Roof Insulation Calculator
Find the R-value required for your climate zone, then calculate the insulation depth and material quantity needed to meet current energy code requirements.
Attic & Climate Details
sq ft
R
Additional R-Value Needed
R-49 more
Current R-ValueR-11
DOE MinimumR-38
DOE RecommendedR-60
Depth to Add13.2"
Material Needed398 bags
Installation Details
Blown Cellulose (3.7 R/inch)13.2" depth needed
Material quantity398 bags
Attic area1,200 sq ft
Final R-valueR-60
Cost & Energy Savings
Estimated material cost$795
Estimated labor (DIY alt.)$300
Total project cost$1,095
Est. annual energy savings$18
Simple payback period62.1 years
Link copied to clipboard
How to Use This Calculator
Select your location and insulation type to get tailored recommendations:
- Climate Zone — Select your IECC climate zone (1–8) or state. Zones 1–2 are the warmest (Florida, Hawaii), Zone 8 is the coldest (Alaska). The calculator maps your zone to the correct minimum and recommended R-values.
- Insulation Type — Choose blown fiberglass, blown cellulose, open-cell spray foam, closed-cell spray foam, mineral wool batts, or rigid foam board. Each has a different R-value per inch.
- Attic Floor Area — Enter your attic's square footage to calculate total material quantity needed.
- Existing Insulation — Enter any existing R-value so the calculator only tells you how much to add.
The Formula
Required Additional R-Value = Target R-Value - Existing R-Value
Depth Needed = Required R-Value / (R-value per inch)
R-Values per Inch (approximate):
Blown Fiberglass: R-2.2 per inch
Blown Cellulose: R-3.7 per inch
Open-Cell SPF: R-3.8 per inch
Closed-Cell SPF: R-6.5 per inch
Rigid EPS Foam: R-4.0 per inch
Rigid XPS Foam: R-5.0 per inch
Material Quantity = Attic Area (sq ft) × Depth (inches) / Coverage per bag
(Blown cellulose: ~40 sq ft at 1-inch depth per bag)
Depth Needed = Required R-Value / (R-value per inch)
R-Values per Inch (approximate):
Blown Fiberglass: R-2.2 per inch
Blown Cellulose: R-3.7 per inch
Open-Cell SPF: R-3.8 per inch
Closed-Cell SPF: R-6.5 per inch
Rigid EPS Foam: R-4.0 per inch
Rigid XPS Foam: R-5.0 per inch
Material Quantity = Attic Area (sq ft) × Depth (inches) / Coverage per bag
(Blown cellulose: ~40 sq ft at 1-inch depth per bag)
Example: Zone 5 Home Adding Blown Cellulose
Example: Climate Zone 5 (Chicago area), 1,200 sq ft attic, existing R-11 batts
Target R-Value (Zone 5)R-49 recommended
Existing R-ValueR-11
Additional R-Value NeededR-49 - R-11 = R-38
Insulation TypeBlown Cellulose (R-3.7/inch)
Depth to AddR-38 / 3.7 = 10.3 inches
Attic Area1,200 sq ft
Bags Needed~31 bags (30 sq ft per bag at 10 inches)
Estimated Cost$450–$900 DIY, $1,200–$2,400 installed
Upgrading from R-11 to R-49 in a Zone 5 climate can save $300–$600/year on heating and cooling bills. The payback period is typically 3–5 years even at installed costs.
Frequently Asked Questions
IECC minimums: Zone 1–2: R-30. Zone 3: R-38. Zone 4: R-38 to R-49. Zone 5–8: R-49. Energy Star recommends R-49 to R-60 for most of the US. This calculator shows both the code minimum and Energy Star recommendation for your zone.
Going from R-38 to R-49 saves noticeably more energy. Going from R-49 to R-60 has diminishing returns. In Zone 5+, R-60 is worth it if the attic is easy to access and cellulose or fiberglass is inexpensive in your area. Beyond R-60, marginal returns are minimal.
Insulate the attic floor (ceiling of living space) if you have a vented attic — this is standard and most cost-effective. Insulate the roof deck (creating a conditioned attic) if you have HVAC equipment in the attic, use the attic as living space, or have complex ventilation issues. Roof deck insulation typically uses closed-cell spray foam or rigid foam above the deck.
Measure the depth of your current insulation and identify the type. Use the R-value per inch to calculate your current total R-value. Compare to your zone's recommended value. Signs of inadequate insulation: ice dams in winter, very hot attic in summer, high heating/cooling bills, temperature differences room-to-room.
Blown insulation DIY kits are available at home improvement stores for accessible attic floors — a realistic weekend project that can save significant money. Spray foam requires professional equipment and should be professionally installed. Always install ventilation baffles at eaves first to maintain soffit vent airflow.