⌂ RoofingCalculatorHQ

Attic Insulation Calculator

Calculate top-up depth, bag count and material cost for blown cellulose, blown fibreglass, batts or mineral wool. IECC 2021 climate-zone targets, R-19 to R-60.

Attic Insulation Calculator

Calculate top-up depth, bag count and material cost from attic area, climate-zone target R-value and existing insulation. IECC 2021 baseline.

Top-up depth required
8.6 in
R-value gap to fill: R-30 (5.28 m²K/W)
Equivalent U-value: 0.11 W/m²K
Material count
50 bags
Material cost (estimate)
$700
Reference standard
IECC 2021 Tbl R402.1.3 / DOE Energy Star

What this calculator does

This tool tells you how much insulation to add to your attic to hit a target R-value, how many bags or batts you’ll need, and roughly what the material will cost in 2026 US dollars. It’s built around the IECC 2021 prescriptive minimums for climate zones and the bag-coverage tables published by the major loose-fill manufacturers (Owens Corning, Johns Manville, Knauf, GreenFiber, CertainTeed).

Enter your attic floor area, the target R-value for your climate zone, and the R-value of the insulation already in place. The calculator returns the R-value gap (target − existing), the top-up depth required for your chosen insulation material, the bag count, and the material-only cost at typical 2026 big-box pricing.

How to use it

  1. Measure the attic floor area in square feet. For a simple gable on a 30 × 50 ft footprint, that’s 1,500 sq ft. For a hip or complex roof, sum the projected (plan-view) footprint, not the sloped surface.
  2. Pick the target R-value based on your IECC climate zone. Zones 1–2: R-30. Zones 3–5: R-49. Zones 6–8: R-60. The DOE recommends R-60 across all zones for cost-effective new construction.
  3. Enter your existing R-value. A flat layer of fibreglass batt at 6.25 inches is R-19. A 10-inch layer of older blown fibreglass that has settled is closer to R-22 — measure depth with a yardstick at three points and use the average.
  4. Choose your insulation type. Blown cellulose for R-per-dollar champion. Blown fibreglass for moisture-prone or fire-sensitive retrofits. Batts for DIY-friendly small areas under 600 sq ft. Mineral wool for tight-to-eaves applications where rodent or fire resistance matters.
  5. Read the result. The big number is the top-up depth in inches. The cards below show units of material to buy and the estimated material cost.

How the math works

The tool starts with the R-value gap — the target R minus the existing R. It then divides that gap by the R-per-inch of the chosen material to compute installed depth:

depth_in = R_gap / R_per_inch

R-per-inch values come from each manufacturer’s NRCA-listed installed-density data. Cellulose at residential blown density gives R-3.5 per inch; loose-fill fibreglass gives R-2.5; high-density batts give R-3.2 (Kraft-faced) to R-4.0 (mineral wool).

Bag count is derived from each manufacturer’s coverage table at standard density. A 19-lb bag of cellulose blown at R-30 covers 30 sq ft. At R-49 it covers proportionally less (30 × 30/49 ≈ 18 sq ft). The calculator adjusts coverage to match the gap-R you’re filling, then rounds up.

Material cost uses 2026 average prices from Home Depot, Lowe’s and Menards: $14/bag for blown cellulose (GreenFiber Cocoon-R), $19/bag for blown fibreglass (Owens Corning AttiCat), $38/bundle for R-30 paper-faced batts, $42/bundle for mineral wool. Costs vary by region — Pacific Northwest and Mountain West typically run 8–15% higher than the Mid-Atlantic baseline.

Climate zones and IECC 2021 targets

The 2021 International Energy Conservation Code splits the US into eight climate zones based on heating degree days (HDD) and moisture index. The attic-floor minimums are:

Climate zoneHeating degree daysAttic R-minEquivalent depth (cellulose)
1 (Miami, Hawaii)0 – 1,800R-308.6 in
2 (Houston, Phoenix)1,800 – 2,700R-4914 in
3 (Memphis, Atlanta)2,700 – 3,600R-4914 in
4 (Baltimore, KC)3,600 – 5,400R-4914 in
5 (Chicago, Boston)5,400 – 7,200R-4914 in
6 (Minneapolis, Burlington VT)7,200 – 9,000R-6017 in
7 (Duluth, Anchorage)9,000 – 12,600R-6017 in
8 (Fairbanks)> 12,600R-6017 in

State amendments override the IECC in roughly 30 states. California’s Title 24 sets a stricter R-38 ceiling minimum in conditioned attics (because the duct system stays inside the thermal envelope). Florida amends Zone 2 down to R-30 over masonry. Always check your state energy code, not just the IECC, before specifying.

Existing-insulation depth lookup

If you don’t know what’s already in your attic, drop a yardstick into the loose-fill at three or four points and average the depth. Then use this lookup:

  • Blown cellulose, undisturbed: R-3.5 per inch
  • Blown cellulose, settled (10+ years): R-3.2 per inch
  • Blown fibreglass, undisturbed: R-2.5 per inch
  • Blown fibreglass, settled: R-2.2 per inch
  • Vermiculite (pre-1990 homes): R-2.4 per inch — but stop and test for asbestos before disturbing any vermiculite that may be Libby-source Zonolite
  • Fibreglass batt, kraft-faced: R-3.1 per inch
  • Mineral wool batt: R-3.7 per inch
  • Closed-cell spray foam: R-6.5 per inch

For a 6-inch flat layer of blown cellulose that’s been there 12 years, multiply 6 × 3.2 = R-19. For a 10-inch tired blown fibreglass layer, 10 × 2.2 = R-22.

Air-seal before you blow

The single biggest mistake DIY attic insulators make is blowing cellulose over an unsealed top-plate, recessed light, or duct boot — and then watching their winter heating bill barely move. Loose-fill insulation has an air permeance much higher than people assume, and the building’s stack effect drives warm interior air straight through the new R-49 layer if the air barrier underneath has holes.

The Department of Energy’s Air Sealing a Home field study (PDF) shows that pairing R-30 of insulation with proper top-plate caulking outperforms R-60 of unsealed insulation in 84% of measured homes. Energy Star’s Home Sealing checklist hits the standard targets:

  1. Replace recessed lights with airtight IC-rated LED retrofits or build a fire-rated dam.
  2. Caulk or low-expansion-foam every interior-wall top plate.
  3. Mastic every duct boot, plenum joint, and HVAC penetration.
  4. Foam around plumbing stacks, electrical conduits and the bathroom-fan housing.
  5. Replace the attic hatch with an insulated, weatherstripped lid.
  6. Install soffit baffles to maintain ventilation gap before blowing loose-fill.

Federal and state incentives

The Section 25C Energy Efficient Home Improvement Credit, extended through 2032 by the Inflation Reduction Act, refunds 30% of insulation material cost up to a $1,200 annual cap. Labour doesn’t qualify. To claim, file Form 5695 with the manufacturer’s NRCA-listed product number on your tax return.

The High-Efficiency Electric Home Rebate Act (HEEHRA) layers on top, providing means-tested rebates up to $1,600 specifically for insulation in households at or below 150% of area median income. Implementation varies by state; check your state energy office. Many electric utilities (Pacific Power, ConEd, Eversource, FPL) offer additional $0.50/sq ft rebates for blown-insulation top-ups verified by a Building Performance Institute auditor.

When to call a contractor

Most 1,200–2,000 sq ft single-storey attics are weekend DIY projects with a rented blower from Home Depot. Hire a contractor when: the existing insulation contains vermiculite (asbestos test required); the attic has active rodent activity (remediate first); you’re doing dense-pack walls, cathedral ceilings, or rim joists (specialised technique); or your climate zone requires R-60 in a tight-to-eaves attic where blowing the corners requires a long-reach hose. Expect $0.85–$1.50/sq ft installed all-in for blown cellulose top-ups, $1.20–$2.10 for blown fibreglass.

Frequently asked questions

How much insulation do I need in my attic?
The 2021 IECC sets attic insulation minimums by climate zone: R-30 in Zones 1–2 (Florida, southern Texas, Hawaii), R-49 in Zones 3–5 (most of the southern, central and Mid-Atlantic US), and R-60 in Zones 6–8 (northern New England, the Upper Midwest, Mountain West, Alaska). Department of Energy Energy Star pushes R-60 across all zones for new builds. To meet R-49 with blown cellulose at R-3.5 per inch, you need about 14 inches loose-laid; to meet R-60 with the same material you need about 17 inches. The calculator above subtracts your existing R-value from the target to compute the top-up depth required.
How many bags of blown insulation do I need for a 1,500 sq ft attic?
Bag coverage scales with the depth you're laying. A standard 19-pound bag of blown cellulose covers about 30 sq ft when installed at R-30 equivalent (about 8.5 inches loose-fill density). To top up a 1,500 sq ft attic from R-19 to R-49 (a 30-point gap) you need about 50 bags at $14 each, roughly $700 in material before equipment rental. Blown fibreglass covers more sq ft per bag at the same R-value (about 42 sq ft at R-30) but settles more, so installers typically over-blow by 10–15% to compensate for two-year settle.
Is blown cellulose or fibreglass better for attics?
Blown cellulose has a higher R-per-inch (3.5 vs 2.5 for fibreglass), is made from 80%+ recycled newsprint, and resists air leakage better at the same density. Blown fibreglass is non-combustible, doesn't absorb water, and ships lighter (a benefit when you have a 600-pound bag of cellulose to drag through a 22-inch hatch). For most US attics, cellulose wins on R-value-per-dollar and on draft-stop performance. Fibreglass wins for moisture-prone retrofits and where the existing batt insulation is already fibreglass and you want to keep the assembly homogeneous. Both meet ASTM C739 (cellulose) or ASTM C764 (fibreglass) when installed by a manufacturer-trained installer.
Can I add insulation on top of existing batts?
Yes — adding loose-fill blown insulation over existing fibreglass or rockwool batts is the standard top-up method, and it's R-additive: existing R-19 + new R-30 blown = R-49 effective. Two cautions. First, don't compress the existing batts; if they're flat and tired, leave them and blow over. Second, never bury can lights, bath fans or other heat-emitting fixtures unless they're rated IC (insulation contact) and AT (air-tight) — wrap or build a fire-rated dam around non-rated lights. Verify the existing assembly is dry; topping over wet batts traps moisture against the deck and risks rot. NRCA recommends a vapour-retarder check on the warm side before top-ups in Zones 5+.
How long does blown insulation take to install?
A two-person crew with a rented blower can top up a 1,500 sq ft attic from R-19 to R-49 in 4–6 hours, including dragging bags up the ladder, baffling soffit vents, blocking can lights, and depth-marking. Big-box stores (Home Depot, Lowe's) lend the blower free with 10+ bag purchases. Professional installers complete the same job in 2–3 hours and charge $0.85–$1.50/sq ft installed, all-in. The single biggest time sink is air-sealing top plates and recessed-light penetrations before blowing — skip that and you'll lose 20–30% of your effective R-value to convective bypass.
Does attic insulation qualify for the federal energy tax credit?
Yes. The Inflation Reduction Act extended the Section 25C Energy Efficient Home Improvement Credit through 2032: 30% of insulation material cost up to $1,200 per year. Labour does not qualify, but materials (bags, batts, air-sealing foam) do, and the credit is non-refundable but rolls into the next year's tax filing. Eligible products must meet the IECC 2021 prescriptive requirements for your climate zone — most blown cellulose and fibreglass products qualify automatically. Save your receipts and the manufacturer's IRS Form 5695 reference. State and utility rebates stack on top of the federal credit; check the DSIRE database for your zip code.
What's the difference between R-value and U-value?
R-value (resistance) and U-value (conductance) are reciprocals of each other in steady-state heat transfer: U = 1/R-total, where R-total includes the assembly R-value plus surface air films. R-value is the conventional unit in the US and Canada (h·ft²·°F/Btu), while U-value is the conventional unit in the UK, EU, Australia and most international codes (W/m²K). To convert: R-49 (imperial) = 8.6 m²K/W = U-0.117 W/m²K. The 2021 IECC mandates an attic R-49 in Zone 4–5, which is equivalent to roughly U-0.026 in metric — far stricter than current European new-build requirements but on par with Passivhaus standards.
Should I air-seal before insulating?
Always. Department of Energy field studies consistently show that air-sealing the attic floor before insulating delivers 20–40% better whole-house energy savings than insulation alone. The priorities are: caulk or foam top plates of interior walls, seal duct boots with mastic, weatherstrip and insulate the attic hatch, foam around plumbing and electrical penetrations, and replace non-IC-rated recessed lights with airtight LED retrofits. Energy Star's Home Sealing Quick Reference (PDF download) is the standard checklist. NRCA and ARMA both call air-sealing the precondition for any attic insulation upgrade — without it you're paying for insulation that the building's stack effect bypasses.

Related calculators

📋 Embed this calculator on your site (free, attribution required)

Free to embed on any non-commercial or commercial site, provided the attribution link remains visible. No tracking, no email capture, just the calculator.