How to Use This Calculator
Select your insulation material: blown cellulose, blown fiberglass, or fiberglass batts. Blown materials are used in open attic floors and wall retrofits. Batts are used in new construction walls and floors between joists. Enter the area in square feet. Set your target R-value: the R-value you want to achieve after installation. If you have existing insulation, enter its current R-value in the existing R-value field; the calculator finds only the additional material needed. Results show additional depth in inches, bag or roll count with 10% waste, total final R-value, and a cost range. The Attic Insulation Calculator page includes a climate zone R-value guide by US region.
How R-Value Determines Insulation Depth and Quantity
R-value and depth are directly related: R-value = Depth (inches) × R-value per inch of the material. To find required depth: Depth = R-value target ÷ R-value per inch.
| Insulation type | R per inch | Inches for R-38 | Inches for R-49 | Inches for R-60 |
|---|---|---|---|---|
| Blown cellulose | 3.7 | 10.3 | 13.2 | 16.2 |
| Blown fiberglass | 2.5 | 15.2 | 19.6 | 24.0 |
| Fiberglass batts | 3.14 | 12.1 | 15.6 | 19.1 |
Example: existing insulation of R-11, target R-49, blown cellulose. Additional R needed = 49 - 11 = 38. Depth to add = 38 ÷ 3.7 = 10.3 inches. For 1,000 sq ft attic: Volume = 1,000 × (10.3 ÷ 12) = 858 cu ft. At 1.5 lb/cu ft: 1,287 lbs. At 30 lbs per bag × 1.10 waste: 47.2 bags, order 48.
Choosing the Right R-Value for Your Project
Focus on air sealing before chasing higher R-values. Increasing attic insulation from R-38 to R-60 saves less energy per dollar than sealing air leaks in a home at R-38. Warm air carries moisture and heat; an air leak through a penetration the size of a quarter can offset the benefit of several inches of additional insulation. Prioritize the R-value recommendation for your zone, then address air sealing.
R-values are additive but thermal bridging is real. The stated R-value of an insulation batt is for the insulation material alone. In a wall, wood studs conduct heat at about R-1 per inch. A 2 × 4 stud wall insulated with R-13 batts has an effective whole-wall R-value of about R-11 to R-12 because studs make up roughly 15% of the wall area. This framing factor is why continuous exterior insulation is so effective at improving whole-wall performance.
For attic top-ups, there is no need to remove existing batts. Blown insulation installs directly over existing fiberglass batts. If existing batts are kraft-paper faced, the paper facing should be on the warm side (facing down). Blown insulation settles slightly around and between existing batts, improving coverage. The calculator accounts for your existing R-value and calculates only the additional material needed.
What to Buy
For blown insulation, the bag label tells you everything you need: R-value per inch, bags per 1,000 sq ft at your target R-value, and bag weight. Use the label's own coverage table alongside the calculator result; if they differ by more than 5 to 10%, verify your inputs. Major brands (Owens Corning, CertainTeed, GreenFiber) all produce reliable products with consistent performance.
For fiberglass batts, match the batt to your framing. For 2 × 4 walls: 15-inch wide batts for 16-inch on-center framing, 23-inch wide batts for 24-inch on-center. For 2 × 6 walls: same widths, but use R-19 or R-21 batts for the deeper cavity. For attic floors over joists: use unfaced batts placed perpendicular to the existing insulation direction. Faced batts are for new construction where vapor control location is specified by code.