How to Determine the R-Value of Insulation | Today's Homeowner

How can I determine the R-value of my attic insulation? -Anita

Hi Anita,

The R-value of insulation is a measure of how well it reduces the flow of heat and cold into and out of your house. The higher the R-value, per inch of thickness, the better job the insulation will do when you heat or cool your home.

While you can’t have too much insulation, older homes often don’t have enough. To determine the R-value of the insulation in your attic, you need to know the R-value of the type of insulation in your home and the thickness of the insulation.

R-Value of Insulation by Type

Here are the R-values per inch of the most common types of insulation:

• Fiberglass (blown): 2.2 – 2.9
• Fiberglass (batts): 2.9 – 3.8
• Cellulose (blown): 3.1 – 3.8
• Rock Wool (loose): 2.2 – 3.3
• Foam (sprayed): 3.6 – 8.2

As you can see, there’s some variation in R-value depending on the particular type and brand of products used. Depending on where you live, it’s recommended that attics have a minimum R-value of R-30 in warm climates to R-60 in cold climates.

Recommended Insulation by Climate

By dividing the recommended R-value for your area by the R-value of your particular type of insulation, you find that an attic should have on average:

Warm Climates (R-30 to R-49):

• Fiberglass (blown): 14” – 18”
• Fiberglass (batts): 11” – 14”
• Cellulose (blown): 11” – 13”
• Rock Wool (loose): 12” – 18”
• Foam (sprayed): 5” – 11”

Moderate Climates (R-38 to R-60):

• Fiberglass (blown): 17” – 22”
• Fiberglass (batts): 13” – 17”
• Cellulose (blown): 13” – 16”
• Rock Wool (loose): 15” – 22”
• Foam (sprayed): 6” – 14”

Cold Climates (R-49 to R-60):

• Fiberglass (blown): 19” – 25”
• Fiberglass (batts): 14” – 19”
• Cellulose (blown): 14” – 18”
• Rock Wool (loose): 17” – 25”
• Foam (sprayed): 7” – 15”

To find the R-value of the existing insulation in your attic, multiply the number of inches of insulation by the R-value for your particular type.