Building Science

The Voodoo of Insulation R-Value

Pho­to Cred­it: www​.pub​licbroad​cast​ing​.net

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.