Building Science

The Voodoo of Insulation R-Value

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The movie The Big Short opens with a fictional line falsely ascribed to Mark Twain that goes, “It ain’t what we do not know that causes all our problems. It’s what we do know that ain’t so.” While Mark Twain never penned those words, the truth of that statement still hits home. In many ways, this perfectly describes our common misunderstanding of R-values and its relationship to building energy efficiency.

It is perfectly clear why, as an industry, we have come to this common misunderstanding. It is very attractive to boil down something as complex as energy efficiency performance of a building to a simple number. R-value. It makes comparing two different products or two different projects very easy, much like miles per gallon (MPG) helps us compare the performance of two different cars. Unfortunately, while MPG does a fairly good job of informing a buyer about the expected mileage of a vehicle, R-value is not nearly as helpful to buyers of buildings who want maximum energy efficiency performance.

Thermal Resistance

For starters, R-value is a measurement of a single variable among many variables that impact energy efficiency of a building. Technically it is the measurement of the capacity of an insulating material to resist heat flow. The higher the R-value, the greater the insulating power. However, a building’s envelope (the boundary between outside and inside) is typically constructed of several materials and is often not consistent throughout the entire building.

For light-framed buildings, roughly 25% of a typical wall has no insulation in it because there are wood or steel frame members in the wall that interrupts the insulation layer. While you may be paying for an R-13 cavity insulation for your building, according to ASHRAE 90.1, these stud members create thermal bridges (interruptions in insulation layer where thermal energy is conducted through the envelope) that degrade the performance to as little as R-5.9 as on a conventional metal stud building.

Adding insult to injury, we further reduce the performance of the building’s thermal envelope when we create openings for windows and doors. Even the highest performing windows on the market today has a meager value of around R-6. The amount of glazing on a building will heavily impact energy performance. The performance of many building is also further constrained by designs that cause penetrations to be made through the insulation layer that conduct energy through the wall, such as metal structural elements and fasteners.

One last point on R-value is to consider how much insulation is optimal. In a linear world, if R-13 is good, R-19 is better, it would stand to reason that R-35 would be outstanding. While this may be true in a laboratory under controlled conditions, unfortunately it is not true in the real world with real buildings operating in real climates by real people. A 2013 study by the National Concrete Masonry Association (NCMA) perfectly illustrates the diminishing returns to added insulation. More is not always better, so we can’t simply rely on R-value numbers.

Air Tightness

Another important factor contributing to building energy efficiency is air infiltration. While not considered much until recently, it makes perfect sense. How many times did your parents tell you to close the refrigerator or the front door to the house? If you let the good conditioned air out and the bad unconditioned air in it doesn’t matter as much what R-value of insulation you have in your building.

For typical buildings constructed in the last three decades, the entire volume of air inside is being replaced with outside air every 6 to 10 minutes. That’s 6 to 10 air changes per hour (ACH50) that has to be conditioned and dehumidified by your mechanical system. The penalty for this “leakiness” can cause an increase in energy consumption of up to 18 to 20%.

Current IECC building code (2015 International Energy Conservation Code) requires that residential buildings in Central Texas have no more than 5 air changes per hour and buildings in North Texas no more than 3 air changes per hour. This increased performance requirement will significantly improve the performance of buildings. However, none of these gains are directly impacted by the R-value of the insulation used on the project.

Thermal Mass

If you take a deeper look at Chapter 4 of the 2015 IECC, you will see a table that shows what level of R-value that buildings need to have in different climates in order to be considered code-compliant. For example, here in Central Texas there are some buildings that require R-20 insulation and others that only require R-5.7 to meet code. The two buildings are considered comparable in performance yet have over 350% difference in R-value between them.

So why the difference? The first case requiring R-20 is for commercial buildings constructed with wood framing. The second case is for commercial buildings that utilize a “mass wall” assembly for the building envelope. A mass wall is any wall material or system that that has the capacity to store and release large amounts of thermal energy over time. These include, for example, concrete, concrete masonry (CMU), insulated concrete forms (ICF) and insulated concrete block (ICB).

A mass wall’s ability to store and release thermal energy significantly compounds the insulating effect of a wall system by delaying the impact on the interior climate. This is why building codes allow buildings that utilize mass walls to have less added insulation to achieve the same result. To drive this point home, the ICF Manufacturer’s Association (ICFMA) recently published a large-scale study that demonstrated that a wood framed wall with R-20 insulation required 149% more energy to maintain temperature over an ICF with similar R-value.

Turning the Volume To 11

As consumers, we love simple statistics on which to base our decisions because it makes life easier. Miles per gallon, FICO scores, 4K televisions, and so on. Sometimes these rules of thumb are meaningful to the buying decision, sometimes not. In the faux documentary This is Spinal Tap, Nigel Tufnel, a lead character of the fictional band by the same name, explains to Rob Reiner why their Marshall amplifiers have controls that go from 0 to 11, while most amps only go to 10. He says, “It’s one louder isn’t it. If we need that extra push over the cliff, we put it up to 11.”

Ultimately, while R-value is an important consideration when making decisions about your building, it is only one factor among many that will impact the energy efficiency of your project. Not all building materials perform the same, and R-value is not the best way to compare the performance of these systems. Therefore, we need to be willing to dig a little bit deeper into building science behind energy performance in order to avoid being misled by the voodoo of insulation R-values.

Bautex Block Wall System

The Bautex™ Block Wall System provides continuous insulation and thermal mass that 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.