Insulation Calculator
Work out exactly how many batt packages or bags of blown-in insulation you need for an attic floor or wall cavities. Add your own prices for a full cost estimate — nothing here uses a stale national average.
How this calculator works
Batts are priced by the package, so the calculator just divides your total area by the package's coverage. Blown-in is priced by the bag, but bag charts list coverage at a fixed 1 inch depth, so the calculator scales that down to your target depth first:
// Batts: packages = ceil(area / coverage_per_package) // Blown-in: bags = ceil( area / (coverage_sqft_per_bag_at_1in / target_depth_in) )
A deeper target depth means each bag covers less floor area, since the same bag of material is being spread thicker over a smaller footprint — that's why bags needed rises with target depth even though the attic floor area hasn't changed.
Worked example
A 1,200 sq ft attic floor (e.g. two 40 ft × 15 ft sections):
Batts — R-38 batts at 40 sq ft per package:
- Packages: ceil(1,200 ÷ 40) = 30 packages.
- At $50 per package: 30 × $50 = $1,500.
Blown-in — a bag rated 360 sq ft at 1 inch, targeting 14 inches for roughly R-49:
- Coverage at 14 in: 360 ÷ 14 ≈ 25.7 sq ft per bag.
- Bags: ceil(1,200 ÷ 25.7) = 47 bags.
- At $18 per bag: 47 × $18 = $846.
The two methods aren't directly comparable on cost alone here, since they're insulating to different R-values — use the R-value your climate calls for, then compare batts vs blown-in at that R-value.
Typical batt coverage by R-value
| R-value | Typical use | Approx. coverage per package |
|---|---|---|
| R-13 | 2×4 wall | ~100 sq ft |
| R-19 | 2×6 wall | ~75 sq ft |
| R-30 | Attic | ~50 sq ft |
| R-38 | Attic | ~40 sq ft |
| R-49 | Attic, cold climate | ~32 sq ft |
These are rough starting points, not a specific product's numbers — coverage varies by brand, batt thickness and package size, so always check the label and adjust the coverage field above if it differs.
Measuring your space
- Attic floor — measure the usable floor area, length × width, in each section.
- Wall cavities — measure the wall's overall length × height; batts fit standard stud spacing, so waste is usually minor.
- Obstructions — don't subtract for wiring or small penetrations; do subtract large permanent obstructions like a chimney chase.
- Target depth — for blown-in, use the depth your bag chart lists for your target R-value, not a guess.
How much does insulation cost?
Rather than quoting a national average that's stale the day it's published, this calculator prices the job from what you actually enter. Give a price per package for batts, or per bag for blown-in, and it's multiplied straight through. The estimate doesn't include rental of a blowing machine, baffles, or air sealing work, which are often worth budgeting for separately.
Frequently asked questions
What R-value do I need for my attic vs walls?
Attics generally need a higher R-value than walls, since a roof loses and gains far more heat. Walls commonly run R-13 to R-21; attics commonly run R-30 to R-38 in milder climates and R-49 to R-60 in colder ones. This is general guidance — check your local energy code for the exact requirement.
Batts vs blown-in — which covers a given area more efficiently?
It depends on the shape of the space. Batts fit standard stud and joist bays well with little waste. Blown-in fills irregular spaces more completely and covers an open attic floor faster. Attic floors typically favor blown-in; framed wall cavities typically favor batts.
How many inches of blown-in insulation is standard for an attic?
It depends on your target R-value, but many attics land around 12 to 15 inches for R-38, and 16 to 20 inches or more for R-49 to R-60 in colder climates. Your bag's coverage chart will list the exact depth for a given R-value for that specific product.
Do I need a vapor barrier?
It depends on climate. Cold climates typically use a vapor barrier on the warm-in-winter side of a wall; hot, humid climates often handle it differently, since the moisture drive reverses. Attics generally rely on ventilation instead. Check local building code guidance rather than a blanket rule.