Insulation Calculator: Batts, Blown Bags & Target R-Value Sizing
Use this insulation calculator to determine the exact insulation quantity, thickness depth, and package counts required to reach energy code target R-values. Covers fiberglass batts, loose-fill blown cellulose, open and closed-cell spray foam, and rigid foam board across US Climate Zones 1 through 7.
Install 10.90 inches of Blown Cellulose (34 Bags (25-lb)) to achieve R-38 across 1,000 sq ft.
| Surface Area to Insulate | 1,000 sq ft |
|---|---|
| Target Thermal Performance | R-38 (Current in place: R-0) |
| Additional R-Value Required | R-38 |
| Material Thermal Resistance | R-3.5 per inch of thickness |
| Required Insulation Thickness | 10.9 inches (27.6 cm) |
| Settling / Cut Waste | 5% allowance |
| Estimated Material Purchase | 34 Bags (25-lb) |
📐 Show Formula and Default Assumptions
We believe all home improvement estimates should be transparent and verifiable. Below is the exact math running in this browser tool:
-
Required Insulation Depth Formula:
Required Depth in Inches = (Target R-Value - Existing R-Value) ÷ R-Value per InchCalculates physical layer depth required to achieve desired thermal resistance. -
Material R-Value per Inch Standards:
Cellulose = R-3.5/in | Blown Fiberglass = R-2.5/in | Batt = R-3.2/in | Closed-Cell Foam = R-6.5/in | Polyiso = R-6.0/inCertified thermal conductivity ratings under ASTM C518 testing standards. -
Blown Cellulose Bag Coverage Standard:
Bags = ceil( Area ÷ Coverage per Bag at Target R-Value )A standard 25-lb bag of cellulose covers ~32 sq ft at R-38 (11 inches depth). -
Fiberglass Batt Package Coverage:
Batt Packages = ceil( Area ÷ Package Sq Ft Coverage )Standard retail packages contain 40 to 88 sq ft depending on R-value thickness.
Note: All default assumptions (waste %, coverage rates, container yields) can be adjusted directly in the input fields above to match your project specifications.
How to Measure Attics and Walls for Thermal Insulation
- Measure attic floor or wall square footage: Multiply length by width of the space, deducting large chimneys, access hatches, and HVAC mechanical platforms.
- Determine existing insulation R-value: Measure depth of current attic insulation with a ruler. Multiply depth by R-value: old yellow fiberglass is ~R-2.5/inch; grey cellulose is ~R-3.2/inch.
- Identify your regional DOE Climate Zone target: Southern zones (Zone 1–3) require R-30 to R-38 in attics; northern freezing zones (Zone 4–7) require R-49 to R-60.
- Air-seal all ceiling penetrations before insulating: Expanding foam air-sealing around plumbing stacks, recessed light canisters, electrical wires, and top plates saves twice as much energy as insulation alone.
- Install attic rafter baffles (vent chutes): Staple foam or cardboard baffles into every rafter bay along the eaves to prevent blown insulation from blocking soffit airflow.
- Always maintain 3 inches of clearance from heat sources: Keep loose insulation away from metal stove flues, masonry chimneys, and non-IC rated recessed lighting.
How the Insulation Calculation Works (Worked Example)
Consider topping up an uninsulated 1,000 sq ft attic to energy code R-38 in Climate Zone 4 using blown-in cellulose insulation (R-3.5 per inch):
- 1. Attic Floor Area: 1,000 square feet.
- 2. Target R-Value: R-38 (with 0 existing insulation).
- 3. Required Thickness Depth: 38 ÷ 3.5 R-per-inch = 10.86 inches -> Settle depth is ~11.0 inches.
- 4. Bag Coverage at R-38: Manufacturer standard for a 25-lb bag at R-38 is 31.8 square feet per bag.
- 5. Calculate Bags Needed: 1,000 sq ft ÷ 31.8 sq ft/bag = 31.45 bags.
- 6. Add 5% Settling Allowance: 31.45 × 1.05 = 33.02 -> Round UP to 34 bags.
- 7. Blower Machine Rental: Most home improvement centers provide free machine rental with purchase of 20+ bags.
- 8. Attic Vent Baffles: 1,000 sq ft attic (~80 ft perimeter) requires approximately 20 rafter baffles along the eaves.
Insulation Materials, R-Values per Inch & Recommended Applications
| Insulation Material | R-Value / Inch | Moisture Resistance | Primary Building Application |
|---|---|---|---|
| Blown-In Cellulose | R-3.5 to R-3.8 | Treated with borate fire retardant | Attic floors, dense-pack closed walls |
| Blown-In Fiberglass | R-2.5 to R-2.8 | Non-combustible, moisture resistant | Open blow attics, cathedral ceilings |
| Fiberglass Batts / Rolls | R-3.1 to R-3.7 | Low (requires vapor retarder) | Standard 2x4 (R-13/15) and 2x6 (R-19/21) walls |
| Closed-Cell Spray Foam | R-6.5 to R-7.0 | 100% moisture vapor barrier | Basement rims, metal buildings, flash-and-batt |
| Open-Cell Spray Foam | R-3.6 to R-3.8 | Air barrier, vapor permeable | Roof rafters, interior acoustic soundproofing |
| Rigid Foam Board (Polyiso) | R-6.0 to R-6.5 | Foil-faced radiant vapor barrier | Exterior wall continuous sheathing, roofs |
| Rigid Foam Board (XPS) | R-5.0 | High water resistant (closed cell) | Basement foundation walls, slab underlayment |
Frequently Asked Questions About Insulation Calculator
What is R-value in home insulation?
R-value measures thermal resistance to conductive heat flow. The higher the R-value number, the greater the insulating power. A higher R-value keeps summer heat outside and prevents winter furnace heat from escaping through your roof.
Which is better for attics: blown-in cellulose or fiberglass batts?
Blown-in cellulose is generally superior for attic floors. Loose-fill cellulose flows around joists, wiring, and cross-bracing to create a monolithic blanket with zero voids. Fiberglass batts leave gaps around wires and pipes that allow convection air leaks.
Can you put new insulation directly on top of old insulation?
Yes, as long as the old insulation is dry and free of mold or rodent feces. However, new insulation installed on top of existing layers must NEVER have a Kraft paper or foil vapor barrier facing, which would trap moisture inside the lower layer.
How thick is R-38 and R-49 attic insulation?
With blown cellulose (R-3.5/in), R-38 is approximately 11 inches deep, and R-49 is about 14 inches deep. With blown fiberglass (R-2.5/in), R-38 requires about 15 inches, and R-49 requires 19 to 20 inches.
Why are attic baffles (vent chutes) necessary before adding insulation?
Attic baffles staple between roof rafters directly above the exterior wall top plates. They maintain a clear 2-inch airway so fresh air can enter soffit intake vents and flow up to the ridge vent, preventing winter ice dams and roof moisture rot.