Roof Insulation Calculator
Calculate required insulation thickness, R-value, and material quantity for attics and roofs. Compare Fiberglass, Spray Foam, and Cellulose.
About
Proper roof insulation is the single most effective way to reduce energy costs in residential structures. Heat loss through the roof accounts for a significant percentage of total thermal inefficiency. This calculator determines the necessary thickness of insulation material to achieve a specific Target R-Value. It accounts for the thermal resistance per inch of various materials, including fiberglass batts, loose-fill cellulose, and closed-cell spray foam.
Building codes vary by climate zone, often requiring R-values between R-30 and R-60 for attics. Choosing the right material involves balancing depth constraints (rafter size) with thermal performance. For instance, closed-cell foam offers high resistance in shallow spaces, whereas blown cellulose is cost-effective for deep attic floors. This tool provides material-specific volume and unit purchasing estimates based on standard coverage metrics.
Formulas
The calculation for required thickness is derived from the target thermal resistance (Rtarget) and the material's resistance per unit of length (rinch):
Total volume required is calculated based on the area (A):
For loose-fill materials, the number of bags depends on the coverage rate (C) at the specified depth:
Reference Data
| Material | R-Value per Inch | Typical Density | Application |
|---|---|---|---|
| Fiberglass Batts | R-3.1 to R-4.3 | Low | Joist cavities, rafters |
| Loose-Fill Fiberglass | R-2.2 to R-2.9 | 0.5 - 1.0 lb/ft³ | Attic floors (blown) |
| Cellulose (Loose-Fill) | R-3.2 to R-3.8 | 1.5 - 2.0 lb/ft³ | Attic floors (blown) |
| Mineral Wool Batts | R-4.0 to R-4.5 | Medium | Fire resistance, sound |
| Open-Cell Spray Foam | R-3.5 to R-3.8 | 0.5 lb/ft³ | Roof decks, walls |
| Closed-Cell Spray Foam | R-6.0 to R-7.0 | 2.0 lb/ft³ | Roof decks, moisture barrier |
| Rigid Foam (XPS) | R-5.0 | High | Sheathing, below grade |
| Rigid Foam (Polyiso) | R-6.0 to R-6.8 | High | Roofing underlayment |
| Cotton Batts (Blue Jean) | R-3.5 | Medium | Eco-friendly option |
| Aerogel | R-10.3 | Very High | Specialized, space-constrained |