Insulation Calculator
Calculate fiberglass batt bundles, blown-in cellulose bag counts, cubic volume, and target DOE climate zone R-values for attics, walls, and floors.
Calculation Results
Total flat square footage of attic floor, wall cavities, or crawlspace.
Expected installed thickness in inches to achieve the target R-value.
Total cubic volume of insulating material needed.
Full multi-roll or multi-batt contractor packaging bags (based on standard factory package coverage).
Standard 25-30 lb bags of loose-fill cellulose or expanding fiberglass for attic blowers.
Estimated total insulation material cost based on square footage.
How This Calculator Works
Determine the right insulation requirements for your home in 4 steps:
- Measure the Envelope: Enter the length and width of the attic floor, exterior wall run, or crawlspace ceiling.
- Select Target R-Value: Pick the appropriate R-value based on your local building code and US Department of Energy climate zone.
- Choose Framing Spacing: Standard residential framing is 16" on-center (takes 15" batts); newer energy-efficient framing uses 24" on-center (takes 23" batts).
- Select Purchase Method: Check the output for pre-cut batt bags if installing by hand, or loose-fill bags if renting an attic blowing machine.
Formula & Calculations
Insulation calculations determine thermal resistance (R-value), installed depth, and package quantities for batts or loose-fill:
1. Area & Cubic Volume
Surface Area (sq ft) = Length × Width
Material Volume (cu ft) = Surface Area × (Installed Thickness in inches / 12)
2. Department of Energy (DOE) R-Value Recommendations
- Climate Zones 1-3 (Southern US): Attic R-38 (approx 12" deep); 2x4 Walls R-13 to R-15; Floors R-19.
- Climate Zones 4-5 (Mid-Atlantic & Central US): Attic R-49 (approx 15.5" deep); 2x6 Walls R-19 to R-21; Floors R-25 to R-30.
- Climate Zones 6-7 (Northern US & Canada): Attic R-49 to R-60 (approx 16" to 19" deep); 2x6 Walls R-21+; Floors R-30.
3. Blown-In Bag Coverage Rule
One standard bag of loose-fill cellulose (25 lbs) yields roughly 1,350 R-sqft of thermal resistance. Dividing (Area × Target R-value) by 1,350 produces the exact number of blower bags required including settled density.
Step-by-Step Calculation Examples
Attic Retrofit in Climate Zone 4 (40ft x 25ft, R-49)
Insulating a 1,000 sq ft attic floor to R-49 using blown-in cellulose at $1.25/sq ft.
- length: 40
- width: 25
- target_r_value: 49
- stud_spacing: 16
- cost_per_sqft: 1.25
- net_area: 1000.0 sq ft
- nominal_thickness: 15.5 in
- volume_cuft: 1291.7 cu ft
- batt_packages: 25
- blown_in_bags: 37
- estimated_material_cost: $1250.00
Frequently Asked Questions
What R-value do I need in my attic?
According to the US Department of Energy and 2021 International Energy Conservation Code (IECC), homes in warm southern climates (Zones 1-3) require R-38 (roughly 12 inches of insulation). Homes in moderate to cold climates (Zones 4-7) require R-49 to R-60 (roughly 15.5 to 19 inches).
What is the difference between faced and unfaced insulation?
Faced insulation has a Kraft paper vapor retarder attached to one side. It should always be installed facing the warm-in-winter interior living space to prevent indoor humidity from condensing inside the wall cavity. Unfaced insulation has no vapor retarder and is used for layering on top of existing attic insulation or interior soundproofing.
Can I add new insulation on top of old insulation?
Yes, as long as the existing insulation is dry, mold-free, and not compressed. Always use UNFACED insulation when adding an extra layer over old batts; installing a second vapor barrier will trap moisture inside the lower layer and cause structural rot.
How many bags of blown-in insulation do I need for a 1,000 sq ft attic?
To achieve R-38 (12 inches) over 1,000 sq ft, you will need approximately 28 to 30 bags of loose-fill cellulose. To achieve R-49 (15.5 inches), you will need approximately 36 to 38 bags, depending on the manufacturer coverage chart.
Why should I never compress fiberglass insulation batts?
Fiberglass insulates by trapping millions of tiny dead air pockets between glass filaments. If you squeeze a 6-inch R-19 batt into a 3.5-inch 2x4 stud wall, you eliminate those air pockets and degrade its thermal resistance down to roughly R-13, wasting money and material.
What is the difference between fiberglass batts and mineral wool (Rockwool)?
Fiberglass is lightweight, economical, and standard for residential framing. Mineral wool (spun from basalt rock and slag) has higher fire resistance (withstands over 2,150°F), superior acoustic sound dampening, hydrophobic water repellency, and higher density that friction-fits tightly without sagging.