Hummers on the homefront: at 4,600 square feet, is it an eco-house?

It's home-show time in Portland, Oregon, and people are lining up to gawk at the "Oregon Dream" one of eight show homes in a yearly promotion organized by local builders. They shuffle on tile floors, gape at distant ceilings. Women linger by a convoluted tub with innumerable tempting jets; men look wistfully at a gas grill as massive and shiny as a firetruck.

Every home in the show is on the market, and "green" features are getting the hard sell. Native plants show up in the landscaping, scruffy beside freshly unrolled turf. An environmental nonprofit, the Energy Trust of Oregon, is a major sponsor of the show. And the Oregon Dream is "Energy Star for Homes" certified--meaning it uses 15 percent less energy than a standard "code" house of similar size.

There's just one fly in the ointment: the size is gigantic. At 4,624 square feet, the house is like a hybrid SUV: efficient only in comparison to other behemoths. Despite the Energy Star label, the house has two full-size water heaters.

It's a perfect demonstration of the battle between two major trends in American housing. In the past few decades, houses have gotten greener, but they've gotten bigger too, leaving lingering questions: Is super-sized housing defeating conservation efforts? Can McMansions truly be green?

Houses are a major place to look for environmental gains. Besides consuming materials like lumber, the residential sector uses 21 percent of the nation's energy, according to the Energy Information Administration (EIA). EIA reports indicate that over the past 20 or 30 years, energy-saving measures like efficient windows and refrigerators have become commonplace.

Meanwhile, homes have steadily grown from sedan- to Hummer-sized. According to the National Association of Home Builders, the average new single-family home was 983 square feet in 1950, 1,500 square feet in 1970, and 2,434 square feet in 2005. This occurred even as the average household shrunk from 3.4 to 2.6 people.

The net effect is troubling. Despite widespread use of efficient technology, a new study by scientists from the Department of Energy's Lawrence Berkeley Laboratory shows that from 1985 to 2002, total residential energy consumption per capita climbed eight percent, and residential consumption for the nation--the figure most relevant to global effects like carbon dioxide (CO2) emissions--climbed 32 percent.

While reasons for these increases may be complex, the Berkeley report singles out house size as a key factor. A 2005 report from the EIA comes to the same conclusions, showing that households with 3,000 or more square feet use 40 percent more energy than those with 2,000 to 2,400 square feet. In that context, it's fair to wonder if a home like the Oregon Dream deserves the green halo of an environmental certification.

The Energy Star label is appropriate, suggests Bob Stull, senior program manager for the Energy Trust of Oregon's Efficient New Homes Program. He says the label's key criterion is 15 percent improvement over a similar code-built house. Size by itself is not judged. "I'm not saying it's a good thing somebody is buying a 4,600-square-foot home," he says. "But telling people what size home to buy is not what our program is about."

Size is a touchy subject in the exploding field of green building, suggests lay Hall, lead consultant to the U.S. Green Building Council's pilot LEED for Homes program. LEED-H, as it's called, is just one of dozens of American programs offering green building guidelines and certifications. Most score buildings across a broad range of environmental concerns, from indoor air quality to flaming methods. But LEED-H is one of the bold few to take the subject of size head on.

Since bigger homes generally use more energy and materials, LEED-H's pilot scoring system rewards small homes and penalizes large ones. Bigger houses can accumulate credits in other areas--for example, landscaping--to get certified, but at some point the penalty gets too large to overcome. "We've received a huge amount of criticism for that," says Hall, "in particular from high-end custom home builders."

Fortunately not every builder thinks bigger is better. The Cottage Company, a developer in the Seattle area, is widely lauded for its "cottage communities" of finely detailed and certified-green small houses, some less than 1,000 square feet, most less than 2,000. Co-owner Linda Pruitt says her houses "live as big" as McMansions because they're better designed, with features like vaulted ceilings and abundant built-ins. "It's kind of like the design of a yacht," she says. The theme is quality of space, not quantity. Cottage Company's homes around expensive Puget Sound start at $500,000.

Decidedly cheaper are the homes of people like Kathy Dolphin and Tim Johnson. Dolphin has shared a 600-square-foot San Diego house with a husband and daughter for decades; Johnson lives in a 200-square-foot house--with three kids half the time--on a Missouri prairie. The two are worlds apart, but have a lot in common.

They love the challenge of living small, making every restriction into a puzzle. They proudly cheat the electric company with solar panels. And they don't like debt. Dolphin paid her mortgage off early, and Johnson built his house for the cost of parts.

Just like this year's model homes, Johnson's tiny pad features DSL and a large LCD TV. And every day he wakes up to this year's must-have feature: a bedroom coffee station, guaranteed to generate jealous oohs and aahs. CONTACT: Energy Trust of Oregon, (866) ENTRUST, www.energytrust.org.