Attic insulation calculator: bags by R-value
Every bag of loose-fill insulation is printed with a coverage chart, and the calculator uses the two most common DIY products' current charts — Owens Corning's AttiCat fiberglass (May 2026 data sheet) and Greenfiber cellulose (May 2026 DIY guide). Each cellulose bag covers less area because cellulose is denser, so you carry roughly three times as many bags up the ladder for the same R-value.
| R-value | Fiberglass bags / 1,000 sq ft | Fiberglass depth | Cellulose bags / 1,000 sq ft | Cellulose depth |
|---|---|---|---|---|
| R-19 | 9 | 7 in | 22.6 | 5.7 in |
| R-30 | 14.6 | 10-3/4 in | 38.6 | 8.9 in |
| R-38 | 19 | 13-1/2 in | 52.6 | 11.2 in |
| R-49 | 25 | 17 in | 70.8 | 14.3 in |
| R-60 | 31.5 | 20-1/2 in | 86 | 17.3 in |
Both manufacturers say the same thing in bold on the bag: you need both the minimum number of bags and the minimum installed depth to get the rated R-value. Fluffing the material to hit the depth with fewer bags gives you less insulation than the label says.
How much insulation should be in your attic?
ENERGY STAR recommends R-30 in climate zone 1, R-49 in zones 2–3 and R-60 in zones 4–8 for an attic floor. Leave Target R-value on Zone and the calculator uses that level; pick R-38 or R-49 if you're working to a code minimum or a budget.
As a quick check, measure what's there now. Less than about 11 inches of fiberglass or 8 inches of cellulose means you're below R-30 and will notice the difference on your heating and cooling bills. The electricity cost calculator can put a number on what your HVAC currently costs to run.
Topping up existing insulation
This is where most people overbuy. If you already have insulation, the calculator estimates its R-value from the depth — about R-2.5 per inch for old loose fiberglass, R-3.1 for fiberglass batts and R-3.5 for cellulose — subtracts it from your target, and sizes only the difference. Greenfiber's instructions use exactly this subtract-what-you-have method.
Example: a 30 × 40 ft attic in zone 4 with 4 inches of old fiberglass. Existing ≈ R-10, target R-60, so you add R-50: about 31 bags of fiberglass, blown roughly 17 inches deep on top.
Fiberglass or cellulose?
- Fiberglass — fewer, lighter bags to handle; doesn't absorb water; needs more depth for the same R-value.
- Cellulose — about 3.5 R per inch, so it fits low-clearance attics better, and it's denser, which helps block air movement; it's heavier to haul.
Either way, install baffles at the eaves so soffit vents stay open, build a dam around the attic hatch, and keep insulation at least 3 inches from recessed lights that aren't IC-rated and from chimneys and flues. Air-seal the top plates and wiring holes before you blow — insulation doesn't stop air leaks. Prefer batts for an unfinished attic you'll be walking on? The insulation calculator handles two-layer batt jobs.