Insulation Calculator - Calculate Batts, R-Value, and Thickness
Free insulation calculator. Calculate batts, thickness, and R-value for attics, walls, floors. Supports fiberglass, cellulose, XPS, and spray foam.
Area Configuration
Existing Insulation
Measure existing insulation thickness
Enter area dimensions and insulation details to see results.
About This Calculator
How to Calculate Insulation Needed
Measuring Tips
- Measure length and height of walls, or length and width of floors/ceilings
- For attics, measure the floor area, not the roof area
- Subtract windows, doors, and other openings from wall calculations
- Measure cavity depth to determine insulation thickness needed
- Account for obstructions like wiring, pipes, and ductwork
Insulation Material Guide
| Material | R-Value per Inch | Best For |
|---|---|---|
| Fiberglass Batts | R-3.1 | Standard walls, DIY friendly |
| Cellulose (Blown) | R-3.5 | Attics, retrofitting existing walls |
| XPS Foam Board | R-5.0 | Exterior walls, basements |
| Spray Foam | R-6.5 | Air sealing, irregular cavities |
| Mineral Wool | R-3.3 | Fire resistance, soundproofing |
Tips for Best Results
- Seal air leaks before installing insulation for maximum effectiveness
- Don't compress fiberglass batts - it reduces R-value
- Use vapor barriers in cold climates to prevent moisture issues
- Ensure proper ventilation in attics to prevent ice dams
- Wear protective gear when handling fiberglass insulation
Frequently Asked Questions
How do I calculate how much insulation I need?
Measure the length and height of each wall or the length and width of floors/ceilings to get square footage. Subtract windows, doors, and other openings. Divide by the coverage area of one batt or roll to determine how many you need. Most standard batts cover 24-40 square feet each.
What R-value do I need for my attic?
For attics, the DOE recommends R-38 to R-60 depending on your climate zone. Zones 1-3 need R-30 to R-49, zones 4-6 need R-38 to R-60, and zones 7-8 need R-49 to R-60. Higher R-values provide better insulation but require more thickness.
How thick should insulation be?
Thickness depends on the material and desired R-value. Fiberglass batts are typically 3.5" for R-13 (2x4 walls) and 5.5" for R-19 (2x6 walls). Cellulose blown-in requires about 10-14" for R-38. XPS foam board is about 1" per R-5.
What is the difference between fiberglass and cellulose insulation?
Fiberglass is made from glass fibers, is moisture resistant, and comes in pre-cut batts. Cellulose is made from recycled paper, has better air sealing properties, and is typically blown in. Cellulose has a slightly higher R-value per inch (R-3.5) compared to fiberglass (R-3.1).
How many batts of insulation do I need per room?
Calculate the total square footage of all walls and ceilings in the room, subtract windows and doors, then divide by the coverage of one batt. A standard 15" × 48" batt covers about 5 square feet. A typical 12×12 room with 8-foot ceilings needs approximately 20-30 batts for walls.