What Does R-Value Mean In Insulation?

As you started researching insulation, you've come across many new terms. One of these is the term R-value. What is R-value? Does R-value measure something, or is it something you'll need to measure? Do you have to understand R-value to purchase insulation?

South Central Services has insulated hundreds of projects with spray foam insulation. R-value is an important concept to understand when discussing insulation. Whether you're researching spray foam, cellulose, or fiberglass, you'll want to know how R-value works. After reading this article, you'll have the information you need to make informed insulation decisions according to R-value.

By the time you finish reading, you will know:

• What R-value is
• How R-value is calculated
• Frequently asked questions about R-value

What Is R-Value?

R-value, or resistance value, is a basic measurement of how well a material insulates. While other factors can influence an insulation's efficiency, R-value has traditionally been used to measure insulating performance. R-value measures how well a material can resist thermal heat transfer.

As you research insulation, you will see the R-value listed in the product description and specifications. For example, a fiberglass batt on Home Depot's website might list R30 or R-30 in the product name. This is the R-value.

R-value is usually represented by collective R-value in traditional insulations, whereas spray foam R-value is represented per inch. For batt insulation, a R-38 batt is thicker than an R-21 batt because it has a higher R-value. For spray foam, total R-value can be calculated based on the depth installed. A closed cell foam may have an R-value of R-7, giving it the same R-value as a R-21 batt with only 3 inches of foam.

How Is R-Value Calculated?

The R-value calculation is a complex one. Luckily, you don't need to understand how to calculate R-value to choose the right insulation. However, if you’re interested in the science behind R-value, we’ll break it down.

R-value is calculated using thermal conductivity, as the thickness of an insulation divided by its thermal conductivity. The thickness of the insulation is measured in inches, and thermal conductivity is measured by how much heat was transferred. The heat transfer is measured over time to arrive at an accurate calculation.

R-Value FAQs

Here are some top questions people ask about R-value when researching insulation.

Is An Insulation Product’s R-Value Always the Same?

R-value will vary depending on the insulation manufacturer and some other factors. For example, many companies like Carlisle and SWD manufacture closed cell spray foam. Just because the product is closed cell spray foam does not mean it has the same R-value. The R-value will likely be similar but may vary based on differences in the manufacturing process.

Similarly, fiberglass batts do not have a universal R-value. If you research fiberglass for purchase online, you'll see batts and blankets listed at R-15, R-30, R-38, and many other R-values.

The R-value of a blown-in product like fiberglass or cellulose is tricky to determine. Blown-in R-values vary based on the thickness of the product applied. A bag of loose-fill fiberglass may claim to cover 200 square feet at R-19, but if the R-value needs to be higher, the fiberglass will cover half that square footage.

Is the Required R-Value of An Insulation Project Always the Same?

Do attics always require R-38 or higher? Is there a universal rule for the R-value of an external wall? No, the R-values of projects vary based on climate and other factors. At South Central Services, we work in Climate Zone 5. This climate zone includes southern Pennsylvania and areas of Maryland, Virginia, and West Virginia.

Need help determining what R-value your project will require? This climate zone map from the Department of Energy is the best place to start.

What Makes a Good Insulation R-Value?

If you're researching insulation products and wondering if the R-value is good, you must first research the required R-value for your project. A good R-value meets or exceeds the required R-value of a project.

In our climate zone, Climate Zone 5, attics require R-38 or higher. For homeowners in southern Pennsylvania, R-38 is a good R-value for their project. Anyone living further north than our service area may need a higher R-value. What would be a good R-value for a Maryland exterior wall may be different from a good R-value further south.

Are Aged R-Value and Advertised R-Value Different?

R-value can change over time. Aged R-value represents the R-value you can expect over the lifetime of a product. For example, the R-value of blown-in cellulose in an attic will decrease over 15 years. If the R-value dips below what's needed for the attic, the insulation will need to be topped off to meet that expected R-value.

When considering a product, know the difference between the advertised R-value and the aged R-value. Blown-in products settle over time, so the advertised R-value is not the aged R-value.

Spray foam insulation also experiences a decrease in performance that R-value can measure. When spray foam is initially installed, it has a high R-value. After the spray foam ages, the R-value lowers and stays consistent at that diminished value. Spray foam manufacturers should advertise using the aged R-value, which lets you know what R-value you can expect over the product's lifetime.

The Bottom Line About Insulation R-Value

R-value is a key concept to understand when researching and selecting insulation. While a higher R-value technically translates to a higher performance, the right R-value depends on what you are insulating.

Looking for more fundamentals about insulation? Check out these resources next.

• How Spray Foam Insulation Is Made
• Pros and Cons of Spray Foam Insulation
• Fiberglass: Batts vs. Blown-In