A giant will o’ the wisp is going to light up in Kensington Gardens this week. Above the Serpentine Gallery’s summer pavilion will float a huge, translucent inflatable ovoid canopy nearly 80ft (24m) high. The maverick Dutch architect Rem Koolhaas has teamed up with Cecil Balmond, the innovative structural designer, and his team from Arup to construct the pavilion and its shimmering canopy — a “cosmic egg”, as Balmond described it when we met to talk over the drawings.

You’ll be able to sip coffee there, watch videos of events and there will be two 24-hour debating marathons, one this month about the arts in London, the second, in October, about international culture. The first starts at 6pm on July 28, when you’ll be able to hear Koolhaas and Hans Ulrich Obrist, the Serpentine’s curator, interview a range of figures from politicians to philosophers, artists to economists, until 6pm the following day.

It will be quite a beacon in the wild, and one can only hope that, unlike the traditional will o’ the wisps, this one doesn’t befuddle passing travellers and lead to a pile-up on Exhibition Road.

Although this will be the first time the Dutchman has constructed something in London, Balmond and Koolhaas have formed a tight partnership since the mid-1980s. “At that time I was breaking away from traditional design methods,” Balmond says, “and trying to develop a new description of space. Rem was interested in breaking with traditional architectural models, so we found fertile ground.”

They’ve worked on the startling glass prism that is the Seattle Central Library, the massive bulk of the new concert hall in Porto; and, closer to home, Balmond is well-respected for his work with Anish Kapoor on Marsyas, his extraordinary trumpet-uterus-ear thing for Tate Modern in 2002, and he’s currently working on the redevelopment of Battersea Power Station. It’s a “horse-jockey relationship,” he says. “Sometimes I lead, sometimes Rem does.”

This time Koolhaas is very much on Balmond’s turf as Arup’s man has worked closely on four of the previous five pavilions, each time pushing them towards something new. “Each year we think, ‘What next?’ And Rem’s first thought this year was: ‘Why don’t we build a non-pavilion?’ So the first time we met on the site in February we thought of trying to sling some kind of something over the area, a temporary cover — almost a nothing.”

One can imagine the Serpentine’s enthusiasm for this, as originally the gallery had envisaged that this year Koolhaas’s Dutch rivals MVRDV would construct a 200-tonne, 75ft grass-coated hill over the gallery. It was meant to happen in 2004 but was postponed, and it has subsequently been abandoned because of “outstanding technical and financial difficulties” (ie, it was insane).

Simpler though their project may sound, Koolhaas and Balmond have devised something with similarly grand structural ambition: helium will inflate the semi-transparent membrane of their roof structure — the “egg”. The canopy will come down to rest on a series of similarly translucent polycarbonate wall panels which form a double layer of walls. As it has not only to keep off the sun in the summer but to resist the wind and the rain in the autumn, cables and winches will secure it to the ground.

Inside, meanwhile, a generously high ceiling space will be provided by a large cubic void at the centre of the canopy, which will itself be decorated with a repeating pattern of ivy designed by the German artist Thomas Demand, who is showing similar patterns in his solo exhibition in the gallery. (The fact that a walkway will lead directly from the gallery into the pavilion is also a novelty of this year’s project.)

Balmond says it all evolved from simple forms. “In a way this has classic geometry. It started with a square within a circle, and originally it was level, with a glass skirt around it to protect you from the wind. But gradually it became three-dimensional, the square became the cube inside and the circle became the egg.”

And, in some sense, it encapsulates the way their partnership works. “My own work is more non-linear, curvilinear, but Rem is very prismatic in what he likes. Seattle, for instance, is just a cube, with every level pushed laterally on its side. Similarly, the concert hall in Porto has another prismatic form being shaped.”

Previous architects have responded in myriad ways to the simple brief of this project. Toyo Ito, from Japan, brought mathematical theory to bear on it, and constructed a rectangular box that seemed as if cut with a knife and reconstructed, geometric planes and openings going this way and that. But Zaha Hadid’s design in 2000 was the closest to the principle of a tent, employing a canopy of angular flat planes.

Balmond denies that their design is a return to the idea of the pavilion as a grand tent, though he admits that there are parallels in its use of fabric. Also, their design is perhaps the most redeployable of all the pavilions to date. That’s great, I said, and asked whether our readers could perhaps hire it for weddings and family camping holidays. I must say he didn’t seem all that opposed to the idea — he just politely pointed out that, a little like Rome, his cosmic egg wasn’t built in a day.

Serpentine Gallery Pavilion 2006: Rem Koolhaas and Cecil Balmond with Arup, July 13-Oct 15, Serpentine, London W2. Opens tomorrow (www.serpentinegallery.org; 020-7402 6075)