Insulation Calculator — R-Value, Batts & Bags
Under the 2021 IECC, climate zone 5 requires R-60 in the ceiling — a 1,200 ft² attic takes about 60 bags of blown fiberglass. This insulation calculator finds the R-value your climate zone requires and converts it into batts, blown-in bags or rigid board sheets. Requirements from IECC Table R402.1.3, verified 2026-08-04.
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Insulation Calculator — materials list
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Prepared with MaterialMath — https://estimatormath.com. Quantities are estimates based on the dimensions entered and the published figures cited on the tool page (verified 2026-08-04). Verify against your own measurements and your supplier's actual pack sizes before ordering. Not professional engineering, construction or financial advice.
How is insulation quantity calculated?
Three different products, three different arithmetic. Batts are counted by cavity, loose-fill by the bag's published coverage at your target R-value, and rigid board by sheet.
The 4½ inches that trip up every other calculator
A wall framed to an 8 ft ceiling is not 96 inches of open cavity. A single bottom plate and a double top plate are 1½ inches each — 4½ inches in total — leaving about 92⅝ inches between them. That is precisely why fibreglass batts are manufactured 93 inches long.
Skip that deduction and 96 ÷ 93 rounds up to two batts per bay, and the calculator tells you to buy twice what you need. This one deducts the plates, which is why a 40 ft wall at 16" centres comes back as 30 batts rather than 60.
A worked example: a 1,200 ft² attic in climate zone 5
Step by step
- Code requirement, zone 5 ceiling
- IECC 2021 Table R402.1.3 → R-60
- Attic area
- 1,200 ft²
- Blown fiberglass coverage at R-60
- 21.3 ft² per bag
- Bags before waste
- 1,200 ÷ 21.3 = 56.3
- With 5% waste
- 60 bags
- Settled depth
- about 22 inches
The depth figure is the one to check on site. Blown insulation settles, so the coverage chart gives both an initial installed depth and a settled depth. Blow to the initial depth, not the settled one, or you finish a full R-value short. Attic rulers — required at one per 300 ft² by IRC §N1101.10.1.1 — exist so that you and the inspector can both verify what actually went in.
IECC 2021 R-value requirements
| Climate zone | Ceiling | Wood wall | Floor | Basement wall |
|---|---|---|---|---|
| Zone 1 | R-30 | R-13 | R-13 | — |
| Zone 2 | R-49 | R-13 | R-13 | — |
| Zone 3 | R-49 | R-20 or 13 + 5 continuous | R-19 | R-5 |
| Zone 4 (except Marine) | R-60 | R-30 or 20 + 5 continuous | R-19 | R-10 |
| Zone 5 and Marine 4 | R-60 | R-30 or 20 + 5 continuous | R-30 | R-15 |
| Zone 6 | R-60 | R-30 or 20 + 5 continuous | R-30 | R-15 |
| Zone 7 and 8 | R-60 | R-30 or 20 + 5 continuous | R-38 | R-15 |
From IECC 2021 Table R402.1.3. Adoption lags. Energy code is adopted state by state, and as of 2026-08-04 a number of states still enforce the 2009, 2015 or 2018 edition — under which several of these figures are considerably lower. A few states have no statewide residential energy code at all. Your building department is the authority.
Who this calculator is for
- Homeowners topping up an attic, which is the highest return-on-investment energy retrofit in most US housing.
- Renovators with open walls — the one moment when insulating and air sealing are cheap.
- Anyone checking a contractor's proposal against what the code in their zone actually asks for.
- People sizing HVAC. Insulate first, then run the BTU calculator with a better envelope quality — the equipment you avoid buying often pays for the insulation.
What this calculator does not account for
- Air sealing. Not a quantity, and more important than the R-value. Insulation slows conduction and does nothing about airflow.
- Thermal bridging. R-values here are for the insulation alone. Framing occupies about 15% of a 16" on centre wall and conducts around it — nominal R-13 cavity delivers roughly R-11 as an assembly. This is exactly why the 2021 IECC expects continuous exterior insulation in zone 4 and above.
- Vapour retarders and moisture. Where the vapour retarder goes depends on climate zone, and getting it wrong traps moisture inside the assembly. This is a genuine building-science question, not a quantity.
- Ventilation. Attic ventilation is required and insulation must not block the soffit vents. Baffles are listed as an optional line but the requirement is not optional.
- Spray foam. Priced and installed by board-foot coverage by a contractor, with a different air-sealing role. Not modelled here.
- Existing insulation. If you are topping up, you need the R-value already there. Measure the depth and identify the material before ordering.
Batt thickness by R-value
| R-value | Thickness | Fits |
|---|---|---|
| R-13 | 3-1/2" | 2×4 wall |
| R-15 high density | 3-1/2" | 2×4 wall |
| R-19 | 6-1/4" | 2×6 wall or floor |
| R-21 high density | 5-1/2" | 2×6 wall |
| R-30 | 9-1/2" | Floor or attic |
| R-38 | 12" | Attic |
| R-49 | 15-1/2" | Attic |
Note R-19 and R-21: both fit a 2×6 wall, but R-19 is 6¼″ thick and gets compressed to 5½″ in the cavity, where it performs at about R-18. R-21 is a high-density product made to fill the same space uncompressed.
Common questions
What R-value do I need in my attic?
Under the 2021 IECC, R-49 in climate zones 2 and 3, and R-60 in zones 4 through 8. Zone 1 — south Florida and Hawaii — is R-30.
These went up sharply in the 2021 edition. Zone 4 moved from R-49 to R-60 and zone 2 from R-38 to R-49. But energy code adoption is a state decision and lags badly: as of 2026 a number of states still enforce the 2009, 2015 or 2018 edition, and a few have no statewide residential energy code at all. Check with your building department before ordering.
How many bags of blown insulation do I need?
Divide the attic area by the coverage figure for your target R-value. A 1,200 ft² attic to R-49 in blown fiberglass takes about 60 bags at roughly 26.6 ft² per bag.
Cellulose is denser and covers less per bag — around 20.6 ft² at R-49 — so the same attic takes closer to 60 bags. Federal rules require the coverage chart to be printed on every bag; use YOUR bag's chart for the final count, because it varies between manufacturers by 15% or more at the same R-value.
How many batts do I need for a wall?
One batt per stud cavity, and the cavity count is the wall length divided by the stud spacing. A 40 ft wall at 16" on centre has 30 cavities, so 30 batts of 15" × 93".
The subtlety is that a wall framed to an 8 ft ceiling has only about 92-5/8" of open cavity, because a single bottom plate and a double top plate consume 4-1/2". That is exactly why batts come 93" long, and why one batt fills one bay rather than one-and-a-bit. This calculator deducts the plates; most do not, and end up telling you to buy twice as many.
Does compressing insulation reduce its R-value?
Yes. R-value comes from trapped air, and compressing the batt removes some of it. An R-19 batt squeezed into a 5-1/2 inch 2×6 cavity performs at about R-18 — which is precisely why R-21 exists as a high-density product for the same cavity.
The same applies to insulation stuffed behind pipes and wires. Split the batt around the obstruction instead of compressing it, or you lose more than you gain.
Why do walls now need continuous exterior insulation?
Because studs conduct heat around the insulation. Framing occupies roughly 15% of a 16" on centre wall, and wood is a much better conductor than fibreglass. Nominal cavity R-13 delivers an assembly performance of about R-11 once that thermal bridge is counted.
A continuous exterior layer covers the studs as well as the cavities, which is why the 2021 IECC allows zone 4 and above walls to be met either as R-30 cavity or as "R-20 + 5 continuous". The second route usually performs better in the real building, and it keeps the sheathing warmer, which reduces condensation risk.
Should I air seal before insulating?
Always. Insulation slows conduction; it does nothing about airflow. An R-60 attic with unsealed top plates, recessed lights and a bath fan penetration performs like a far lower R-value, because warm moist air simply goes around it.
Air sealing is also cheap relative to insulation, and it is only accessible before the insulation goes down. Seal the penetrations, box in the recessed lights that are not IC-rated, install baffles at the eaves so the soffit vents stay clear, then insulate.
Sources for the figures used
Every rate, coverage figure and code limit in this calculator comes from a published source. Where a figure is a convention rather than a standard, it says so.
- 2021 International Energy Conservation Code, Table R402.1.3 — R-value requirements by climate zone and assembly (checked 2026-08-04)
- IECC Figure R301.1 — climate zone map — Zone assignments by county (checked 2026-08-04)
- DOE / Oak Ridge National Laboratory insulation guidance — Framing factor and thermal bridging effects on assembly R-value (checked 2026-08-04)
- Manufacturer bag coverage charts — Federal rule requires the coverage chart to be printed on every bag of loose-fill; the figures here are typical and should be checked against your product (checked 2026-08-04)