Roof & Attic Insulation Calculator — ft, R-value, cost
How to use this calculator
Enter your attic floor area (length x width or custom area), select insulation type and target R-value. If you have existing insulation, enter its depth and type to calculate the additional insulation needed. Results update instantly with bags/rolls, depth to add, and optional vapor barrier and ventilation baffles.
Insulation types — blown fiberglass (R-2.5/in), blown cellulose (R-3.7/in), batt fiberglass (R-3.2/in), batt mineral wool (R-3.3/in).
Cost — price per bag or roll plus optional vapor barrier and ventilation baffles.
Saved Calculations
| Time | Type | Area ft² | Depth | R-Value | Qty | Cost |
|---|
How to Calculate Attic Insulation
This calculator estimates the insulation depth, number of bags or rolls, and cost to reach your target R-value. Enter attic floor dimensions in feet and inches, select insulation type (blown fiberglass R-2.5/in, blown cellulose R-3.7/in, batt fiberglass R-3.2/in, or batt mineral wool R-3.3/in), and set your target R-value per IECC climate zone requirements.
Formulas
Depth needed = (Target R-value − Existing R-value) / R-value per inch. Bags = Area / Coverage per bag at target depth. Coverage per bag decreases as depth increases — manufacturer tables are used for blown products. Batt rolls are calculated by area / roll coverage.
Climate Zone Guide (IECC 2021)
Zone 1-3: R-30 minimum. Zone 4: R-38. Zone 5-6: R-49. Zone 7-8: R-60. Adding insulation on top of existing is acceptable — the R-values are additive.
FAQ
Can I mix insulation types? Yes. Blown insulation can be added over existing batts. R-values add together regardless of type. Do I need a vapor barrier? In cold climates (zones 5+), a vapor barrier on the warm side is recommended. In mixed climates, consult local codes.
