insulation can be combined. For example, two layers of R-19 insulation provide R-38 total.
Recommended R-Values by Region
The U.S. Department of Energy provides recommended R-values based on climate zone:
- Attic: R-38 to R-60 (colder climates need higher values)
- Exterior Walls: R-13 to R-21
- Floors over Unheated Spaces: R-25 to R-30
- Crawl Space Walls: R-13 to R-25
Material Comparisons
Fiberglass batts are the most common and affordable option, available in pre-cut widths to fit standard stud spacing (16” or 24” on center). Blown-in cellulose is made from recycled paper and is effective for filling irregular cavities and adding attic insulation. Spray foam provides the highest R-value per inch and also acts as an air barrier, but it costs significantly more. Mineral wool (also called rock wool) offers good fire resistance and sound dampening properties.
Material Estimation
The bag and roll estimates are based on a standard coverage of approximately 40 square feet per bag at a baseline thickness of 3.5 inches (R-13 equivalent). As the required thickness increases, more material is needed per square foot, and the bag count scales proportionally.
Worked Example
To insulate 1,000 square feet of attic to R-38 using fiberglass batts: Thickness = 38 / 3.14 = 12.10 inches. This is equivalent to two layers of R-19 fiberglass batts (each about 6 inches thick). Material estimate: thickness ratio = 12.10 / 3.5 = 3.46. Bags needed = ceil((1,000 / 40) x 3.46) = ceil(25 x 3.46) = ceil(86.5) = 87 bags. For the same area using spray foam at R-6.50 per inch: thickness = 38 / 6.50 = 5.85 inches, requiring far less material thickness but at a higher cost per square foot.