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The Centre for Alternative Technology is one of the most venerable organisations on the green scene. Nestled in the lush yet inexpensive hills of Snowdonia, many miles from the industrialised world, it has been a hippie haven for years. But the rationale behind it was not escapism: when it was set up in the 1970s, the goal was to develop technologies to solve the planet's problems rather than sit around campfires singing about them.

In 1975 CAT threw its doors open to the public. Visitors were greeted by men with beards and colourful signs describing the technologies on display, including creaky wind turbines and strange panels that generated electricity from the sun. It all seemed very alternative and few members of the construction industry made the trek to Machynlleth. Today it's a different story. CAT has a turnover of £3m and employs 100 people. Its long experience of experimenting with renewable energy sources and sustainable construction methods makes it a site of pilgrimage for many in construction. And starting from next week there is an even more compelling reason to go - CAT is starting to build one of its most ambitious project to date, a £6m education centre. This will distil more than 30 years of sustainable construction thinking.

Called the Welsh Institute for Sustainable Education, or WISE, the building is needed precisely because CAT has become so successful. It runs several courses ranging from practical instruction for electricians and plumbers on installing renewable technologies to an MSc in Advanced Environmental and Energy Studies, taught in conjunction with the University of East London. Student numbers have risen from 250 a year in the 1990s to 1600 a year today. Students on the MSc stay at CAT for a week every month and there isn't enough space for them.

"It's really pushing the facilities to the limit as we can only sleep 60," says Phil Horton, CAT's special projects officer. CAT also wants to broaden its appeal to more establishment types. "We want local authorities and businesses to come as well, so we had to improve the quality of the accommodation."

At the heart of WISE will be a circular 200-seat lecture theatre with rammed earth walls. There will also be seminar rooms and workshops, office space and 24 ensuite bedrooms and a restaurant. It will be two storeys high with the second storey set back so rooflights can naturally light the ground floor. "We have tried to create a modest building with a courtyard and domestic scale sized rooms for natural light and ventilation pivoting around the lecture space," explains architect Pat Borer who has worked with architect David Lea on the building's design.

The information centre features rammed earth walls separating the corridor from the shop. These walls have a shiny, textured finish similar to an expensive polished plaster. Like WISE, this has a timber-framed external envelope but with a much more complex infill.

The brief was suitably radical. "It's as close as possible to zero carbon with good insulation and powered by renewable energy," explains Borer, who has worked on other buildings at CAT. But the scale of the building meant a departure from CAT's usual construction method - self build. "We are trying to get beyond the craft stage to a modern form of construction that is affordable," says Borer. Accordingly, the design was adapted to make it friendly to mainstream construction.

The materials will have an embodied energy content that is as E E low as possible. That's a far cry from one of the earliest buildings at CAT, the Low Energy House. Constructed in 1978 by Wates Homes, it had conventional masonry walls, and therefore a high level of embodied energy, but with 450 mm of insulation in the cavity and quadruple glazing. "I think the ideas have moved on a lot - the early focus was on energy saving," says Horton. "But no consideration was given to comfort levels and no account was taken of embodied energy. Now there's more of a focus on lifetime issues."

Rammed earth is being used to construct the lecture theatre walls. These are 7 m high with a diameter of 15 m and will support the roof over the double height space. "Earth is probably the lowest impact material you can have," says Borer. "These will probably be the highest rammed earth walls in Britain."

Interestingly, they won't be exposed to the elements, unlike other recent rammed earth buildings such as the Eden Project's visitor centre. "The Eden Project rammed earth has had problems with the weather," says Toby Hodsdon, Buro Happold's structural engineer on WISE. "The approach here is to use rammed earth as the primary load-bearing structure, and for stability. It will use timber frame as an insulating and weatherproof envelope."

At WISE, a circular corridor separates the rammed earth lecture theatre walls from the timber framed external wall. This will be extensively glazed on the south side to maximise solar gain, which will warm the rammed earth wall. "The idea is you don't put rammed earth out in the wind and rain but where it's useful for thermal mass and acoustics," says Hodsdon.

This approach also means the rammed earth walls become an architectural feature. They are a deep, rich brown and, because the earth was rammed into smooth-faced formwork, they have a shiny, textured finish similar to an expensive polished plaster.

This method was used in CAT's information centre, completed in 1990, but the exterior envelope is quite different. Like the information centre it's constructed from a glulam-beam-and-post system, similar to a steel frame, with the posts 5 m apart. But the information centre had a complex infill. From the inside out, it consisted of clay plaster on plasterboard, a vapour barrier, sheep's wool insulation, a second vapour barrier, an outer sheathing board, followed finally by a lime render.

The education centre at the Centre for Alternative Technology is designed to minimise its use of resources. It has a lecture theatre built with rammed earth walls (far left) and the student accommodation (on the right) is designed to benefit from natural lighting.

"We are trying to make this easier and cheaper to do," says Borer. The solution is elegant in its simplicity. A mixture of lime and treated hemp will be used as the infill between the posts. It's an excellent, low-energy insulator, and it allows moisture to pass easily through the wall.

It's also very simple to construct, and familiar to a mainstream contractor. "A machine sprays it into position," explains Borer. "It will be a bit like pouring concrete into a frame." The wall is finished on both sides with a lime render. The walls will be 450 mm thick and according to Hodsdon have an impressive U value of 0.17 Wm2/K.

They had another neat idea for the intermediate floors: using low-grade materials for a high-grade purpose. "The first floor is constructed from 150 mm deep joists screwed together so they are solid, with a plywood deck on top," says Hodsdon. "This has the benefit of sequestering a lot of carbon and means a lot of short lengths of low grade timber can be used." Because these joists are screwed together to form a composite deck, lengths shorter than the 5 m span can be used, so they can be sourced locally, unlike the deeper, longer conventional method.

Despite its radical construction techniques the building is not expensive. According to Horton it should come in at £1700-1800 a square metre. "It doesn't have to be expensive doing eco-building," he says.

Involvement of the whole team was vital at an early stage, so the PPC2000 partnering contract is being used. "It's good for everybody as it means they have an input into its buildability," he explains.

The building's minimal energy needs will be supplied by CAT's own centralised power supply (see the "What else is at CAT?" box above) According to Horton it will be closely monitored to check its actual resource consumption. Visiting students will be be able to see how much energy and water they use in their bedrooms with the idea of demonstrating they can enjoy the comfort levels of a conventional hotel with fewer resources.

In the meantime anyone who wants to experience this first hand should get down to Machynlleth, as visitors can observe the construction process and see if CAT's brand of sustainability has finally hit the mainstream.