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.