Roof Insulation Calculator
Enter attic floor area, insulation type, and R-value target to calculate the number of blown-in bags or batt packages needed for IECC code compliance.
Find your insulation quantity fast. Enter attic area, insulation type, and your target R-value below to get the material quantity.
Flat ceiling area — not the sloped roof surface.
Blown-in cellulose or loose fiberglass — result in bags.
IECC Zone 4–8 minimum: R-60. Zone 2–3: R-49.
Check manufacturer’s bag label for coverage at your target R-value.
Enter attic area and
R-value to calculate
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Insulation formulas
Insulation calculations operate on the flat attic floor area — not the sloped roof surface. The R-value target and the insulation’s efficiency per inch determine the required physical depth, and the manufacturer’s coverage data converts depth to bags or packages.
Attic Floor Area
A = Length × Width
Bags Needed (blown-in)
Bags = A ÷ (Coverage per bag at target R-value)
Required Depth
Depth = Target R ÷ R-value per inch
Worked example. 1,500 sq ft attic, Climate Zone 5 (R-60 target), blown-in cellulose at R-3.5/inch. Depth = 60 ÷ 3.5 = 17.14 inches. If the manufacturer’s label states 40 sq ft per bag at R-60: Bags = 1,500 ÷ 40 = 37.5, round up to 38 bags. Add 10% settling buffer: order 42 bags. Use the roof ventilation calculator after specifying insulation — heavy insulation must be paired with adequate attic ventilation to prevent moisture problems.
R-value targets by climate zone
| Climate zone | States (examples) | IECC R-value (attic) | Rating |
|---|---|---|---|
| Zone 1–2 | FL, HI, TX (south) | R-30 to R-38 | Warm climate |
| Zone 3 | GA, SC, AZ, NM | R-49 | Mixed climate |
| Zone 4 | VA, MO, KS, OR | R-49 to R-60 | Mixed-humid |
| Zone 5 | IL, OH, PA, NY, WA | R-60 | Cold climate |
| Zone 6–8 | MN, MT, AK, ND | R-60 | Very cold |
Material cost benchmarks
| Type | R-value/inch | Cost per sq ft (installed) |
|---|---|---|
| Blown-in fiberglass | R-2.5 to R-3.0 | $1.20–$1.80 |
| Blown-in cellulose | R-3.5 to R-3.7 | $1.20–$2.00 |
| Fiberglass batts | R-2.9 to R-3.8 | $1.50–$2.50 |
| Spray foam (open-cell) | R-3.7 to R-3.9 | $3.50–$6.00 |
| Spray foam (closed-cell) | R-6.0 to R-7.0 | $6.00–$14.00 |
Frequently asked questions
Do I use the attic floor area or the roof surface area for insulation?
Always use the attic floor area — the flat horizontal surface of the ceiling below the attic. The IECC and all manufacturer coverage specs calculate insulation by the horizontal area it covers, not the sloped roof surface area. Using sloped area significantly overstates the quantity and cost.
What R-value do I need for my attic?
The IECC 2021 requires R-60 for attics in Climate Zones 4 through 8, which covers most of the northern and central US. Zones 1 through 3 (southern states) require R-30 to R-49. Use the DOE Climate Zone map to find your zone by zip code. Many states have adopted higher minimums than the IECC baseline — check local energy codes.
Why do I need to add 10% extra to the bag count?
Blown-in loose-fill insulation settles under gravity and loses up to 10 percent of its initial volume within the first year. Installers blow it heavier — intentionally over-installing — to ensure the R-value remains at target after settling. The 10% buffer accounts for this inevitable compaction. Batts do not settle but require 5% extra for trimming and fitting around obstructions.
Can I add blown-in insulation over existing fiberglass batts?
Yes, this is standard practice and safe. Blown-in cellulose can be installed directly over existing fiberglass batts as long as the existing insulation is dry, undamaged, and not harboring mold. The total R-value is additive. Air-seal all penetrations (pipe boots, wire chases, light fixtures) before blowing for maximum effectiveness.
What is the R-value of fiberglass batts versus blown-in?
Standard fiberglass batts provide R-2.9 to R-3.8 per inch. Blown-in fiberglass offers R-2.5 to R-3.0 per inch. Dense-pack cellulose provides R-3.5 to R-3.7 per inch. Closed-cell spray foam delivers the highest at R-6.0 to R-7.0 per inch — but at 3 to 7 times the installed cost of blown-in materials.
How thick is R-60 blown-in cellulose insulation?
R-60 using blown-in cellulose at R-3.5 per inch requires approximately 17 to 18 inches of depth. For blown-in fiberglass at R-2.7 per inch, achieving R-60 requires 22 to 24 inches of depth. This deep installation is physically impossible in a 2x8 rafter cavity (7.25 inches) — attic insulation achieves these depths because it sits on top of the ceiling joists without depth constraints.
Does insulation R-value affect ventilation requirements?
Yes, critically. The IRC requires that attic insulation installed to or above the ceiling joist tops must maintain a minimum 1-inch air space between the top of the insulation and the underside of the roof sheathing. Baffles (rafter vents) are required at each rafter bay to maintain this airway and allow outside air to flow from soffit to ridge. Without baffles, insulation blocks the ventilation path, creating moisture and mold problems.
References
US Department of Energy. (2024). Insulation Fact Sheet. DOE Office of Energy Efficiency. energy.gov
International Code Council. (2021). International Energy Conservation Code. ICC. codes.iccsafe.org
North American Insulation Manufacturers Association. (2024). Model Energy Code Compliance Guide. NAIMA.