Uniqlo’s HeatTech underwear is 15 years old, and this week launches a new collaboration with Alexander Wang. The company’s head of design, Katsuta-San, explains why it’s all we want to wear in winter

Another week, another designer/high street collaboration – although this week there happened to be two.

In one corner, there was the much-hyped Moschino collaboration with high street brand H&M, which had a small but visible queue outside its flagship store on Thursday morning – the big sellers were gold logo earrings the size of a saucer.

In the other corner was Uniqlo, the Japanese retail giant, who on the same day launched its HeatTech collaboration with the Chinese-American designer, Alexander Wang, to slightly less fanfare. Here, there were no scraps, no ticketed queuing systems and no draconian rules as to how much time you could spend inside or how many items you could buy, as has become the norm with designer/high street collaborations. Instead, it was like any other day. Inside Uniqlo’s multi-level Oxford Street store in London, a small but substantial group of people buzzed around the underwear section. “It’s always like this,” explained Terry, a sales assistant, folding knits with scholarly discipline. “The minute it gets cold, this stuff flies.”

Japan is not known for being especially cold, but Uniqlo has become known for its thermal credibility. They won’t reveal current sales figures but at the very start, at least, it’s thought they were selling more than 10m of these underwear garments a year. Wang or no Wang, come winter, it’s Uniqlo that keeps the world warm.

Facebook Twitter Pinterest One of Uniqlo’s ultra light down jackets. Photograph: press image

Uniqlo’s cold-weather clothing has now taken on a cult-like status. While much of the high street wanes, Uniqlo’s sales are up 8.4% year on year, with the majority of their revenue coming from its cold-weather clothing. Katsuta-San, Uniqlo’s head of design, has been responsible for tweaking these pieces over the years. “When people need to make clothes to stay warm, they are inclined to go thicker. We have done the opposite. We’re not all about high fashion, but sometimes we can do both.” These days he adds colours or changes the scoop of a neckline, but otherwise the pieces seem to sell themselves.

Uniqlo in its modern form has been around since the mid-80s, and is owned by Tadashi Yanai, a billionaire who is the richest man in Japan, and president of Fast Retailing, which owns the brand. In 1984, Yanai opened Unique Clothing Warehouse in Hiroshima, although there are now more than 2,000 stores worldwide. Sweden, known for its challenging weather, is its latest market, although in Japan, Uniqlo is so common it has coined the expression unibare, which means: “We all know you’re wearing Uniqlo, and it’s really lame.”

Collaborations have become Uniqlo’s bread and butter, choosing designers who fit their aesthetic (rather than the other way round). In the last few years, they have worked with Jil Sander and Tomas Maier. The aim is to harness some of their quiet minimalism, make it mass and translate it all into sales. Past collaborations have

Uniqlo’s ascendancy really began 15 years ago with the launch of HeatTech. Thermal clothing didn’t really exist on the high street; at least, not anything you would want to be seen in. Suddenly, layering from the inside out became a possibility. A second flourish came in 2009 with the introduction of the ultra light down jacket, another bafflingly thin garment that seems to do the impossible while scrunching into a fist-sized ball. Fashion, too, has had a hand, and the gradual move towards streetwear and anti-fast fashion helped – this stuff is anti-trend, so “designed to be worn year in year out,” explains Katsuta-San.

Facebook Twitter Pinterest Katsuta-San, Uniqlo’s head of design. Photograph: Max Lakner/BFA/REX/Shutterstock

More recently, public obsession with living an anti-cluttered life, helped no end by its “guru” Marie Kondo, has been pivotal. Speaking backstage at the launch in New York, Katsuta-San says he finds this global interest in all things Japanese curious. “I don’t know why everyone is so interested in the way we do things, [but maybe it is because] we never want to do normal things?” He shrugs. “I don’t know. Or maybe it is because we like to do things simply, to not follow fashion, and to do the impossible: to prove two opposite things in one design.”

Designed to be worn under clothes, the new collaboration includes 12 pieces in ribbed fabric, lots of black and some neon that adds a certain couch-to-club element. Some pieces are designed to be seen. “But I’m not going to show my underwear to anyone,” he says.