In Washington, D.C., it takes a village to build a solar house

There are green buildings, and then there are really green buildings.

I just returned from the Solar Decathlon in Washington, D.C., a competition sponsored by the U.S. Department of Energy where 20 universities build houses powered entirely by the sun. That's right, off the grid on the National Mall for almost two weeks.

I will be hosting an Ask the Editors forum on green buildings this Thursday, October 18 at 11:00 a.m. PDT, where I can share some of the things I saw at the Solar Decathlon and answer any other green building questions you have. Please attend with your questions! Click here for more.

In the meantime, here's a photo gallery of the event and links to the videos we did from the Solar Decathlon. Below that, is a list of some of the green building technologies that the competitors used.

Video: In the nation's capital, a village powered by the sun. Solar Decathlon competition showcases new solar technology and sharp design.

Video: Solar power as cheap as fossil fuels? Applied Material's solar guru says growing scale of solar means lower prices.

Video: Solar Decathlon reigning champs look for repeat. University of Colorado at Boulder has pursued a highly modular and flexible design for its green home.

Video: 'Underdog' Santa Clara University competes in Solar Decathlon. Web-connected sensors and sustainable building materials used.

Video:Seeding solar homes of the future. Andy Karsner, the U.S. assistant secretary for energy efficiency and renewable energy, at the Solar Decathlon in Washington D.C.

Video A 'plug and play' green home: Carnegie Mellon students create a modular solar-powered home with computer control system.

Internet tech meets building tech
Solar-powered homes have been around for decades--the Massachusetts Institute of Technology called its Solar Decathlon entry "MIT Solar 7" because it's the seventh solar home made on campus. The first was in 1939.

Things have gotten a bit more high-tech. Many of the homes featured networks of sensors and control systems to regulate light or heating and cooling. Carnegie Mellon University and Santa Clara even developed software for the residents to track energy usage in real time and centrally control home appliances.

Another common theme: modularity. The teams had to build their houses, which were 700 square feet, to be transported to Washington, D.C. so they had to be disassembled and reassembled easily by nonprofessional builders.

Many of the homes had a "core" where plumbing and electrical wiring are housed. Other rooms are bolted on to the core. This approach, which Carnegie Mellon University called "plug and play," gives homeowners and builders more flexibility. If you want to expand with another bedroom, you attach that room to the core structure.

The University of Colorado at Boulder, which won the last two competitions, has taken that to an extreme. Its entrant can be expanded to 2200 square feet and used in a wide range of locations, from cities to country, they say.

Advanced materials. The choice of insulator can make a huge difference in a building's energy efficiency.

Technische Universitat from Darmstadt, Germany used a lightweight and air-tight insulation that saved on building space because it didn't need to use thick insulation in its walls.

A few schools used aerogels, a very insulating material that's also translucent. MIT and the Georgia Institute of Technology built walls using aerogels which insulate while they let natural light in.

This is an example of where students with access to cutting-edge technology have gotten ahead of the average consumer. Aerogels, for example, are not used in home construction now because of the cost.

Meanwhile, bamboo was the material of choice for flooring. Bamboo is a sustainable material because it's a grass that grows fast. It's also hard and has a consistent color through the board. Santa Clara University used joists made from bamboo, one of several technology firsts at the competition.

Of course, every home had solar photovoltaics and solar heating. The solar panels need to generate more electricity than the house actually consumes in case it's overcast so homes have a few days' worth of battery charge.

The technology of choice for solar thermal was evacuated tubes. The hot water they generate from sunlight not only makes water for laundry and appliances, it's also used for heating, typically radiant heat under the floors.

Architecture and design. I can't say I'd want to live in all the houses I saw, but there are some very attractive buildings, both on the inside and out. A lot were inspired by nature, so you will see a lot of plantings integrated into the homes' layouts.

As Charlie Gay, the general manager of Applied Materials' solar business, said during our interview, the architecture has come a long way. "You feel comfortable in the house--it's not hale bales and straw," he said.

For all the competitors, this is a serious undertaking. Students and their advisers spend months, if not years, on these houses and they need to raise hundreds of thousands of dollars to complete them.

New technologies, like a solar-powered home-cooling system or photovoltaic blinds, are developed just for the event. But a lot of it comes down to smart design--integrating solar intelligently and making the space livable, even better than what most of us have today.

Even after a day and half there, I could easily have spent more time looking at and learning about the homes. Before the event officially opened to the public (and before all the homes were finished), lines were forming to see the insides.

The competition ends later this week, but you can check the standings right now and the 10 contests each school is judged on. It looks like Maryland is in the lead, but we still have a few more days to go.

Update: We have a winner: Germany's Technische Universitat Darmstadt beat out second-place finishers Maryland and third place team University of Santa Clara.