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🧊 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​

ZoneAtticWallsFloors
Zone 1 (Hot)R30-R49R13-R15R13
Zone 2 (Warm)R30-R60R13-R21R13-R19
Zone 3 (Moderate)R30-R60R13-R21R19-R25
Zone 4 (Cool)R38-R60R15-R21R25-R30
Zone 5 (Cold)R49-R60R20-R21R25-R30
Zone 6-8 (Very Cold)R49-R60R20-R21R30-R38

Common Insulation Types​

Fiberglass Batts​

ThicknessR-ValueCoverage (15" wide)Best For
3-1/2"R11-R1340-93 sq ft2×4 walls
5-1/2"R15-R2140-93 sq ft2×6 walls
6-1/4"R19-R2140-93 sq ft2×6 walls
8-1/4"R25-R3040-93 sq ft2×8 walls
10-1/4"R30-R3840-93 sq ftAttics

Blown-In Fiberglass​

DepthR-ValueCoverage per BagBest For
6"R19~40 sq ftAttics
8"R25~30 sq ftAttics
10"R30~24 sq ftAttics
12"R38~20 sq ftAttics
16"R49~15 sq ftAttics

Spray Foam​

ThicknessR-ValueCoverage per KitBest For
1"R6-R7VariesSealing, small areas
2"R12-R14VariesWalls, rim joists
3"R18-R21VariesWalls
4"R24-R28VariesWalls, attics

Rigid Foam Board​

ThicknessR-ValueSheet SizeBest For
1"R5-R6.54×8'Exterior, continuous
2"R10-R134×8'Exterior, continuous
3"R15-R19.54×8'Exterior, continuous
4"R20-R264×8'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
ApplicationWaste Factor
Walls5-10%
Attics5%
Floors10-15%
Irregular spaces15-20%

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​

MaterialR-Value per Inch
Fiberglass (batt)R3.1-R3.8
Blown fiberglassR2.2-R2.9
Blown celluloseR3.1-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​

  1. Check local codes - R-value requirements vary
  2. Fill cavities completely - No gaps or voids
  3. Don't compress - Compression reduces R-value
  4. Seal air leaks first - Before insulating
  5. Consider moisture - Prevent condensation issues

Common Mistakes​

MistakeProblemSolution
Compressing insulationReduces R-valueDon't compress
Gaps around edgesAir leakageFill completely
Wrong R-valueCode violationCheck requirements
No vapor barrierMoisture issuesInstall if required
Insufficient depthLow R-valueMeet minimum requirements
Air Sealing First

Seal air leaks before insulating. Air leaks can reduce insulation effectiveness by 50% or more.