Yikes. Just the other day I was trying to get across to my teenage sons precisely why we were hanging the wash on the line instead of firing up the clothes dryer. You know: trade fossil fuels for sunlight, save money, avoid emissions — not to mention, get outdoors.

But today two New York Times blogs, Green and Dot Earth, are flogging a new study that chides energy-conserving Americans for succumbing to a set of “myths” prioritizing behavioral changes over efficiency upgrades. One supposed myth is that line-drying saves more energy than washing the clothes in cooler water.

Huh? Avoiding an hour’s worth of electric or gas drying doesn’t save more Btu’s than running the washing machine with the thermostat turned down? How can that be?

Well, it can’t be, as I confirmed with a couple of calculations, and as the study authors — a team from Columbia University’s Earth Institute, Ohio State, and Carnegie Mellon — could have seen for themselves if they had stopped to ponder the physics of clothes washing and drying.

The study, which has just been published in the Proceedings of the National Academy of Sciences, examined Americans’ perceptions of lifestyle changes vs. tech upgrades in 15 different household arenas. Line-drying vs. cool-water washing was one arena; others concerned turning off lights vs. converting to compact fluorescents, bicycling vs. buying a hybrid, and so forth. (Though … why cast these as either-or choices? Why not dump incandescents and turn off the CFLs when you leave the room? Why not bike in town and rent a hybrid for longer trips, of which you’ll make fewer now that you’ve geared into a cycling lifestyle?)

My corrective calculations are straightforward, but to spare readers the gory details I’ve off-loaded them here. Basically, according to Department of Energy, a dryer consumes power at a rate between 1.8 and 5 kW, while a washing machine runs between 0.35 and 0.5 kW. Compounding this 5-to-10-fold difference, a load of laundry takes longer to dry than wash. Since energy equals power integrated over time, bypassing the dryer for a clothesline probably saves 10 or more times as much energy as fractionally lowering the wash temperature.

What the study authors billed as a myth, then — the notion that line-drying trumps cool-water washing in saving energy — is fact!

(I’ve emailed my calculations to the lead author, who hasn’t yet replied. The outsize figures for energy savings from washer-thermostat turndown that appear in the study’s supporting document are sourced to a Rocky Mountain Institute study that doesn’t appear to be retrievable from the link provided. Could the authors have inadvertently applied an RMI annual energy savings figure to a single washload?)

“Delusions Abound on Energy Savings, Study Says,” the Green blog proclaimed. “Misperceived Paths to Energy Savings,” Dot Earth chimed in. But in this one case, it appears the delusions belong to the study authors, while Average Joe & Jane have their energy caps on straight.

The Dot Earth post concludes with a call to boost energy literacy. Sure, but it brings to mind that old saw about people in glass houses.