Whole-Wall R-Value: A Best Practice for Evaluating a Structure’s Potential Energy-Efficiency

Bautex insulated concrete block (ICB) mass walls provide con­tin­u­ous insu­la­tion and prevent thermal bridging. These attrib­ut­es create a high and accurate whole-wall R‑value, which ensures an energy efficient and com­fort­able home or building.

Which R‑Value Estimation is Most Accurate?

Con­trac­tors and archi­tects must consider many factors when building and designing a high-per­form­ing, energy-efficient com­mer­cial building or home, including the R‑value (or, resis­tance value). The R‑value rep­re­sents a mate­ri­al’s or system’s resis­tance to heat flow by con­vec­tion, con­duc­tion, and radiation.

Several methods estimate the R‑value; unfor­tu­nate­ly, each may give com­plete­ly different results. The question is, which method is most accurate. Four common ways to report the R‑values include the R‑value of insu­la­tion, and the whole-wall, clear-wall, and center-of-cavity R‑values.

R‑Value of the Insulation

The R‑value of insu­la­tion is the product’s resis­tance to heat flow through a given thickness of a material. The Inter­na­tion­al Energy Con­ser­va­tion Code (IECC) mandates the installed R‑values of insu­la­tion required to achieve best practices to insulate homes and buildings according to their climate zones. However, there are several problems with the R‑value of insulation.

The R‑value of the insu­la­tion neglects the insu­la­tion value of the other layers of the wall assembly, like the shealthy, drywall cladding, and air barriers. Fur­ther­more, the R‑value of insu­la­tion ignores thermal bridging.

Whole Wall System R‑Value, Clear-Wall R‑Value, and Center-of-Cavity R‑Value

During the 1990s, a report by the Oak Ridge National Lab­o­ra­to­ry (ORNL) Buildings Tech­nol­o­gy Center (BTC), evaluated the accu­ra­cies of R‑values by studying the impact of all the indi­vid­ual elements of the building envelope on the R‑value.

The study came up with three new ways to estimate R‑values, besides the R‑value of insu­la­tion: whole wall system R‑value, clear-wall R‑value, and center-of-cavity R‑value.

  • The whole wall system is the R‑value for the whole opaque system including all addi­tion­al struc­tur­al com­po­nents (such as double studs), and enclosure interface details, including wall /​roof, wall/​wall (corners), and wall/​floor con­nec­tions. Whole wall R‑value also considers the impact of thermal bridging based on the amount of framing in the wall compared to the amount of insu­la­tion as well as the effec­tive­ness of the sealing of the enclosure at lowering air infil­tra­tion. Cal­cu­lat­ing the whole wall R‑value is with an area-weighted average R‑value for clear wall and wall interface details.
  • Clear-wall R‑value includes the exterior wall area that contains only insu­la­tion and necessary framing products for a clear section. It is a section with no corners, windows, doors, or con­nec­tions with foun­da­tion and roofs. The clear-wall R‑value ignores thermal bridging asso­ci­at­ed with corners, windows, doors, or con­nec­tions with foun­da­tion and roofs.
  • Center-of-cavity R‑value estimates the R‑value at the spot in the wall con­tain­ing the most insu­la­tion. So, there is 0 percent framing factor and no account­ing for any of the thermal short circuits through the framing. The center-of-cavity R‑value does not take into account thermal bridging. The center-of-cavity R‑values are often sig­nif­i­cant­ly higher than clear-wall R‑values.

The Benefits of the Whole Wall R‑Value

The whole wall R‑value is the most precise mea­sure­ment of a wall’s thermal resis­tance over clear-wall, center-of-cavity R‑values, or R‑value of insu­la­tion. The whole wall R‑value takes into account the whole opaque wall system; therefore, it is often con­sid­er­ably lower than the clear-wall, center-of-cavity, or insu­la­tion R‑values. In fact, the whole wall R‑value on the same wall can be as much as 40 percent less than the clear-wall value. 

A report by Christian and Jan Kosny 1999 compared clear-wall and whole-wall R‑values of forty different wall systems. In every case, the clear-wall R‑value was higher than the whole-wall R‑value. For example,

  • Insulated concrete form (ICF) wall whole-frame R‑values were 4 percent less than clear wall R‑values. From a whole-wall per­spec­tive, an ICF wall performs up to the adver­tised R‑values because thermal bridging is absent.
  • Standard 2 X 4 wood-frame R‑values was 7.6 percent less than clear-wall R‑value.
  • Steel frame C‑stud whole-frame R‑value was 23.3 percent less than clear-wall R‑value.

Results of this study conclude that mass walls, like ICF walls, with con­tin­u­ous insu­la­tion and without thermal bridging, can produce an insu­la­tion value for the entire wall system closer to the man­u­fac­tur­ers claimed R‑value than either wood- or steel-framed wall systems.

Bautex Wall Assembly Provides High Whole-Wall R‑Values

Con­struc­tion with a mass wall, like the Bautex ICB, out­per­forms framed buildings with com­pa­ra­ble stated R‑values because ICB walls provide con­tin­u­ous layers of insu­la­tion with no thermal bridges. These are both attrib­ut­es that produce reliable and accurate high whole-wall R‑values.

The Bautex Block is a high thermal mass material that creates a tight building envelope for a high-per­form­ing energy-efficient home or building. The R‑value of Bautex Blocks per inch is 1.84. The Blocks provide a high-level of con­tin­u­ous insu­la­tion with a whole wall R‑value of 14 that resists the flow of heat by con­vec­tion, con­duc­tion, and radiation.

Fur­ther­more, the Bautex Wall System meets the thermal per­for­mance dictated by the Inter­na­tion­al Res­i­den­tial Code (IRC) and Inter­na­tion­al Building Codes (IBC) and provides an R‑value that exceeds the ASHRAE 90.1 require­ments for climate zones one through six. However, a building or home built with Bautex Block and a brick veneer achieves a whole-wall R‑18 system; exceeding the ASHRAE 90.1 rec­om­men­da­tions for zone seven.

The whole-wall method of esti­mat­ing the R‑value of a wall system provides a true mea­sure­ment of a wall’s resis­tance to heat flow. Therefore, selecting products or systems that report whole-wall R‑values, like Bautex wall system, will provide a more reliable estimate of a home or building potential energy-effi­cien­cy than products that report R‑values based on other methods.