Building Science: Crunching the (Good) Numbers

It’s easy to think of “significant” numbers. But often the numbers burned most acutely into our psyches tend to be associated with war, disaster or some kind of misfortune: December 7: Pearl Harbor Day; 9-11: the day everything changed; 20: the age of the gunman who murdered 20 elementary school kids last month; 2012: the end of the world (or not, if you’re reading this). $3.59: the cost of a gallon of gas at this writing.


Perhaps its our morbid interest in misfortune and suffering that draws us to the “negative” power of numbers, but we’re fortunate to work in a field where numbers tend to bring good news. Why talk about yesterday’s R-11 walls when today R-19 is the standard? Why bemoan an old, single-pane window’s negligible 0.9 U-value, when a new, triple-glazed argon-filled glazing blows it away, at U-.31? In the green building world, there’s nothing newsworthy about yesterday’s numbers. Who cares that an old, poorly insulated house has a blower door score of 6,500 to 8,500 cfm50, when a new, tight home averages about 1,200 cfm50? The latter number is the one that matters—at least until it, too, is replaced by a better score.

In general, the only time we mention a number in connection with environmental building and products (unless we’re talking about climate change) is to demonstrate an improvement or innovation in building science. A tract home builder isn’t likely to boast about installing a 6-SEER furnace in a new home, but you can bet she’ll mention that 16-SEER model.

In our January 2013 issue, we zeroed in on some of the most important numbers in home construction: wall R-values, heating and cooling systems and so on. We’ll look at the latest research from Building America, and take on a side-by-side comparison of how a prototype home performs with different wall systems. We worked with Dietrich’s 3D-CAD/CAM Software to compare R-values and energy efficiency to a high level of accuracy. What you’ll see are the final heating energy numbers for each system, but those figures came from thousands of calculations, not seat-of-the-pants guesswork.

The integration of computerized design makes it easy to tweak green building numbers. But building science is complicated. Some numbers can be boosted easily for minimal cost. Others lose their effectiveness at a certain threshhold. For example, Building America found that putting more than two inches of foam on a home’s exterior wasn’t worth the cost—when compared with increasing furnace efficiency or other options.

uvalueBecause building systems are measured by different standards, there’s no magical number that represents optimal sustainability: we want SEER to go up, U-value to go down, AFUE to go up, air infiltration to go down. The closest we can get to a quantifiable goal: we want homes to achieve zero-net-energy (or better).

On the building science front, the numbers are all moving in the right direction—some of them quite quickly. The next great challenge is to get mainstream Americans interested in sustainability numbers. How many hundreds of gallons of water can they save? How much can they reduce their energy bill? By how many thousands can they improve their resale value? How many kilowatts can they sell back to their local utility? Numbers can be a force for the good. Let’s give them a push in that direction.