Updating An Incomplete Measure Of Energy Performance

by Joseph Garvey

R-values are our friends. They make our job much easier. They help us explan complicated things to customers who want them even simpler than we do. Unfortunately, the R-values many of us use are also obsolete. At best, they form only part of the energy performance picture. In a construction market where energy efficiency counts more and more, the trusty old number might no longer suffice to persuade an informed homebuyer. So more than ever savvy concrete builders need to open up discussion and see the total energy value of their permanent structures.

R-value measures a material's capacity to impede or resist heat flow. The "R" thus stands for Resistance, with a higher value denoting a higher capacity for this desirable performance factor. So far, so good, as long as we're talking about building materials in their uninstalled state. To construct a building, however, those materials have to get stacked, glued, screwed, pounded, or poured into shape. Any assembly method typically involves seams, joints, holes, grooves, channels, and countless other connections through which the heat flow resistance capacity of the off-the-shelf material gets compromised.

Even if you could monolithically produce the entire structure, the best building material would still have inconsistencies sufficient to lower its R-value here and there. And no building is without windows and doors all varying in their ability to resist heat flow, and none of them doing a very good job of it. Once you factor in the dynamics of climate, the prospects of accurately predicting energy performance seem dimmer than ever.

The upshot is that the R-value of a building material delivered to the jobsite is not the same as the R-value of the wall that same material goes to form. As Andre Desjarlais, program manager for Building Envelope Research at the Oak Ridge National Laboratory (ORNL), TN, points out, they are two -- or three -- total different measures.

Between themselves, consumers, and the gatepost, insulation manfacturers call the R-value stamped on the batt of glass wool, for instance, the "material R-value." Yet in a standard 2x4 frames wall, that "R-11" may prove a high estimate, mostly due to thermal bridges created by the studs and other framing. With time, batt insulation can also sag, compress, or otherwise deteriorate. Add a nice listtle family of mice who set up shop above a firestop, and that wall will soon rate far lower than R-11.

To account more accurately for heat-flow resistance, ORNL's research team coined the terms "clear-wall R-value" and "whole-wall R-value," distinguishing the ideal performance of insulating materials in a typical section of wall, uninterrupted by architectural details such as window or door openings, from that of the fully framed and finished wall as a whole, minus only the actual glass. Wholewall R-value deducts for typical thermal bridges, openings, and other design or construction elements that can dminish heat flow resistance capacity. Thus, whole-wall R-value never amounts to more than a percentage of the clear-wall value.

So how in the world do you figure whole-wall R-value? Luckily, you don't have to. Desjarlais and his colleagues have already done the legwork. Just dial up their webpage (www.ornl.gov/roofs+walls/whole_wall), click on "Whole-Wall Thermal Performance Calculator," and follow the simple instructions. This conveniently interactive method determines whole-wall value for almost all the current wall systems -- including wood, steel, and hybrid studs; structural insulated panels (SIPS), and concrete masonry units (CMUs).

Just for fun, let's run the program for a hypothetical ranch house, framed with standard 2x4s. Punch in the parameters, and instantly, the cold, hard facts are staring a would-be stickbuyer in the face.

Clear-Wall R-value = 11.43 h-sqft-def-degF/Btu
Whole-Wall R-value = 9.86 h-sqft-degF/Btu
(86.3% of Clear-Wall R-Value)

All that sawing and hammering, and supply problems, and the environment, and termites, and fure threat -- and stickbuilders can't even break R-10? Tsk, tsk!

Four insulating concrete form (ICF) brands are also listed, but only one has an active link. This ICF maker's own website lists a definite R-value for the foam only -- 4.17/inch, which we take as material R-value. Quadrupling that for a minimum thickness of 4", counting both sides of the form, means a foam-alone value of 16.68. Pour six inches of concrete at ORNL's R-0.1/inch (for what must be the least energy-friendly mix in the world), and we get a material R-value of 17, plus change. Granted, nothing earth-shattering, but then not too shabby either.

Yet when we went through the motions for a structure of this same manufacturer's ICF, the ORNL calculator return downright dismal results. Once we chose "Concrete-Filled EPS Form" as building materials, the next step was to okay the given interior and exterior finishes -- 1/2" gypsum (i.e. drywall) and 1/2" stucco, respectively. Step three invited us to select a floor plan. For fair comparison, we went for the same "Ranch House #," a somewhat cramped-looking three-bedroom, two-bath which at 1,732 sf was the largest option. Finally, we clicked "continue," let the numbers crunch (which takes but a millisecond), and --

Clear-Wall R-value = 11.96 h-sqft-degF/Btu
Whole-Wall R-value = 10.29 h-sqft-degF/Btu
(86% of Clear-Wall R-value)

WHOA!! -- That's a tad higher than the stickbuilt house, but nowhere near even R-17. If we can't do better than that, we might as well call off the pump truck and save the foam for coffee cups!

Challenged to explain this discrepency, Desjarlais replied, "We stand by our data." In his team's view, this particular form has two drawbacks. First, the foam is "dispersed non-uniformly." Second, the through-form ties are of heat-conducting material. Thus, ORNL believes the uneven inside foam surface possibly diminishes -- or at least does not add to -- R-value, and the ties make thermal bridges. The scientists' verdict is not, however, the last word. For one thing, Desjarlais freely acknowledges that whole-wall R-value is yet an incomplete measure of energy performance.

But then, concrete builders -- and their growing hordes of happy customers -- have long known about the added benefits of thermal mass and airtightness. Our own "quick-n-dirty" survey shows R-value claims posted online by seven randomly-selected ICF manufacturers as ranging from an "actual" 20 to an "equivilent" 50, with R-28.3 the average. Suitably, the website of the same ICF manufacturer rated low by ORNL offers a typically comprehensive view of real-world energy efficiency. The manufacturer states that their installed product's "high R-value, high-thermal mass and reduced air infiltration provides a wall that out-performs an R-40+ wood frame wall." Who couldn't live with that?

Asked for his side of the R-story, the company's president stresses that their higher equivilent performance is borne out of "boatloads of statistics on actual houses with actual people living in them." Needless to say, he feels no fondness for the ORNL calculator.

Actually, ORNL does not recognize the need to think outside the R-value box. The Oak Ridge boys aim to round out their site with the eventual addition of a calculate for thermal mass -- pending funding approval. Since, however, Desjarlais already cautions that the benefits of thermal mass "are climate specific and cannot be claimed universally," this might be a good time to write a letter to your congressperson.

Meanwhile, the rest of us can look forward to the results ORNL's own ongoing comparative study of two Tennessee Habitat for Humanity houses -- one wood frame, the other ICF from a different manufacturer. We'll keep our eye on the bottom line, which after all, is what interests our customers most. Direct yours, meanwhile, to the January 2001 issue of Permanent Buildings & Foundations for the very encouraging figures from a recent 8-year study that tracked ICF residential energy savings down to the penny.

Incidentally, what was the highest wall system featured in ORNL's current online calculator? The so-called "Larsen Truss" scores a whole-wall R-value of 31.77 -- 80.4% of its clear-wall value of R-39.53. To help close your next sale, show your prospect a sketch of that contraption you say you can stickbuild it for about ten times the labor cost of a stay-in-place home. (Offer to throw in the remote-control thermostat for free!)

Remember, if you're not selling the total energy performance of your preferred concrete building method, you're selling all of us way short.