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Real R-values for Whole Walls
(published by JLC September 1997)

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Scientists with the Department of Energy have developed a new method for measuring the effective R-values of wall assemblies. The technique, the brainchild of Jeffrey Christian and Jan Kosny of Tennessee's Oak Ridge National Laboratory (ORNL), takes into account not just the performance of insulation, but also the heat losses that happen at corners, areas around windows, and joints where walls meet roofs, floors, or foundations.

Depending on how a wall is built, these thermal bypasses can cut the assembly's total heat-flow resistance to far below the R-value of the insulation it contains. Most builders realize that the framing in a 2x6 wood-frame wall, for instance, reduces its actual insulating value below the R-19 label on the fiberglass batts used to insulate it. But "framing factor" rules of thumb often underestimate this R-value drop. Christian and Kosny's new technique allows builders to estimate a typical wall's real performance much more closely, and to make meaningful comparisons to alternative wall systems like steel framing, foam-core structural panels, or foam-formed poured-in-place concrete walls.

Testing and simulations. The Oak Ridge method uses a computer model to predict wall performance based on the known properties of the materials that make up the wall. For each type of assembly, Christian and Kosny test the computer's output against the measured performance of a "clear wall" section that has no windows or connection to other components. Once the computer's accuracy has been verified, the pair apply the model to estimate the wall's performance as part of an actual house, with connections to corners, floors, roofs, and window and door openings.

Simple size-ups. Christian and Kosny have already published figures on two dozen traditional and innovative wall systems. A few sample comparisons reveal some interesting implications (see chart).

For one thing, you get less than you pay for when you jump from 2x4 to 2x6 walls. A 2x4 wall has a true clear-wall R-value of 10.6, close to its advertised R-11. Its whole-wall value isn't much lower at 9.6. But an "R-19" 2x6 wall, even with 2-foot o.c. stud spacing, gives you only R-16.4 in clear-wall terms, and earns just R-13.7 for a whole-wall rating. So 2 extra inches of thickness bumps your wall's practical insulating value up by only R-4.1.

Typical steel-framed wall sections fare even worse. A basic metal-framed stud wall with R-11 batt insulation bottoms out at a measly R-6.1, while even the sophisticated foam-sheathed steel-wall system used on NAHB's Energy Conservation House, whose clear-wall section rates R-14.8, earns a whole-wall score of only R-10.9 — barely a point above a simple 2x4 stick-built wall.

But fans of steel framing shouldn't lose heart. Christian has high hopes for ORNL's newly invented Stud Snuggler, an insulating-foam wrap for steel studs that economizes on the costly foam by installing it right around the studs, where it does the most good. Christian has been stingy with details, but indications are that the new device may put steel walls into the same energy-efficiency ballpark as wood-frame assemblies.

Structural panels loom large. The big winner so far in the Oak Ridge ratings is the 6-inch-thick structural insulated panel assembly. Starting with a clear-wall value of 24.7, it retains 88% of that performance in the whole-wall evaluation at R-21.6. This is 58% better than a 2x6 stick-framed wall, and head and shoulders above any other system Oak Ridge has so far tested. While foam-core construction still comes at a premium, energy-conscious buyers might find this real insulating value to be worth the price.

Wall system comparisons online. For details on the full range of wall systems tested, you can consult long articles Christian and Kosny have published in Home Energy magazine ("Wall R-values That Tell It Like It Is," April 1997) and the ASHRAE Journal ("Thermal Performance and Wall Ratings," March 1996).

Both articles are available on the World Wide Web. Try these URLs:
www.cad.ornl.gov/kch/articles.html and
www.homeeenergy.org/homeenergy/297wall.html.