What Is the Blown-In Insulation Calculator?
The Blown-In Insulation Calculator is a free online tool designed for contractors and DIY builders who need quick, accurate calculations in the construction and building space. By entering your area to insulate, insulation type, desired r-value, you get instant results including bags needed, depth required, r-value added. No formulas to memorize, no spreadsheets to build — just enter your numbers and get the answer in seconds. Whether you're a beginner or experienced professional, this calculator saves you time and eliminates guesswork.
Why This Calculation Matters
Getting bags needed right can make the difference between success and costly mistakes. In construction and building, small errors compound quickly. Manual calculations are error-prone and time-consuming, especially under pressure. This calculator applies proven formulas used by contractors and DIY builders worldwide, giving you confidence that your numbers are correct. Use it to plan your projects with precision and avoid common pitfalls that trip up beginners.
When Should You Use This Calculator?
This tool is most useful when you know your area to insulate and need to find the right bags needed. It's also great for quick estimates before committing to a decision, and to double-check manual calculations or professional quotes, and when comparing different scenarios side by side. Bookmark this page and come back whenever you need a fast, reliable answer — the calculator is always free and requires no signup.
Blown-In Insulation Calculator
How to Use This Calculator
- Enter Your Area to Insulate (sq ft): Start by entering your area to insulate — this is the primary input for the calculation.
- Fill In Additional Details: Complete the remaining fields: insulation type, desired r-value, existing insulation r-value, price per bag. Each value refines the calculation for greater accuracy.
- Click Calculate: Hit the Calculate button to run the numbers. Results appear instantly below.
- Review Your Results: Check your bags needed, depth required, r-value added. Use these figures to inform your next decision or compare against alternative scenarios.
How to Calculate Blown-In Insulation
Determine the R-value gap (target minus existing), calculate the depth needed, then figure bags from area and coverage per bag.
- R-value needed: target R-value − existing R-value
- Depth: R-value needed ÷ R-per-inch
- Cellulose: R-3.5/inch, ~27 sq ft/bag at R-38
- Fiberglass: R-2.7/inch, ~22 sq ft/bag at R-38
- Settling: cellulose settles ~20%, fiberglass ~5-10%
- Coverage per bag varies by depth — check the manufacturer's chart
Tips & Considerations
- Double-check your area to insulate before calculating — even small input errors can significantly change your results.
- Run the calculator with different values to compare scenarios and find the optimal approach for your situation.
- Pay attention to both bags needed and depth required — they work together to give you the full picture.
- Bookmark this page for quick access next time you need to plan your projects.
- If you're unsure about your price per bag, start with a conservative estimate and adjust from there.
Frequently Asked Questions
How many bags do I need?
For cellulose at R-38: about 37 bags per 1,000 sq ft. For fiberglass at R-38: about 46 bags per 1,000 sq ft. Exact counts depend on the brand and bag size.
How deep should blown-in insulation be?
Cellulose at R-38: 10.9". Fiberglass at R-38: 14.1". Add depth for settling — cellulose settles about 20% in the first year.
Fiberglass vs cellulose?
Cellulose is denser, better at blocking air, and higher R-value per inch. Fiberglass is lighter, moisture-resistant, and settles less. Cellulose is usually cheaper.
Can I add over existing insulation?
Yes. Subtract existing R-value from target to find R-value needed. You can mix types, though matching is ideal.
How much does it cost?
Cellulose: $10-$15/bag. Fiberglass: $25-$40/bag. Many stores offer free blower rental with 10+ bags. Pro install adds $0.50-$1.50/sq ft.
Does it settle over time?
Cellulose settles 15-20%. Fiberglass settles 5-10%. Install at the manufacturer's listed initial thickness to account for settling.