Insulation Calculator

The Insulation Calculator helps homeowners, contractors, and energy auditors determine the right insulation R-value, thickness, material quantity, and estimated cost for any building component. Based on DOE/IECC 2021 climate zone recommendations, it compares seven popular insulation types and estimates annual energy savings with payback periods.

What Is R-Value?

R-value is a measure of thermal resistance — how well a material resists the flow of heat. A higher R-value means better insulating performance. R-value is additive: stacking two R-10 layers gives R-20. It depends on the material type and thickness:

Different insulation materials have different R-values per inch. Closed-cell spray foam leads at R-6.5 per inch, while blown-in fiberglass provides R-2.5 per inch but at a much lower cost.

Understanding Climate Zones

The U.S. Department of Energy (DOE) divides the country into eight climate zones based on temperature and heating degree days (HDD). Colder zones require higher R-values to maintain energy efficiency:

How to Use This Calculator

1. Select your climate zone — Find your DOE zone based on your state or city. Zone 1 is the hottest (southern Florida, Hawaii) and Zone 8 is the coldest (interior and northern Alaska).
2. Choose the building component — Select attic floor, cathedral ceiling, exterior wall, floor over garage, basement wall, or crawlspace wall.
3. Pick an insulation type — Choose from fiberglass batt, blown-in fiberglass, blown-in cellulose, open-cell spray foam, closed-cell spray foam, rigid foam board (XPS), or mineral wool.
4. Enter the area in square feet — Measure the total area to be insulated. For walls, multiply wall height by total wall length.
5. Optional: enter your current R-value — If upgrading existing insulation, enter the current R-value to calculate the additional insulation needed.

Insulation Types Compared

Fiberglass Batt

The most common and affordable insulation. Pre-cut panels fit between standard wall studs (16" or 24" on center). R-3.2 per inch. Best for new construction or open cavities. Easy DIY installation but must be cut carefully to avoid gaps.

Blown-In Cellulose

Made from recycled newspaper treated with fire retardant. R-3.5 per inch. Excellent for attic floors and retrofitting enclosed walls. Settles about 20% over time, so professionals typically over-install. Eco-friendly choice with a lower carbon footprint.

Closed-Cell Spray Foam

The highest-performing insulation at R-6.5 per inch. Also serves as an air barrier and vapor retarder. Ideal for cathedral ceilings, rim joists, and below-grade walls. More expensive but requires less thickness, saving space in wall cavities.

Rigid Foam Board (XPS)

Extruded polystyrene boards at R-5 per inch. Waterproof and moisture-resistant, making them ideal for basement walls and exterior foundation insulation. Available in standard 4×8 ft sheets in 1" to 3" thicknesses.

Mineral Wool (Rockwool)

Made from volcanic rock or steel slag. R-3.15 per inch. Naturally fire-resistant (withstands temperatures over 2,000°F) and provides excellent soundproofing. Denser than fiberglass, making it easier to cut and fit without compressing.

Energy Savings Formula

The calculator estimates annual energy savings by comparing heat loss before and after insulation, using heating degree days (HDD) for your climate zone:

Where A is the insulated area (sq ft), HDD is annual heating degree days, and C_fuel is the cost per BTU of heating fuel. This provides a rough estimate — actual savings depend on HVAC efficiency, air sealing, and local energy prices.

Frequently Asked Questions

What is R-value in insulation?

R-value measures an insulation material's resistance to heat flow. The higher the R-value, the better the insulating effectiveness. R-value is expressed per inch of thickness, and different materials have different R-values per inch. Total R-value is the R-per-inch multiplied by the thickness in inches.

What R-value do I need for my attic?

Attic R-value recommendations depend on your climate zone. Zone 1 needs R-30, zones 2-3 need R-49, and zones 4-8 need R-60. Colder climates require higher R-values to prevent heat loss through the roof, which is the largest source of energy loss in most homes.

What is the best insulation type for walls?

For existing walls, closed-cell spray foam offers the highest R-value per inch (R-6.5) and acts as a vapor barrier, making it ideal when cavity space is limited. For new construction, fiberglass batts or mineral wool are cost-effective options. Blown-in cellulose works well for retrofitting enclosed wall cavities through small access holes.

How do climate zones affect insulation requirements?

The DOE divides the US into 8 climate zones. Zone 1 is the hottest (minimal insulation needed) and Zone 8 is the coldest (maximum insulation required). Each zone has specific R-value recommendations for attics, walls, floors, and basements based on heating degree days. Using the correct zone ensures you neither under-insulate (wasting energy) nor over-insulate (wasting money).

How much does insulation cost per square foot?

Insulation costs vary by type and thickness. Fiberglass batts cost about $0.10 to $0.16 per square foot per inch of thickness. Blown cellulose costs $0.10 to $0.18. Closed-cell spray foam is the most expensive at $0.70 to $1.20 per square foot per inch, but provides the highest R-value per inch, requiring less thickness overall.