How Much Insulation Do I Need? Best R-Value Guide

What Is Insulation R Value, Anyway?

R-value measures your insulation’s ability to resist the transfer of heat. The higher your insulation’s R-value, the better it protects against heat transfer in 2 directions:

• Heat loss from your home to the outside, and
• Heat gain from the outside into your home.

The R-value system was designed to standardize the amount of insulation needed in different parts of your home in different regions of the country.

The higher the required R-value (AKA thermal resistance rating), the more insulation you need in that part of your home. For example, attics require a higher R-value than flooring because more heat is transferred through your attic than anywhere else in your home.

Knowing Your R-Value Increases Home Energy Efficiency

Knowing your R-value directly leads to an increase in energy efficiency and reduced home energy bills. The EPA reports homeowners save an average of 15% on heating and cooling costs with proper home insulation and air sealing. You'll also save roughly 11% on your total home energy costs. 

This table displays average home energy savings by properly insulating your house in each IECC climate zone (data via EnergyStar).

TABLE: Average U.S. Home Energy Savings With Proper Insulation, by IECC Zone

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The average American household spends nearly $2,100 per year on home energy bills. Properly insulating and air sealing your home saves you $231 every year.

Want to know how much home insulation installation costs in your area? Check out HOMR's updated Cost Guide to Insulation Installation for current pricing data anywhere in the United States.

R-Value Calculator: How Much Insulation Do I Need?

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The amount of home insulation you need depends on 3 factors:

• The type of insulation you’re using,
• The IECC climate zone you live in, and
• The part of your house you’re insulating.

If you’re unsure of your IECC climate zone, ask your local building department. 

Table: Recommended Home Insulation Per IECC Climate Zone

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Let's talk about how much insulation you need based on the type of insulation you're using, and the insulation R-value per inch.

Fiberglass Batt Insulation R-Value

Fiberglass is the most commonly used material in batt insulation. To standardize this section, we will focus on fiberglass insulation as the primary batt material.

Fiberglass Batt R-Value Per Inch

The average R-value per inch of fiberglass batt insulation is R-3.1 to R-3.4. You need roughly 14.4”-15.8” of fiberglass batt to achieve an R-value of R-49.

Cellulose Blown In Insulation R-Value

Most blown-in insulation projects use recycled cellulose. To standardize this section, we will focus on cellulose insulation as the primary blown in insulation material.

Blown In Cellulose R-Value Per Inch

The average R-value per inch of blown in cellulose insulation is R-3.5. You need roughly 14” of blown cellulose insulation to achieve an R-value of R-49.

Already picked your insulation material? Get a free quote on the installation here!

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Open Cell Spray Foam Insulation R-Value

Open cell spray foam consists of cells that are deliberately left open to create a soft material that expands up to 3 inches when applied. Open cell spray foam is more flexible than closed cell foam, but only offers about half the overall R-value.

Open Cell Spray Foam R-Value Per Inch

The average R-value per inch of open cell spray foam insulation is R-3.8. You need roughly 13” of open cell spray foam to achieve an R-value of R-49.

Closed Cell Spray Foam Insulation R-Value

The cells in closed cell spray foam are densely pressed together to prevent the passage of moisture and air. While closed cell is  less suitable for moist areas than open cell foam, its rigid density provides exceptionally high R-values — great for attics and exterior walls.

Closed Cell Spray Foam R-Value Per Inch

The average R-value per inch of closed cell spray foam insulation is R-6.5 to R-7. You need roughly 7”-7.5” of closed cell spray foam to achieve an R-value of R-49.

Polyiso Foam Board Insulation R-Value

Polyisocyanurate foam board (polyiso) is an extremely rigid, closed cell material that is bonded with facers on both sides. It's primarily installed underneath exterior home siding to help homeowners with pre-existing insulation get their homes up to R-values in the current building code.

Foam Board R-Value Per Inch

The average R-value per inch of polyiso foam board is between R-6 and R-7. The exact R-value of your foam board depends on the brand.

DALLAS HOMEOWNERS: HOMR selected Brighton Remodeling as the top exterior remodeling company in Dallas — read our updated DFW Remodeling buying guide and learn why they’re the right choice to install foam board under your siding.

Insulation Thickness Chart For Different Areas of Your Home

So, how much insulation do you need in different parts of your home?

Your attic requires the thickest insulation due to high levels of heat and moisture passing through attic and roof spaces. Interior 2/4 wall insulation often has the lowest minimum requirements, as they aren’t exposed to exterior heat transfer.

TABLE: Average Insulation Thickness Required to Achieve Home R-Value Standards

Attic Insulation Thickness

Every attic in American homes requires insulation with an R-value between R-30 and R-60. 

• Fiberglass Batt Insulation: between 9 ½” and 17 ¾”
• Cellulose Blown In Insulation: between 8 ½” and 16 ¾”
• Open Cell Spray Foam Insulation: between 8” and 15 ¾”
• Polyiso Foam Board Insulation*: between 4 ¾” and 9 ¼”
• Closed Cell Spray Foam Insulation: between 4 ¼” and 8 ¾”

*Note: Polyiso foam board is rarely used to fully insulate roof lines and attics. It’s used as a supplement to pre-existing insulation.

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Want to save 20% on energy bills? Message HOMR for a no-hassle quote!

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Exterior Wall Insulation Thickness

Exterior walls are generally 2x6 and require more insulation than 2x4 walls. Exterior walls in American homes require insulation with an R-value between R-13 and R-25. 

• Fiberglass Batt Insulation: between 4” and 7 ½” 
• Cellulose Blown In Insulation: between 3 ¾” and 7” 
• Open Cell Spray Foam Insulation: between 3 ½” and 6 ¾” 
• Polyiso Foam Board Insulation: between 2 ½” and 4 ¼” 
• Closed Cell Spray Foam Insulation: between 2” and 3 ¾” 

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2x4 Wall Insulation Thickness

The proper insulation thickness for most 2x4 walls in American homes is between R-13 and R-15. Most homeowners choose batt insulation for interior walls.

• Fiberglass Batt Insulation: between 4” and 4 ¾” 
• Cellulose Blown In Insulation: between 3 ¾” and 4 ¼” 
• Open Cell Spray Foam Insulation: between 3 ½” and 4” 
• Polyiso Foam Board Insulation: between  2 ½” and 2 ¾” 
• Closed Cell Spray Foam Insulation: between 2” and 2 ¼” 

Floor Insulation Thickness

Almost all U.S. home flooring requires insulation with an R-value between R-19 and R-30.*

• Fiberglass Batt Insulation: between 5 ¾” and 9 ½” 
• Cellulose Blown In Insulation: between 5 ½” and 8 ½” 
• Open Cell Spray Foam Insulation: between 3 ½” and 6 ¾” 
• Polyiso Foam Board Insulation: between 2 ½” and 4 ¼” 
• Closed Cell Spray Foam Insulation: between 2 and 3 ¾” 

*NOTE: IECC climate zone 1 requires an R-value of 13 — refer to the above table for proper insulation thickness.

Crawl Space Insulation Thickness

Most homeowners use fiberglass batt insulation to insulate crawl spaces. Crawl spaces in IECC climate zone 1 require R-13 insulation. Zones 2-3 require R-13 to R-19, and zones 4-8 requires R-25 to R-30.

• Fiberglass Batt Insulation, IECC climate zone 1: 4” 
• Fiberglass Batt Insulation, IECC climate zone 2-3: between 4” and 5 ¾” 
• Fiberglass Batt Insulation, IECC climate zone 4-8: between 7 ½” and 9 ½” 

PRO TIP: If you have an unventilated crawl space, insulate the walls instead of the subfloor above the crawl space.

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