The “R” in R-value Doesn’t Stand for “Real-World”


Conventional consumer wisdom and US energy codes lead us to believe higher R-value equates to better insulating value. In theory, materials with higher R-values should be more effective as insulation and thus be more environmentally efficient. But the reality is that R-value is just one measure of insulating quality—and an imperfect one at that. In order to get an accurate gauge of the actual thermal resistance and energy efficiency of an insulating material there are other factors that should be considered.

What is R-value?

R-value, in technical terms, is a measure of thermal resistance to heat flow through a material. All other conditions being equal, it’s the ratio of the temperature difference across an insulator and the heat flux, or transfer, through it.

As an equation, R-value looks like this:


In other words, R-value equals temperature difference divided by heat flux.

Spray foam insulation has been used by builders for more than 30 years, and true to its longstanding reputation as an energy efficient material it has an exceptionally high R-value per inch: open cell foam scores a 3.5 per inch, while the workhorse closed-cell spray foam insulation (SPF) has an R-value of around 6.5 per inch.

While spray foam insulation (closed cell) does indeed have a high R-value, the simple fact remains that R-value alone is not the best way to properly assess the insulating value of SPF. R-value is a simple metric developed specifically for testing legacy insulations like fiberglass and rock wool, and cannot truly define the insulating value of SPF insulation versus other types of insulation.


The American Society for Testing and Materials (ASTM) is responsible for assigning R-value to types of insulation and other building materials. Their tests are conducted in the highly controlled environment of a lab, where they can get precise measurements on varying materials while controlling all of the outlying environmental factors that might otherwise vary the results (things like wind and humidity). ASTM results give a pretty good idea of a material’s thermal resistance—in a lab.

But the reality is the very factors lab testing eliminates—thermal bridging, air movement, air infiltration, thermal mass, moisture content, solar radiation, and varying performance at extreme temperatures—are factors that directly affect an insulating material’s effectiveness in real world conditions. The only thing the ASTM tests for when coming up with R-value assignments is conduction—the transfer of energy from one plate, through the material being tested, to the other plate. This focused form of testing doesn’t give an altogether accurate gauge of how a material actually performs in a real building over its lifetime, with many varying environmental factors at play.

The Three Metrics for Heat

Insulation’s primary job is to stop the movement of heat in or out of a building, but R-value measures exclusively for conduction, and conduction is only one of the three ways that heat can move. Convection and radiation are the other two ways, but are not tested by the ASTM when determining R-value.

It might surprise you to hear that studies supported by the US Department of Energy (DOE) show that fiberglass batting—another common insulating material—loses R-value before it is even applied, and its R-value continues to degrade once it comes into contact with real-world environmental factors—the kind a building faces every single day.

SPF insulation, on the other hand, does not lose R-value when tested for factors that influence real world performance. When temperatures drop, it actually performs better than its stated R-value. One of the reasons for this is that spray foam, a viscous liquid in its application state, quickly expands to fill a cavity and then cures to a firm solid which does not degrade over time or when exposed to elements or various other factors. In fact, SPF insulation cures into a monolithic blanket that becomes a part of the permanent building envelope. Unlike fiberglass batting, it will never have to be removed or replaced.

“Fiberglass is generally assigned an R-value of approximately 3.5. It will only achieve that R-value if tested in an absolute zero wind and zero moisture environment. Zero wind and zero moisture are not real-world.” David B. South, co-inventor of the monolithic dome, and author of Urethane Foam: Magic Material—and the Best Kept Insulation Secret

Other Flawed Criteria

Aside from not taking into account all types of heat or real-world environmental factors, R-value testing is fundamentally flawed for a few more reasons.

  • The testing is done at a standard air temperature of 75 degrees. While this allows for consistency in the testing, it is somewhat illogical in that most buildings would not be using heat or air conditioning if the outside air temperature were 75 degrees. Testing an insulating material at this temperature is an irrelevant metric because it doesn’t ensure the material will test well at a more realistic temperature.
  • Test criteria demands that the R-value testing is done only once the insulating material has reached a “steady state”—meaning that it has become thermally saturated so that every unit of heat entering it on one side is matched by an equal unit of heat on the other side. The time that it takes a material to reach such a “steady state” is not taken into account, even though that time would occur in the real world.It’s important to note here that traditional fiberglass insulation takes only a few hours to reach a steady state, while spray foam insulation takes about a day. Because spray foam thus has higher thermal inertia, its ability to collect and store heat is also greater.
  • R-value testing doesn’t take into account the gaps and air leaks that traditional insulating materials can leave in a structure. SPF is unique in that in it original liquid state it fills all cracks and crevices, therefore leaving no gaps or air leaks.

Real R-Value, not Lab R-Value and Why SPF is a Much Better Performing Insulations Than Legacy Insulations

R-value is determined in a lab. Your home is not a lab. And although your home might look like a solid piece of construction, even the most well constructed building has a certain amount of tiny gaps and cracks between the various materials—siding, framing, drywall, etc. An insulating material of relatively uniform or geometric shape, like fiberglass batting, can be roughly cut to fit a space, but it won’t account for those tiny little places through which air can leak.

Spray foam insulation, on the other hand, applies as a liquid and flows into every little tiny crack and nook in a cavity, then expands and “locks” into place to form a firm solid that adheres to the surface. Once sealed in place, SPF prevents heat via air infiltration on a micro-level that fiberglass batting and other materials simply cannot. So although these other materials might stand up in a lab test of theoretical R-value, once put in place, they are drastically subpar to spray foam.

In tests conducted at Oak Ridge National Laboratory, commonly installed fiberglass batting was found to have an R-value that was typically 28% less than what the manufacturer claimed because of its lack of air barrier qualities when exposed to a real-world environment that included wind, moisture, and a cold climate. In fact, tests showed that the effective R-value of fiberglass diminished by more than 50% under these conditions—the type of conditions that insulating materials are subjected to constantly in real-world building applications.

R-value is a Myth and SPF is Ultra-Insulation

To put it bluntly, R-value is a poor way to judge insulating value. In his book Urethane Foam: Magic Material—and the Best Kept Insulation Secret, David B. South calls R-value “a modern fairy tale that has been so touted to the American consumer that it now has a chiseled in-stone status.” He argues that it’s impossible to define an insulation type by a single number.

However, spray foam insulation constantly outperforms it’s R-value, and should be considered a special class insulation—a hyper or ultra-insulation in contrast to the stuff we’ve been using and testing and being disappointed by for years.