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Insulation Calculator

Calculate insulation needed by area and desired R-value. Compare costs and quantities across fiberglass, cellulose, spray foam, and rigid foam.

Quick Answer

For a 500 sq ft attic at R-38, you need approximately 12 inches of fiberglass batts or 11 inches of cellulose. Fiberglass batts cost roughly $930, while blown cellulose runs about $1,390. Spray foam is most expensive but provides the highest R-value per inch.

sq ft

Material Comparison (R-38, 500 sq ft)

Fiberglass Batts

R-3.2/inch

$935
Best Value
Thickness
11.9"
Quantity
17 batts (16" x 48")
Cost/sqft
$1.87

Blown Fiberglass

R-2.5/inch

$1,242
Thickness
15.2"
Quantity
37 bags
Cost/sqft
$2.48

Cellulose (Blown)

R-3.5/inch

$1,388
Thickness
10.9"
Quantity
37 bags
Cost/sqft
$2.78

Spray Foam (Open Cell)

R-3.7/inch

$2,192
Thickness
10.3"
Quantity
5136 board feet
Cost/sqft
$4.38

Spray Foam (Closed Cell)

R-6.5/inch

$3,654
Thickness
5.8"
Quantity
2924 board feet
Cost/sqft
$7.31

Rigid Foam (XPS)

R-5/inch

$1,388
Thickness
7.6"
Quantity
46 4x8 sheets
Cost/sqft
$2.78

About This Tool

The Insulation Calculator helps you determine how much insulation material you need for any project and compare costs across different insulation types. Enter the area you want to insulate and your target R-value, and the calculator shows thickness requirements, material quantities, and estimated costs for six common insulation materials. Making an informed choice among insulation types can save hundreds to thousands of dollars while achieving the same thermal performance.

Understanding R-Value

R-value measures a material's resistance to heat flow. The higher the R-value, the better the insulation. Different parts of your home need different R-values. Attics need the most insulation (R-30 to R-60) because heat rises and escapes through the roof. Walls typically need R-13 to R-21 depending on wall thickness. Floors over unconditioned spaces need R-19 to R-30. The US Department of Energy publishes recommended R-values for each of eight climate zones, with colder zones requiring higher values.

Fiberglass Insulation

Fiberglass batts are the most common insulation in residential construction. They come in pre-cut widths (15 inches for 16-inch on-center framing, 23 inches for 24-inch on-center) and standard lengths (48 or 93 inches). Fiberglass is inexpensive, widely available, and easy for DIY installation. Its main drawback is that it does not seal air leaks; it only slows heat transfer. For maximum effectiveness, fiberglass must be installed without compression or gaps. Compressed fiberglass loses R-value proportionally to its compression. Blown fiberglass is used in attics and fills irregular spaces better than batts but has a lower R-value per inch.

Cellulose Insulation

Cellulose is made from recycled newspaper treated with fire retardant. It is blown into attics or dense-packed into wall cavities. Its R-value per inch (3.5) is slightly higher than fiberglass batts (3.2) and it fills irregular spaces more completely. Cellulose is the preferred choice for attic retrofits because it can be blown over existing insulation quickly. It also provides better soundproofing than fiberglass. The main disadvantage is that it absorbs moisture, so it should not be used in areas prone to water intrusion without proper vapor barriers.

Spray Foam Insulation

Spray foam provides the highest performance but at the highest cost. Open-cell foam (R-3.7/inch) is softer, less expensive, and provides excellent air sealing but is vapor-permeable. Closed-cell foam (R-6.5/inch) is rigid, provides a vapor barrier, adds structural strength, and is waterproof. Closed-cell foam is the best choice for basements, crawl spaces, and exterior applications. Spray foam must be professionally installed and requires specialized equipment. Its main advantage is that it simultaneously insulates and air-seals, which can reduce total energy costs by 30-50% compared to fiberglass alone.

Choosing the Right Material

For new construction walls, fiberglass batts or spray foam are the most common choices. For attic retrofits, blown cellulose offers the best combination of cost, coverage, and performance. For basements, rigid foam or closed-cell spray foam provides insulation plus moisture resistance. For tight spaces where high R-value per inch matters (like 2x4 walls that need R-15+), closed-cell spray foam is the only option that fits. Consider hiring a professional energy auditor to assess your specific needs before making a major insulation investment.

Frequently Asked Questions

What R-value do I need for my home?
R-value requirements depend on your climate zone and the part of the house. For attics: R-30 to R-60 (higher in colder climates). For exterior walls: R-13 (2x4 walls) or R-19 to R-21 (2x6 walls). For floors over unheated spaces: R-19 to R-30. For basement walls: R-10 to R-15. Check the US Department of Energy's recommendations for your specific climate zone (zones 1-7) or your local building code.
What is the most cost-effective insulation?
Fiberglass batts are the cheapest to buy and install for standard wall and attic applications. Blown cellulose is the best value for attic retrofits because it fills irregular spaces and is quick to install. Spray foam costs 2-3x more but provides the highest R-value per inch and doubles as an air barrier, which can significantly reduce heating and cooling costs. The best choice depends on your specific application, budget, and whether you are doing new construction or retrofitting.
Can I add insulation on top of existing insulation?
Yes, you can add new insulation on top of existing insulation in most cases. In attics, blown cellulose or unfaced fiberglass batts can be added directly over existing insulation. Never add faced (with vapor barrier) insulation over existing insulation, as this creates a moisture trap. Before adding insulation, check for moisture damage, mold, or pest problems in the existing insulation and address those issues first.
What is the difference between R-value and U-value?
R-value measures thermal resistance (how well a material resists heat flow). Higher R-values mean better insulation. U-value measures thermal transmittance (how easily heat passes through). U-value is the inverse of R-value (U = 1/R). Windows and doors typically use U-value because their insulating properties are lower. A wall with R-19 insulation has a U-value of 0.053.
Does insulation also help with soundproofing?
Yes, all insulation provides some sound dampening, but effectiveness varies. Fiberglass batts and cellulose are good sound absorbers for airborne noise (voices, TV, music). For maximum sound control, use dense-pack cellulose or mineral wool (Rockwool), which has better sound-dampening properties than fiberglass. Spray foam is less effective for sound because its rigidity transmits vibrations. For serious soundproofing, combine insulation with resilient channels, double drywall, or sound-dampening compounds.

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