🧊 Insulation Calculator
Calculate the amount of insulation needed for walls, ceilings, attics, and floors. Determine R-value requirements and material quantities.
Quick Formula
Square Footage Method
Insulation Needed = Area (sq ft) × Coverage per Package (sq ft)
R-Value Method
R-Value Required = Desired R-Value ÷ R-Value per Inch × Thickness Needed
R-Value Requirements
By Climate Zone
| Zone | Attic | Walls | Floors |
|---|---|---|---|
| Zone 1 (Hot) | R30-R49 | R13-R15 | R13 |
| Zone 2 (Warm) | R30-R60 | R13-R21 | R13-R19 |
| Zone 3 (Moderate) | R30-R60 | R13-R21 | R19-R25 |
| Zone 4 (Cool) | R38-R60 | R15-R21 | R25-R30 |
| Zone 5 (Cold) | R49-R60 | R20-R21 | R25-R30 |
| Zone 6-8 (Very Cold) | R49-R60 | R20-R21 | R30-R38 |
Common Insulation Types
Fiberglass Batts
| Thickness | R-Value | Coverage (15" wide) | Best For |
|---|---|---|---|
| 3-1/2" | R11-R13 | 40-93 sq ft | 2×4 walls |
| 5-1/2" | R15-R21 | 40-93 sq ft | 2×6 walls |
| 6-1/4" | R19-R21 | 40-93 sq ft | 2×6 walls |
| 8-1/4" | R25-R30 | 40-93 sq ft | 2×8 walls |
| 10-1/4" | R30-R38 | 40-93 sq ft | Attics |
Blown-In Fiberglass
| Depth | R-Value | Coverage per Bag | Best For |
|---|---|---|---|
| 6" | R19 | ~40 sq ft | Attics |
| 8" | R25 | ~30 sq ft | Attics |
| 10" | R30 | ~24 sq ft | Attics |
| 12" | R38 | ~20 sq ft | Attics |
| 16" | R49 | ~15 sq ft | Attics |
Spray Foam
| Thickness | R-Value | Coverage per Kit | Best For |
|---|---|---|---|
| 1" | R6-R7 | Varies | Sealing, small areas |
| 2" | R12-R14 | Varies | Walls, rim joists |
| 3" | R18-R21 | Varies | Walls |
| 4" | R24-R28 | Varies | Walls, attics |
Rigid Foam Board
| Thickness | R-Value | Sheet Size | Best For |
|---|---|---|---|
| 1" | R5-R6.5 | 4×8' | Exterior, continuous |
| 2" | R10-R13 | 4×8' | Exterior, continuous |
| 3" | R15-R19.5 | 4×8' | Exterior, continuous |
| 4" | R20-R26 | 4×8' | Exterior, continuous |
Wall Insulation
Standard 2×4 Wall (16" OC)
Cavity:
- Batt size: 15" × 93" (R11-R13)
- Cavities per stud bay: 1
- Coverage: ~93 sq ft per package
Calculation:
Wall Area = Length × Height
Batt Packages = Wall Area ÷ 93 sq ft
Example: 20' × 8' wall
- Wall Area = 20 × 8 = 160 sq ft
- Packages = 160 ÷ 93 = 1.72 = 2 packages
Standard 2×6 Wall (16" OC)
Cavity:
- Batt size: 15" × 93" (R15-R21)
- Coverage: ~93 sq ft per package
Calculation: Same as 2×4, but higher R-value
Attic Insulation
Blown-In Method
Formula:
Bags Needed = (Attic Area × Desired Depth) ÷ Coverage per Bag
Example: 1,200 sq ft attic, 12" depth, R38
- Coverage per bag: ~20 sq ft at 12"
- Bags = (1,200 × 12) ÷ (20 × 12) = 60 bags
Batt Method
Formula:
Batt Packages = Attic Area ÷ Coverage per Package
Example: 1,200 sq ft attic
- Coverage: ~93 sq ft per package
- Packages = 1,200 ÷ 93 = 13 packages
Floor Insulation
Crawl Space
Batt insulation between joists:
Batt Packages = Floor Area ÷ Coverage per Package
Example: 800 sq ft floor
- Packages = 800 ÷ 93 = 9 packages
Basement Walls
Rigid foam board:
Sheets Needed = Wall Area ÷ Sheet Coverage
Example: 100 linear feet × 8' high = 800 sq ft
- Sheet coverage: 4×8' = 32 sq ft
- Sheets = 800 ÷ 32 = 25 sheets
Waste Factor
Always add waste for:
- Cutting around obstacles
- Fitting around pipes, wires
- Handling damage
- Irregular spaces
| Application | Waste Factor |
|---|---|
| Walls | 5-10% |
| Attics | 5% |
| Floors | 10-15% |
| Irregular spaces | 15-20% |
Common Calculations
Whole House Example
House: 2,000 sq ft, 2 stories, 1,000 sq ft attic
Walls:
- Perimeter: 180 linear feet
- Wall area: 180 × 18' = 3,240 sq ft
- Batt packages: 3,240 ÷ 93 = 35 packages
Attic:
- Attic area: 1,000 sq ft
- Blown-in bags (R38, 12"): 1,000 ÷ 20 = 50 bags
Total: 35 batt packages + 50 blown-in bags
R-Value per Inch
| Material | R-Value per Inch |
|---|---|
| Fiberglass (batt) | R3.1-R3.8 |
| Blown fiberglass | R2.2-R2.9 |
| Blown cellulose | R3.1-R3.8 |
| Spray foam (open) | R3.5-R4.0 |
| Spray foam (closed) | R6.0-R7.0 |
| Rigid foam (XPS) | R5.0 |
| Rigid foam (EPS) | R3.6-R4.0 |
| Rigid foam (polyiso) | R6.0-R6.5 |
Vapor Barriers
When Required
- Cold climates - Vapor barrier on warm side (interior)
- Hot climates - Vapor barrier on cool side (exterior)
- Mixed climates - May not need vapor barrier
Installation
- Proper side - Vapor barrier on correct side
- Continuous - No gaps or tears
- Sealed - Seams sealed with tape
- Overlapped - Proper overlap at seams
Best Practices
- Check local codes - R-value requirements vary
- Fill cavities completely - No gaps or voids
- Don't compress - Compression reduces R-value
- Seal air leaks first - Before insulating
- Consider moisture - Prevent condensation issues
Common Mistakes
| Mistake | Problem | Solution |
|---|---|---|
| Compressing insulation | Reduces R-value | Don't compress |
| Gaps around edges | Air leakage | Fill completely |
| Wrong R-value | Code violation | Check requirements |
| No vapor barrier | Moisture issues | Install if required |
| Insufficient depth | Low R-value | Meet minimum requirements |
Related Resources
- Construction Math Reference - Area calculations
- Drywall Calculator - Wall finishing
- Paint Calculator - Paint quantities
Air Sealing First
Seal air leaks before insulating. Air leaks can reduce insulation effectiveness by 50% or more.