Attic Insulation Calculator
Enter your attic floor area, what you have now and the R-value you want. The calculator suggests a target from ENERGY STAR’s recommendations for your climate zone, then works out how many inches of insulation to add, the volume, and — using the coverage chart on your product’s bag — how many bags to buy.
Attic Insulation Calculator
Find how much insulation you need to reach a target R-value in your attic: the extra R-value, depth, volume, number of bags and cost.
Your results appear here and update as you change the numbers.
How this calculator works
- R-value to add = target R − existing R (never below 0).
- Depth to add (in.) = R to add ÷ R per inch of the new product.
- Volume (cu ft) = attic area × depth ÷ 12. Spray foam is priced in board feet = area × depth.
- Bags = volume ÷ coverage per bag, rounded up. Use the coverage chart printed on your product — it varies by brand and blowing machine.
- Cost = bags × price per bag + area × labor per sq ft.
Worked example
A 1,000 sq ft attic with R-11 going to R-49 with blown fiberglass (R-2.5/in): add R-38, which is 38 ÷ 2.5 = 15.2 in., or 1,000 × 15.2 ÷ 12 = 1,267 cu ft. With bags that fill 20 cu ft: 1,267 ÷ 20 = 63.3 → 64 bags; at $40 a bag that is $2,560 in material (example prices).
When to get a professional check
Air-seal the attic floor before adding insulation, keep soffit vents clear with baffles, and check local code minimums. Spray foam and knob-and-tube wiring situations need a professional.
Assumptions used — example values, not yet reviewed against current local prices
| Assumption | Value |
|---|---|
| R per inch: blown fiberglass | 2.5 R/in |
| R per inch: blown cellulose | 3.5 R/in |
| R per inch: fiberglass batts | 3.1 R/in |
| R per inch: open-cell spray foam | 3.6 R/in |
| R per inch: closed-cell spray foam | 6.5 R/in |
| Suggested target: zone 1 (ENERGY STAR R30–R49) | 49 R |
| Suggested target: zone 2 (ENERGY STAR R49–R60) | 60 R |
| Suggested target: zone 3 (ENERGY STAR R49–R60) | 60 R |
| Suggested target: zones 4–8 (ENERGY STAR R60) | 60 R |
Sources and references
Ready for real prices? Learn how to find and vet a local roofer.
How to choose a contractorResults are planning estimates, not quotes. Prices vary by location, contractor and the condition of your roof. Verify sizes and code requirements locally before buying or building.
Frequently asked questions
What R-value should my attic have?
ENERGY STAR recommends R30–R49 in zone 1, R49–R60 in zones 2–3 and R60 in zones 4–8 for an uninsulated attic.
How do I find my existing R-value?
Measure the depth and multiply: about 2.5 per inch for loose fiberglass, 3.5 for cellulose, 3.1 for fiberglass batts.
Why do I need the bag’s coverage chart?
Bag coverage depends on the product and blowing machine, so use the manufacturer’s number instead of a generic estimate.
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