When Denise Scott Brown visited Las Vegas for the first time in the 1960s, she was overwhelmed with emotions. But she wasn’t quite sure which ones. “The first thing I felt was a kind of shiver,” she recalls in a new documentary. “Was it horror or was it pleasure?” She was intoxicated by the frenzy of neon signs that “reach out and hit you as you travel down the highway”, and exhilarated by the overload of pure “communication without architecture”. Was there something there, she wondered, that architects could learn from?

Half a century later, we find her back in Vegas, walking around a graveyard of neon signs in a dusty lot with her husband Robert Venturi. Together the duo changed the course of modern architecture, championing everyday popular taste and the “ugly ordinary” over the rarefied, bleached white world of modernism. They brought back wit, colour and meaning, and embraced messy diversity over the bland homogeneity of so much of the built environment. And Sin City was the cradle of their inspiration – proselytised in their seminal 1972 book, Learning from Las Vegas.

“This is the equivalent of Michelangelo’s Piazza del Campidoglio in Rome,” says Venturi, as he wanders between the rusting neon signboards propped like architectural antiques in a salvage yard. “All the holy relics of vulgar commercial America!” He crosses himself and the pair of pensioners let out a mischievous giggle.

It is one of many such charming vignettes captured by their son, Jim Venturi, in the film Stardust, which has its UK premiere at the Barbican next week. More than a decade in the making, the project features numerous commentators who have since died – including critics Brian Sewell, Ada Louise Huxtable and Gavin Stamp, as well as Venturi himself, who died in 2018 – making it a poignant time capsule of voices from beyond the grave. Scott Brown has outlived them all, as feisty as ever at 93.

Stardust is the latest in a niche genre of films about architects made by their children. The punishing profession is clearly something that inspires both a kind of filial awe and morbid curiosity, if you’re forced to grow up immersed in it. Each film seems to ask: ‘Why did my parents do this to themselves?’ The Venturi documentary follows My Architect – Nathaniel Kahn’s 2003 Oscar-nominated quest to learn about his bigamist father, Louis Khan – and Rem, a 2016 hagiographic feature-length music video about the globetrotting starchitect Rem Koolhaas, made by his son Tomas. Jim Venturi has avoided the schmaltz of the former and the introspection of the latter, preferring to stay behind the camera and let his parents and their peers do the talking. Aided by the judicious writing and editing of co-director Anita Naughton, the film skilfully pieces together archive footage, fly-on-the-wall filming and talking heads to paint a multifaceted picture of this fascinating pair, who emerge as complex and contradictory as the buildings they championed. It is a loving portrait, but it doesn’t spare their blushes.

The timing of the London release is fitting, given the brouhaha over the current mutilation of the architects’ only UK work, the Grade I-listed Sainsbury Wing of the National Gallery, at the hands of another US firm, Selldorf Architects. The film doesn’t touch on the controversial plans to smooth over the building’s postmodern quirks with a bland blanket of Scandi good taste, but it does reveal how fractious the original design process was. And quite how big the Venturi-Scott Brown ego could be.

“It was sometimes like dealing with very intelligent children,” says a diplomatic Colin Amery, the late architectural historian who served as an adviser to the project. He recalls having to talk a furious Venturi out of resigning over a disagreement about a window. “He did say to me, ‘They don’t seem to realise they’re dealing with a genius,’” Amery recalls. “Then he made it worse by saying, ‘They wouldn’t do this to Shakespeare.’”

Scott Brown tactfully explains away her husband’s emotional outbursts by putting them down to his Mediterranean roots. “Bob learned to have a Brooks Brothers sheepskin over him,” she says. “But he’s southern Italian opera underneath.” The National Gallery might be thankful that he didn’t get his mobster uncles involved, who feature as a brief, spicy side-note in the movie. (They are described as “the most feared family in the history of Philadelphia” in one archive article – perhaps explaining why a bullet was once shot through the window of the famous house Venturi designed for his mother).

Elsewhere, glorious contradictions abound. The radical duo might have been fans of signs, having championed billboard-smothered “decorated sheds”, but they were clearly not happy when signage interfered with their own buildings. One hilarious scene in the film shows the elderly couple back in London to revisit the Sainsbury Wing, only to find a row of advertising banners hung from poles in front of their masterpiece. “Awful!” barks Venturi, his face a picture of horror. “It’s one of the great facades of the goddam 20th century and they put that in front of it? It’s like putting a billboard in front of [Frank Lloyd Wright’s] Falling Water!” It’s probably a good job he didn’t live to see the building’s makeover.

For a film about a pair of architects, it is interesting that Stardust doesn’t feature their own buildings particularly prominently. Bar the Sainsbury Wing and Venturi’s mother’s house, their work is mostly confined to a series of still images that flash up for a few seconds like postcards. In a way it is apt: the duo were so preoccupied with facades and flatness that a two-dimensional snapshot is perhaps enough. And, as with many architect-theorists, a lot of their own built work never quite lived up to the promise of their writing, and they never achieved great commercial success. They had a tendency to come across as aloof intellectuals, whose uncompromising principles didn’t make for easy working relationships. There is palpable bitterness in a section in the film on Philip Johnson, the savvy, opportunistic postmodern architect who essentially took their ideas and made them commercially successful in a way that Bob and Denise never managed.

“Philip Johnson totally distorted all that we stood for,” Scott Brown says, when asked about the testy term “postmodernism”. “He made it specious and a very big money-making project for architects like him, so that it got a bad reputation.” Johnson is painted as the comedy villain of the piece, and portrayed (not inaccurately) as a misogynist Nazi-lover who had a particular hatred of Denise. As the critic Martin Filler says of Johnson: “You could put everything he disliked into a computer, and out would pop Denise.”

The spectre of misogyny recurs throughout; one of the film’s chief successes is correcting the imbalance of credit that has so often been misapplied to the couple. It opens with a scene from a 2006 lecture – with Bob on stage, Denise in the audience – in which Venturi casually remarks that his 1966 opus, Complexity and Contradiction in Architecture, “was mostly written before I knew Denise”. A voice pipes up from the front row: “No it wasn’t!”

It was the story of their entire professional career. Scott Brown became a partner in the firm Venturi and Rauch in 1969, but her name wasn’t added until 11 years later. When Venturi was awarded the Pritzker architecture prize in 1991, it was he alone who was named. The accolades piled up, while continually spurning Scott Brown. “Bob got an honorary degree from Yale for discovering the popular, everyday environment,” she says at one point. “The thing that really I introduced and supported is ascribed to him.” She adds ruefully: “Maybe there’s a shortsightedness in stardom: you can’t see enough because you’re blinded by the light that you’re generating.”

• Stardust: The Story of Robert Venturi and Denise Scott Brown is at the Barbican cinema, London, on 16, 17 and 21 December