Blown-In Insulation Calculator
Enter your measurements
Results
- Bags needed38 bags
- Attic area1,200 sq ft
- Coverage per bag31.58 sq ft at R-38
- Fill depth14.6 in
Estimated cost
per sq ft (material)
Per sq ft, fiberglass loose-fill material only. The installed range covers a professional crew with a truck-mount blower, including labor and setup; DIY with a rented blower falls between the two. Higher R-value targets need more bags and push costs toward the top of each range.
Estimate only — prices vary by region, supplier, and season. Get a local quote before buying.
This blown-in insulation calculator tells you how many bags of loose-fill insulation you need for your attic, and the fill depth you will reach, based on your floor area and target R-value. It matters because coverage per bag is not fixed: the FTC requires a minimum-coverage chart on every bag, and the square feet a bag covers drops as the target R-value rises, so the bag count for R-49 is far higher than for R-30 over the same floor. That relationship is built into every result here. Loose-fill fiberglass and cellulose are both sold by the bag at big-box stores, typically in 19 to 30 lb bags that cover anywhere from about 17 to 60 sq ft depending on the R-value you target. Most installers add 10 to 15 percent overage for settling, hard-to-reach eaves, and joist bays that swallow more material than open floor. Use this calculator to build that buffer into your bag count before you rent a blower or place a bulk order.
How it’s calculated
Bags = Attic area ÷ coverage per bag. Coverage falls as target R-value rises (more depth = less area per bag).
Worked example
Consider a standard 40 ft by 30 ft attic, 1,200 sq ft of floor area, where the homeowner is targeting R-38. At that R-value each bag covers 31.58 sq ft, so the calculator returns 38 bags and a required fill depth of 14.6 inches. That depth figure is critical: it tells you whether your existing joists will contain the insulation or whether depth markers and eave baffles need to go in first. Note that R-38 is a common attic target, but the 2021 IECC code minimum is R-49 in Climate Zones 4 through 8, so check your local requirement before settling on a number.
Inputs
- Attic length
- 40 ft
- Attic width
- 30 ft
- Target R-value
- R-38
Result
- Bags needed
- 38 bags
- Attic area
- 1,200 sq ft
- Coverage per bag
- 31.58 sq ft at R-38
- Fill depth
- 14.6 in
- Estimated material cost
- $840 – $1,920
Materials & pricing near you
Loose-fill fiberglass is sold by the bag at Lowe's, Home Depot, and regional lumber yards; blower rental is often free with a minimum bag purchase (commonly 10 to 20 bags). Material costs run roughly $0.70 to $1.60 per sq ft depending on target R-value and product line, since coverage per bag shrinks as you blow deeper for a higher R-value. Installed costs with a professional crew running a truck-mount blower typically run $1.40 to $3.20 per sq ft including labor and setup; a DIY job with a rented blower lands between the two. Cellulose is denser per inch and settles more, so its bag math differs, use the cellulose calculator for that material. If your attic is uninsulated, check the ENERGY STAR rebate finder and your utility before buying, as insulation rebates are common.
Frequently asked questions
How do I find the correct coverage per bag for my product?
The FTC R-Value Rule requires manufacturers to print a coverage chart on every bag of loose-fill insulation, listing maximum net coverage area and minimum settled thickness at standard R-values (typically R-13, R-19, R-22, R-30, R-38, and R-49). Find the row matching your target R-value and read across to bags per 1,000 sq ft and coverage per bag. The figure used here is a typical industry average; your specific product may differ by 5 to 10 percent, so always cross-check the label on the bags you actually buy before placing a bulk order.
What R-value do I actually need in my attic?
ENERGY STAR recommends roughly R-30 to R-60 for attics depending on climate zone. Zones 1 to 3 (the southern states and Gulf Coast) generally call for R-30 to R-38; Zones 4 and 5 (mid-Atlantic and lower Midwest) want R-38 to R-49; and Zones 6 to 8 (the northern tier, Mountain states, and Alaska) push to R-49 or R-60. For a permitted job the 2021 IECC sets R-49 as the code minimum in Zones 4 through 8, so confirm with your local building department. These are total values: if you already have R-19 of old insulation, you only need to add the difference.
Can I blow insulation over existing batts or old loose-fill?
Yes, in most cases. If the existing insulation is dry, not moldy, and not matted below its rated thickness, blowing loose-fill on top is an accepted way to add R-value, and you do not need to remove the old material. Run the calculator at your full target R-value, then subtract the R-value of what is already there to find how many bags you actually need to add. Do not lay a new vapor barrier between the layers, and keep insulation clear of recessed lights not rated for contact and of any soffit vents.
Does blown-in insulation settle, and should I add extra bags?
Fiberglass loose-fill coverage charts are stated at settled density, so the labeled R-value already accounts for minor settling and you do not need to over-blow to keep your rating long term. Cellulose settles more noticeably, roughly 10 to 20 percent in the first year, which is why its label lists an initial installed thickness higher than the final settled thickness. Either way, adding about 10 percent to your bag count is a practical buffer for eave infill, blower waste, and measurement rounding, not for settling itself.