"I don't want to waste material," he explains quietly. "If I find some interesting material on the street, I pick it up and bring it to my studio to use for something else."
Ban, 44, is something of an enigma. The sell-out crowd has been treated to a body of work that defies categorisation: the first half of his talk features a range of fashionably modern houses, mostly in his native Japan, built for wealthy private clients.
But the second half is made up of an extraordinary collection of projects – ranging from a pavilion for Hanover's Expo 2000 to a series of temporary shelters built around the world for refugees and earthquake victims – all of which are constructed from cardboard and paper.
The structure of the lecture gives the impression that these two strands of work come from different phases of Ban's career, yet they are contemporaneous. The only difference, Ban jokes, is that he gets paid for the former but not the latter.
"After I finish a house either for a rich person or an earthquake victim, I get the same satisfaction," he says. "And I expend the same energy. Whether or not I'm paid, I'm just happy to do something for someone."
The softly spoken architect, who lives in a self-built house in a forest outside Tokyo, is a rising star. Two years ago, Time magazine nominated him as one of 100 innovators most likely to change the world during the course of this century. He is part of one of the six teams shortlisted last month for the redevelopment of the World Trade Centre in New York.
This summer, he won the World Architecture award for his "Naked House". This is a home in Kawagoe near Tokyo that resembles an industrial greenhouse clad in translucent rice paper, featuring a series of open-ended "rooms" mounted on wheels that can be moved around the cavernous interior.
Yet it is his humanitarian projects that make Ban unique. His involvement in this work began in 1995, after learning of the plight of Rwandan refugees who had fled the genocide in their country. The vast camps set up for the refugees provided inadequate protection from the cold. On top of this, the United Nations' temporary housing kits required timber frames that had to be cut locally, which led to the destruction of swaths of forest. When the UN switched to aluminium frames, the refugees simply sold the aluminium for scrap and carried on cutting down trees.
"I was quite shocked to see the picture of refugees freezing," he recalls. "They were given very poor shelter by the UN. I thought we had to give them better shelter, otherwise the relief operation was useless."
These days I get a fax or an email after earthquakes – they want me to come out to build temporary housing
Ban had been experimenting with industrial paper tubes as a construction material since the 1980s, when he designed a shoestring exhibition on the work of Finnish architect Alvar Aalto and could not afford to use wood. He felt the same tubes could be used to provide cheap, sustainable shelters. He approached the UN Committee for Refugees and, aided by a team of students, developed a simple clip-together frame system and erected 50 experimental shelters in the field. (Ban doesn't mention this in his lecture, but the tubes proved inadequate in the monsoon conditions.)
The same year, he built a street of temporary paper-tube houses for victims of the Kobe earthquake, plus a cardboard church – which still stands – to replace one that was destroyed by the tremors. He raised the money to build the church himself after the priest told him there were no funds available.
Since then, he has been involved in disaster relief projects in Turkey and India. "These days I get a fax or an email after earthquakes – they want me to come out and build temporary housing," he says, smiling humbly.
So how does Ban juggle the humanitarian work with the more lucrative private work? "It's not balanced," he admits. "I always have a problem. It takes a lot of time and I don't get paid. But I'm not so interested in making money." Ban seems entirely genuine, but his disaster projects are clearly good for publicity.
Aesthetically, these projects are a world away from his houses (minimalism with a twist) and his larger public projects such as the Expo pavilion (bran-tub virtuosity). For his disaster jobs, aesthetics make way for efficacy.
In fact, Ban claims he is not interested in aesthetics at all. When questioned by members of the V&A audience at the end of his talk, he refuses to engage in pretentious architect-speak. Instead, he disarms his interrogators by pretending not to understand what they are saying.
Afterwards, Ban tells me: "As architects we have to design beautiful things. But I don't try to design to create an aesthetic – that comes afterwards, it comes naturally. I always try to find the simplest way of designing. The process of thinking and building is very minimal."
"He doesn't have a big ego," says London architect Philip Gumuchdjian, who is working with Ban on his first UK project, a pavilion at Kew Gardens. "He's incredibly open, very easy to work with, and he has a great sense of humour. He's incredibly dedicated; interested in pushing ideas. The discussion about aesthetics is only done through his work.
But he has got a very intriguing aesthetic sensibility that he won't talk about." What is it? "We don't know."
I was shocked to see Rwandan refugees freezing. I thought we had to give them better shelter, otherwise the relief operation was useless
Ban studied architecture at New York's Cooper Union under the tutelage of Peter Eisenmann and Bernand Tschumi – although he claims he was "not a good student". Shortly afterwards, he set up his own practice in Tokyo. "I was quite lucky because I started my own practice without working for someone else – and without reading the rule book.
So for me anything is possible. The worst thing is prejudice or preconception."
He says his interest in humanitarian projects stems from the realisation that architects, unlike other professionals, tend to work exclusively for the wealthy and privileged rather than those who really need their help. "Doctors and lawyers work for people who have problems. We work for happy people."
A concern for his clients is at the heart of his architecture. For example, he tries to avoid concrete pilings when building houses because they are expensive – "and noisy for the neighbours". He once built a house in an earthquake zone out of wooden cupboards because, in a previous tremor, people had survived by climbing inside their wardrobes.
Ban is frugal, searching for cheap solutions that do not consume unnecessary resources. Surprisingly, though, he does not claim to be motivated by environmental concerns. "What is environmentally friendly? I don't know. I'm not trying to be so serious about it. I don't want big slogans. But these are no longer natural disasters; they are man-made disasters. Flooding is caused by deforestation. We cut the trees for the construction industry. In earthquakes, people are killed by collapsing buildings."
Despite the superficial flamboyance of many of his buildings, Ban is painstakingly methodical, testing his ideas until he convinces the authorities they will work. In Japan, he has to get special government dispensation for each paper-tube project and once, to prove a timber-clad office block he designed was fire resistant, he set fire to a series of models.
An engineer who has worked with Ban praises his "quiet persistence", saying: "He doesn't shout and bawl to get his own way but he knows what he wants." Two years after Hanover's Expo 2000, Ban has still not forgiven the German authorities for thwarting him. They forced him to brace his famous cardboard Expo pavilion with extra timber struts that he deemed superfluous. "Even though we tested everything, still the local authorities didn't want to allow something they hadn't seen," he says resignedly. "We had to compromise a lot."
That pavilion is Ban's best-known work – and his most ambitious cardboard structure. The 208 m-long roof is a gridshell made of paper tubes recycled from local factories and held together by fabric ties. The structure was built flat on the ground and raised by hand, with workers slowly raising 1000 props to tease the roof into its undulating, caterpillar form. It was then clad in laminated paper.
Personal effectsWhat are your favourite things?
Travelling – but now it’s getting too much. Enjoying good food. Architecture. Nothing else.
How do you feel about the nomination from Time magazine?
I have to say that I’m not inventing anything new. I’m just using existing materials and existing technology.
How do you pronounce your surname?
However you want. There’s no official way. Peter [Eisenmann] always called me “sugar bear”.