Batt & Fiberglass Insulation Calculator
This free batt insulation calculator tells you how many bags of fiberglass or mineral-wool batts to buy. Enter each wall or attic section, deduct openings, and it applies a cutting-waste factor before dividing by the coverage on your bag (which drops as R-value rises).
It does what the generic insulation calculator doesn’t: it suggests the R-value for your IECC climate zone and assembly (R-30 to R-60 attic, R-13 to R-20 walls), and it works out the faced-vs-unfaced vapor-retarder rule — in cold zones it flags whether to buy faced batts or add a poly sheet on the warm side.
Materials only, no signup. Coverage per bag varies by R-value and width, so the calculator lets you enter the exact number from your bag and cites ASTM C665, IECC 2021 Table R402.1.3, and IRC R702.7 for the R-value and vapor-retarder rules.
Comparing insulation types? Start with the general insulation calculator →
Batt & Fiberglass Insulation Calculator
Estimate bags of fiberglass or mineral-wool batts by wall or attic section — with the IECC R-value for your climate zone, faced-vs-unfaced vapor-retarder logic, and cutting waste.
Areas to insulate
Enter each wall or attic/floor section — length × height (walls) or length × width (attic/floor).
Walls only — subtract openings you won't insulate. Leave 0 for attic/floor.
Material & location
Where you're insulating sets the suggested R-value
Not sure? Look it up with the Climate Zone Calculator.
Facing, coverage & waste
Coverage per bag is on the label — it varies by R-value and width
Coverage drops as R-value rises (R-13 ≈ 88, R-19 ≈ 103, R-38 ≈ 59 sq ft/bag). Enter your bag's number; blank uses 88.
Why batts are bought by the bag, not the square foot
Batts are sold in bags rated for a specific R-value and stud width, and each bag covers a published wall area — so the take-off is your net area (walls minus openings, or the attic footprint), plus a little cutting waste, divided by the coverage on your bag. Coverage falls as R-value rises: an R-13 bag covers about 88 sq ft, an R-38 bag only about 59.
The R-value you need comes from your climate zone and where you're insulating — R-30 to R-60 in the attic, R-13 to R-20 in walls. In cold zones (5–8), unfaced batts on exterior walls also need an interior vapor retarder, so the calculator flags whether to buy faced batts or add a poly sheet.
Calculation Formulas
Sum each wall or attic/floor section, then subtract the window/door area you won’t insulate.
Example:
A 40 × 8 wall (320) less 40 sq ft of openings = 280 sq ft.
Cutting and fit waste — 5% simple, 10% moderate, 15% complex.
Example:
280 × 1.10 = 308 sq ft.
Coverage per bag is the wall area one bag covers at that R-value and width (from the label).
Example:
308 ÷ 88 sq ft/bag (R-13) = 3.5 → 4 bags.
Attic R-30 (Zone 1) to R-60 (Zones 5–8); walls R-13 to R-20+ci; floors R-13 to R-38.
Example:
Zone 5 attic → R-60; Zone 5 wall cavity → R-20 (+ R-5 continuous).
Unfaced batts in Zones 5–8 & Marine 4 need a Class I/II retarder on the warm side (IRC R702.7). Faced batts skip this.
Example:
308 sq ft ÷ 1000 = 1 roll.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Coverage — R-13 (15" wide) | ≈ 88 sq ft/bag | 2×4 walls, 16" o.c. Enter your bag number. |
| Coverage — R-19 | ≈ 103 sq ft/bag | 2×6 walls/floors, 16" o.c. |
| Coverage — R-38 | ≈ 59–64 sq ft/bag | Attic; less area per bag as R rises. |
| Cavity fit — 2×4 | R-13 / R-15 | 3.5" cavity; don’t compress a thicker batt. |
| Cavity fit — 2×6 | R-19 / R-21 | 5.5" cavity. R-19 compressed in 2×6 performs at ~R-17. |
| Attic R-value (IECC 2021) | R-30 to R-60 | R-30 Zone 1 → R-60 Zones 5–8. |
| Wall R-value (IECC 2021) | R-13 to R-20+ci | Cavity plus continuous insulation in cold zones. |
| Vapor-retarder zones | Zones 5–8 & Marine 4 | Interior Class I/II retarder required (IRC R702.7). |
| Framing factor | ~22–25% | Of wall area at 16"/24" o.c. — why published bag coverage already assumes standard framing. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
ASTM C665(ASTM C665)
View StandardStandard specification for mineral-fiber blanket (batt) thermal insulation for light-frame construction.
Key Requirements:
- •Type I unfaced; Type II faced (vapor-retarder or non-vapor); Type III reflective-faced
- •Test methods for thermal resistance, surface burning, and water-vapor permeance
- •Governs the batt product, not the install quantity
IECC 2021 Table R402.1.3(IECC R402.1.3)
View StandardThe energy code’s required R-values (and U-factors) by climate zone and assembly.
Key Requirements:
- •Ceiling R-30 (Zone 1) up to R-60 (Zones 5–8)
- •Wood-frame wall R-13 to R-20 plus continuous insulation in cold zones
- •Verify against your locally adopted edition — 2024 IECC differs
IRC R702.7(IRC R702.7)
View StandardInterior vapor-retarder requirements for exterior frame walls, by climate zone.
Key Requirements:
- •Class I or II vapor retarder on the interior side in Zones 5–8 & Marine 4
- •Kraft facing = Class II (~1 perm); zones 1–4 generally don’t require one
- •Never sandwich a vapor retarder between two impermeable layers
NAIMA(NAIMA)
View StandardNorth American Insulation Manufacturers Association — coverage charts, R-value, and handling guidance.
Key Requirements:
- •Published bag-coverage charts by R-value and width
- •Handling/PPE guidance for fiber insulation
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Cold North (Zones 5–8)
Vapor retarder required
Unfaced batts on exterior walls need an interior Class I/II vapor retarder (or use faced batts), and attic targets hit R-60. Walls often need continuous exterior insulation to make code.
Regional Examples:
Warm South (Zones 1–3)
Unfaced is usually right
An interior vapor retarder generally isn’t required and can trap moisture in cooling climates — unfaced batts are typically correct, with lower attic targets (R-30 to R-49).
Regional Examples:
Mixed / Marine 4
The gray zone
Zone 4 (including Marine 4) is where code sources disagree on the vapor-retarder requirement — check the locally adopted IRC edition before choosing faced vs unfaced.
Regional Examples:
Mineral wool vs fiberglass
Higher R per cavity
Mineral wool packs more R into the same cavity (R-15 in a 2×4 vs R-13/15 fiberglass; R-23 in a 2×6 vs R-19/21) and resists fire and sound better — at a higher cost and with knife-cut install.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Plan disposal before you start
Smaller jobs still produce more debris than a few trash bags can hold. Check what's allowed in a dumpster and which disposal option fits the scope.
See disposal options →
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Blown-In Insulation Calculator
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How to Use This Calculator
- Add each wall or attic/floor section — length × height for walls, length × width for attics and floors.
- Deduct the window and door area on walls (leave 0 for an attic or floor).
- Pick the material (fiberglass or mineral wool) and where you’re insulating — 2×4 wall, 2×6 wall, attic, or floor.
- Choose your IECC climate zone to see the suggested R-value, or look it up with the Climate Zone Calculator.
- Pick faced or unfaced — the calculator flags whether a cold-zone wall needs a vapor retarder.
- Enter the coverage per bag from your bag label (it falls as R-value rises) and a cutting-waste level.
- Click Calculate for bags to buy, the suggested R-value, and any vapor-retarder or support accessories.
What this calculator does not include
Output is materials only — no pricing or labor. Never compress batts to fit (a compressed R-19 performs at ~R-17), and fill every void — gaps and compressed spots cut real R-value sharply. R-value targets are IECC 2021 base text; verify against your locally adopted code, and confirm the vapor-retarder rule (IRC R702.7) for Zone 4/Marine 4 with your building department. Coverage per bag varies by product — use your bag label.
Frequently Asked Questions
How many bags of insulation do I need?
Take your net area (walls minus openings, or the attic footprint), add a cutting-waste factor, then divide by the coverage printed on your bag and round up. Coverage falls as R-value rises — an R-13 bag covers about 88 sq ft, an R-19 bag about 103, and an R-38 bag only about 59. For 308 sq ft at 88 sq ft/bag that's 4 bags. Always use your bag's number.
How many bags of insulation for a 12x12 wall?
A 12×8 wall is 96 sq ft (a 12×12 room has four ~96 sq ft walls ≈ 384 sq ft). Subtract openings, add ~10% waste, and divide by your bag coverage. At R-13 (88 sq ft/bag), 384 sq ft of wall is about 5 bags. Deduct windows and doors first for a tighter count.
What R-value do I need for my attic?
Per the 2021 IECC, attic insulation runs from about R-30 in the warmest climate zone (Zone 1) to R-49 in the middle zones and R-60 in the cold zones (5–8). Set your climate zone in the calculator to see the target, and verify against your locally adopted code — the 2024 IECC and local amendments can differ.
Should I use faced or unfaced insulation?
Faced batts have a built-in vapor retarder and go on exterior walls in cold climates (IECC Zones 5–8 and Marine 4), facing the warm-in-winter side. Unfaced is right in warmer zones (1–4), for interior walls, and any time you're adding over existing insulation — never stack two vapor retarders. The calculator flags which your zone needs.
Can I compress batts to fit a smaller cavity?
No — compressing a batt lowers its R-value. An R-19 batt squeezed into a 2×6 (5.5") cavity performs at only about R-17. Match the batt to the cavity: R-13/R-15 for 2×4 walls, R-19/R-21 for 2×6. Cut batts about an inch long for a friction fit instead.
Is mineral wool better than fiberglass?
Mineral wool (Rockwool) packs more R into the same cavity — R-15 in a 2×4 vs R-13/15 fiberglass, R-23 in a 2×6 vs R-19/21 — and resists fire and sound better, but costs more and must be cut with a knife. Fiberglass is cheaper and tears to fit. The take-off math is the same; pick the material for performance vs budget.
Do I need to subtract framing from the wall area?
Generally no — manufacturer bag-coverage charts already assume standard 16" or 24" o.c. framing, so you can use the gross wall area (minus openings) plus a waste factor. Only subtract framing if you're computing exact cavity area with a cavity-area coverage number, or you'll double-count.
Does this calculator include the cost of insulation?
No — it outputs materials only (bags, plus any vapor retarder or supports), with no pricing or labor. Insulation prices swing widely by R-value and region, so we keep the calculator to quantities you can take straight to any supplier.