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.