Although windows and doors now carry labels showing their resistance to heat flow, it can be tough to gauge the energy-efficiency of the wall that the windows and doors are installed in. For instance, most estimating procedures ask you to describe the wall construction then enter a framing factor (the ratio of stud area to the whole exterior wall area). The framing factor is seldom verified, and is often set lower than it really is. "There's a larger framing factor out there than textbooks tell you to use," says Jeff Christian, a researcher at Oak Ridge National Laboratory. "Some people use 10 percent, but I wouldn't be surprised to find 30 percent" once you add windows, doors, and connections to other walls, says Christian. He figures this extra framing can cut a wall's actual R-value to half the estimate. The problem is made even worse by the growing popularity of steel and engineered-wood framing systems, since most estimating systems assume a wood stud wall. In fact, the lack of an accurate way to measure whole-wall R-values may keep builders and homeowners from considering a new framing system.

Oak Ridge is developing what Christian hopes will become a national standard for determining whole-wall R values for nearly any wall. The system factors in the penalties, or "thermal shorts," incurred by windows, doors, joints, and connections to other walls. It also considers the wall's thermal mass, airtightness, moisture tolerance and sustainability. Oak Ridge has assembled a user-friendly database on the Internet that lets you compare the thermal performance of different wall systems. It includes:

• An archive of experimental results for a number of tested wall systems, including downloadable drawings.
• A database of material thermal properties.
• The ability to enter and estimate the R-value of custom wall systems.

Christian hopes that the rating procedure will spawn consumer labels for walls similar to the ones used on windows and doors. "This will give the industry incentive to develop more efficient systems, while consumers will be better able to see the benefits of spending a little more for a better wall system." A labeling system may also encourage designers, builders and homeowners to look more closely at the efficiency of their walls. Christian thinks this could raise the R-value of the average wall system by 25-50 percent over current standards.