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.