What Is R-Value For Insulation? Explained & Myths Debunked

When it comes to insulation, the term “R-value” is often mentioned, but what exactly does it mean? Unfortunately, it is a bit confusing, and the answer should be more straightforward.

But, once you get some background into how it was created and measured, you’ll easily be able to answer the question when someone asks, “what is R-value?”

First: What Is R-Value?

installing fiberalls batt insulation in an attic

R-value is a measure of a material’s ability to resist heat flow. Before 1945, the U-factor measured this, with lower U-factors being better at resisting heat. 

It was introduced as a solution to marketing difficulties faced by insulation manufacturers, who found it hard to sell products with low U-factors. R-value is calculated as 1/U, so the more significant the R-value, the better the material keeps heat inside.

R-value measures the resistance of a material to all three types of heat transfer: conductive (heat flow from fiber to fiber), radiant (heat transfer from hot fiber to cooler fiber through the air), and convective (heat transfer due to air movement within insulation). Manufacturers claiming otherwise are incorrect.

The standard test for measuring a material’s R-value is ASTM C518, which involves measuring the thermal resistance of a sample between a cold and hot plate.

This makes R-value a valuable tool for comparing insulation products. For example, when testing a fiberglass batt, the test captures heat flow from the hot side to the cold side and the effects of all three heat transfer mechanisms.

Rated Vs Real World R-Values

There are two groups of meaning to R-value: rated R-value and real-world R-value. Both groups measure a material’s resistance to heat flow but have an essential difference.

Rated R-Value

The rated R-value is a regulated number. In response to the energy crisis caused by the Arab oil embargo in 1973, companies began selling insulation products with falsely high R-values, leading to the US Federal Trade Commission (FTC) instituting the FTC R-value Rule. 

The regulation requires insulation to be labeled with a rated R-value derived from one of four approved test methods.

The FTC still enforces the regulation today. For example, in 2013, a company selling insulating paint claimed an R-value of 100, resulting in the FTC issuing a $350,000 fine.

The first thing to note about the rated R-value is that it’s a static number obtained from a test performed under specific conditions. The test for insulation R-value is usually done with a temperature difference of 50 °F on one side and 100 °F on the other, with the material placed horizontally. 

However, these conditions are not representative of real-world use, making rated R-value a limited tool for comparison.

Real World R-Value

Real-world R-value is a dynamic number that changes depending on various factors. The actual R-value of an insulation material in a building can be influenced by the following:

  • Temperature difference
  • Airflow
  • Moisture
  • Compression, missing insulation, and other installation problems

Temperature is the only factor beyond control. For a specific house in a particular location, the temperature difference across the insulation changes with the weather, time of day, and season, thus affecting the insulation’s resistance. 

For instance, an R-13 fiberglass batt may range from R-12 to R-14, with most insulation performing better in cold weather and worse in hot weather.

The other factors related to installation and building failures can be controlled to some extent. For example, some insulation materials, like fiberglass and cellulose, are air permeable and meant to be installed with a certain amount of air between the fibers but often get compressed. 

Compression lowers the R-value; for example, an R-19 fiberglass batt installed in a 2×6 cavity will be compressed and perform as an R-18 batt.

Air and water control layers must do their jobs to avoid these problems. 

In wall cavities, air-permeable insulation must be encapsulated entirely on all sides and fill the cavity completely. Therefore, the real-world R-value of insulation material should be close to the rated R-value if installed correctly. 

Still, the R-value of an assembly or an entire enclosure will average all the layers and their R-values.

Discover the Different Types Of R-Value

insulating an attic with cellulose

When it comes to R-values, most people think of it as the isolated insulation rating. However, the true R-value of a wall or ceiling goes beyond just insulation. 

Other elements such as wood, drywall, sheathing, cladding, and more all play a role.

The R-value of a building assembly is more than just the insulating properties of the insulation. Heat travels through a wall in two ways – through the insulation cavity and through the wood framing. 

These are two separate pathways of heat flow, each with its own layers of materials such as drywall, insulation, sheathing, cladding, air films, and more.

The total R-value is calculated by adding the individual R-values of all these layers. When heat flows through the framing, it travels through all the layers of the wall, including air films, drywall, sheathing, and cladding, but with the addition of wood in the middle, acting as a thermal bridge with only a quarter of the R-value of insulation.

When you consider all the layers and pathways of heat flow, the overall R-value of an assembly becomes different from the R-value of just the insulation material. 

The R-value of a building assembly can be discussed in five different ways. The method depends on various factors such as temperature, airflow, moisture, and installation quality.

Let’s explore the different types of R-values and how they impact the thermal performance of your walls.

  • Material R-Value: This is the basic R-value that building codes refer to. It only considers the R-value of the insulation material itself.
  • Center-of-Cavity R-Value: This is an upgrade from Material R-Value, as it considers all the layers in the wall, from interior air film to exterior air film. However, it neglects the impact of thermal bridging caused by the framing.
  • Clear-Wall R-Value: This is the R-value of the wall without windows, doors, or corners. It accounts for heat flow through the cavities and framing. But remember, you can’t simply average the R-values; you have to convert U-values to R-values.
  • Whole-Wall R-Value: This is the R-value of the complete wall, including all the complexities like wall-to-wall connections, wall-to-roof, and wall-to-floor. But, it does not account for the thermal performance of windows and doors.
  • Overall R-Value: This is the R-value of the entire wall, including fenestration. Unfortunately, windows and doors have lower R-values, making the overall R-value the lowest of all the R-values.

So, what’s the best R-value to consider when comparing walls? The answer is Whole-Wall R-value. It’s the most comprehensive and accurate representation of the thermal performance of a wall, taking into account all the essential elements.

Remember, the R-value of your walls isn’t just about the insulation you put in them. It also changes with temperature and other factors. And the R-value calculated from the definitions above is not a static number.

Why We Shouldnt Just Be Looking at R-Value

R-value is important, but air leakage is also crucial in determining insulation performance. The R-value test can measure the effects of convective loops within insulation but not the amount of air that may leak through the wall assembly once installed. 

Air leakage is affected by factors like insulation density, wind speed, and the presence of air barriers. Despite its usefulness, R-value is among many factors when choosing insulation. 

Fiberglass insulation must be enclosed by air barriers on all six sides to perform optimally, while spray foam’s ability to reduce air leakage sets it apart. Therefore, marketers claiming R-value measurements are meaningless are incorrect. 

It’s important to understand building science principles beyond just R-value to predict heat flow through walls and ceilings. But, unfortunately, no single “magic number” replaces builders’ need to study building science.


A contractor told me radiant heat will pass right through regular insulation, and I need a radiant barrier. Is that correct?

Radiant heat does not pass through solid materials. For example, radiant heat can travel through air or vacuum but cannot travel through solid materials. 

When radiant heat hits a deep layer of insulation, only a small amount can flow through on the other side. 

Heat will always flow from hot to cold, but the more insulation, the slower the rate of heat flow. Therefore, marketers or contractors who claim conventional insulation does not block radiant heat are using a scare tactic and are not accurately representing the properties of insulation materials.

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Davin is a jack-of-all-trades but has professional training and experience in various home and garden subjects. He leans on other experts when needed and edits and fact-checks all articles.