Building Science

More Voodoo: Equivalent and Effective R-Value

In a previous blog post, The Voodoo of Insulation R-Value, we chronicled the horrible ways in which R-value is commonly misunderstood, or, if understood, carelessly used in the building industry. While R-value is an important factor in determining the energy efficiency of a building, it is woefully inadequate to fully explain or predict the actually performance of a completed project. Just ask Energy Conservation Specialists about that after being fined by The Federal Trade Commission (FTC) over unsubstantiated claims of savings on energy bills based on inaccurate R-values.

Unfortunately, the industry continues to stick to their guns in using R-value as an absolute or relative measurement of energy efficiency. The FTC is just as bad in spreading incomplete information. In fact, they have rules about how manufacturers are to test and label their insulation products (16 CFR 460) that overemphasizes the importance of R-value on consumer choice. To make matter worse, the industry has introduced the concept of “Equivalent R-value” and “Effective R-value” in an attempt to rationalize the difference in performance between alternate wall assemblies, which turn out to be inherently more complex than what can be described by a single all-powerful R-value.

What is R-Value and What Does It Tell Me?

In its truest form, R-value is simply the measurement of a material’s ability to resist the flow of heat energy. Better insulators have a higher R-value. A common testing standard for measuring R-value on insulation products is ASTM C 518, “Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus.” The result of the test is an R-value per unit thickness of a material.

Unfortunately, the testing is done under a very specific set of conditions and assumes a steady state temperature of around 70°F (21°C). Of course, the real world does not stay 70 degrees all the time, so the reported R-value will normally be inflated over what we can expect to experience in real-world conditions.

We also make a mistake in assuming that the insulation in the real-world installation is the same as in the test (i.e. the insulation is not compressed, installed with voids, or wetted). We also have to assume the insulation performs the same over time, both of which are rarely found to be the case.

Despite all of these potential pitfalls, the industry continues to devise ways to use R-values to describe the absolute and relative performance we should reasonably expect from different insulating materials and building envelope assemblies.

Light Framing and Effective R-Value

One of the first things we realize when comparing different building envelope designs is the fact that the insulation we are specifying only makes up a portion of the overall assembly. Several other materials make up and contribute to the performance of the assembly. We are not just specifying the R-value of an insulation, we need to take into account how these other materials that make up the rest of the assembly will impact energy efficiency. Some of these materials are highly conductive, and can significantly reduce the performance of the assembly.

In conventionally framed walls, wood or light-gauge steel framing members make up 20-25% of the wall assembly, creating thermal bridges through the envelope. In the spaces occupied by the framing, there is no insulation and the average R-value of that assembly is reduced below the tested R-value of the insulating material itself. The industry has coined the term “Effective R-value” to describe the insulating performance of a light framed wall assembly as a whole.

For example, in the 2015 International Energy Conservation Code (IECC), a 3.5-inch thick metal framed wall spaced on 16-inch centers with a cavity insulation of nominal R-13 is given an Effective R-value of only 5.98 (C402.1.4.1 Thermal resistance of cold-formed steel walls). This is a correction factor of 0.46, which means you are only getting 46% of the value of the insulation you are paying for. Despite the utility of using “effective” R-value in explaining performance of different building envelope systems, using R-values in this way really only serves to confuse the market more.

In his blog post in The Energy Vanguard entitled 4 Types of R-Value, Allison Bailes describes the Effective R-value as “whole-wall R-Value,” referring back to research that was done at Oak Ridge National Laboratories in the 1990’s. In a very clear warning at the end of his article, he cautions us all to be careful how we use R-value as a gauge of energy performance, because R-values we calculate from simple formulas aren’t static and they change dramatically with other factors including outdoor temperatures which can vary significantly across regions. To make things a bit more complicated than that, R-values also don’t account for other performance factors like thermal mass.

Thermal Mass and Equivalent R-Value

In our previous blog post on R-value cited above, we discuss the concept of thermal mass or thermal inertia as being a wall assembly’s ability to absorb and release thermal energy over time, which can significantly increase the energy efficiency of a mass wall assembly. To substantiate and empirically measure this “mass wall” effect, a team at Oak Ridge National Laboratories conducted thermal efficiency tests on a wide array of different wall assemblies in the 1990’s. They published the results in a paper they entitled The Whole Wall Thermal Performance Calculator-On the Net.

In this study, the team used a large-scale hot box test apparatus to measure the actual thermal resistance of 40 different wall assemblies as they are normally constructed. The goal was to develop a uniform “whole wall” R-value metric to be included in a much simplified “Whole-Wall Thermal Performance Calculator” that would allow professionals and consumers alike to compare the expected performance of their wall assemblies.

While this research was very informative and useful in the abstract, many building materials manufacturers took extreme liberties with this data and misused it order to make their products more attractive to the market. As a result, the market has seen many examples of mass wall insulating systems with claims of “effective” R-values of R-45 and even R-60. In reality, the nominal or tested R-value of many of these mass wall systems were actually three or four times lower than their claims. By this point, the market had become totally confused.

In their defense, it is understandable why mass wall system manufacturers did this. They were selling a product with a nominal R-value of R-9 to R-19 and trying to compete with cheaper insulating materials used in light frame construction with similar nominal R-values. The average uneducated consumer would look at the two R-values and conclude the performance of the mass wall and the light framed wall were exactly the same, which is categorically untrue. Unfortunately, in their attempt to right a wrong, the use of “effective” R-values has only served to further confuse an already misinformed public.

Conduction, Convection, and Radiation

To put a final stake in the heart of R-value, it must be pointed out that thermal conductivity (and resistance) is only one measure of energy efficiency of a building envelope assembly. We are completely missing the boat if we don’t also consider thermal convection via air migration through the building envelope, and thermal radiation that is emitted from the surface of materials in a building envelope assembly. In fact, the effects of thermal convection and radiation can, in cases, have a much more significant impact on energy efficiency of a building and the thermal comfort of building occupants.

This MachineDesign article “What’s the Difference Between Conduction, Convection, and Radiation?” and this Jefferson Lab PowerPoint presentation “Understanding Heat Transfer, Conduction, Convection and Radiation” do a great job in explaining the difference between conduction, convection and radiation.

Bautex Block Wall System

The Bautex Block Wall System provides a nominal or tested R-14 continuous insulation that is significantly higher performing than framed walls of R-19 and even higher. In fact, the continuous insulation and thermal mass of the Bautex Wall System exceeds the latest 2015 IECC energy codes by 2-3 times across the State of Texas. For a more in-depth look at the drivers of building energy efficiency, download our Energy Efficiency Whitepaper.