Planning Guide
Attic Insulation Depth Chart
Inches to Approximate R-Value
Depth is the fastest field check for attic insulation. Once you know the material, inches can give you a rough thermal picture.
A depth chart is not a lab test, but it is often good enough for homeowners deciding whether they are obviously under-insulated. The key is to know both the material and the average depth across the attic floor.
Quick Depth Chart
| Material | Approx. R per Inch | 10 in. | 14 in. | 18 in. |
|---|---|---|---|---|
| Fiberglass Batts | ~3.2 | R-32 | R-45 | R-58 |
| Blown-in Fiberglass | ~2.5 | R-25 | R-35 | R-45 |
| Blown-in Cellulose | ~3.5 | R-35 | R-49 | R-63 |
How to Measure Correctly
- Measure several spots, not just the deepest area.
- Ignore piles caused by drift or uneven blowing.
- Check around eaves, hatches, and mechanical areas where depth often drops off.
Why the Same Depth Does Not Mean the Same R-Value
Ten inches of blown fiberglass is not the same as ten inches of cellulose. Compression, age, settling, and installation quality also change real-world performance, so charts are only a starting point.
Compare Depth to Your Climate Zone Target
The depth only becomes meaningful when you compare it to your zone target. For example, a depth that looks decent in Zone 2 may still be far short of what a Zone 6 attic needs.