The Sauna Builder Pushing the Limits of Bathing Outdoors

Saunas are cropping up in some of the most picturesque corners of the world of late, as contemporary bathing culture finds new expression. Among those leading the charge is British design studio Out of the Valley, which debuted a mobile unit in early 2025 that has since appeared everywhere from mountainous lakesides to the Jurassic Coast of Dorset, drawing bathers to gather, sweat, and plunge in the great outdoors.

These mobile saunas are meeting the moment, it seems, as interest in the ancient ritual of sweat bathing continues to surge across the U.S. and U.K. Starting around 2023, cold-water swimmers along British beaches, many of them women who took to the sea during Covid, were seeking post-dip heat. In cities such as London and New York, contrast-therapy hubs and outdoor sauna spots were opening monthly.

“The philosophy is very much that these are an alternative to spas; they offer something affordable, simple, raw. And we’re only at the beginning,” says journalist and author Emma O’Kelly, whose books Sauna: The Power of Deep Heat and Wild Sauna chronicle the phenomenon. “They are all about ‘social wellness’ where people of all ages come together in a hot dark space, phone- and alcohol-free, seminaked and vulnerable to enjoy something that is good for them,” she continues.

I first visited designer-builder Rupert McKelvie, the founder of Out of the Valley, in 2023, at his workshops and studio in a wooded valley on the edge of Dartmoor National Park, deep in the U.K. countryside. It was here that McKelvie had constructed his first timber cabin, as a bolthole for himself some six years earlier. The structure, crafted from oak and cedar, embodied everything that his company stood for at the time: thoughtfully designed and beautifully executed off-grid architecture that worked with the land rather than against it.

He gave me a tour of their workshops and kindly offered to put me up at the original Oak Cabin overnight. It was a joy to wake with the sun to choruses of birdsong, and the low murmur of the river Teign that gently wound through the nearby meadow. Early morning mist hung over the valley, and embers from the cozy cabin’s wood burner glowed from the night before. The experience summed up Out of the Valley’s ethos better than any press release ever could; they were guided by nature, and at home within it.

Two years on, that same design philosophy has taken a new form. McKelvie and business partner Christopher Selman have shifted and upsized their 11-person Devon workshops from crafting private cabins, to producing finely detailed and sustainability-minded prefabricated saunas.

Out of the Valley’s early commissions were deeply private: forest hideaways, bathing pavilions, compact homes designed to disappear into the landscape. “Much of our work was beautiful but unseen,” McKelvie recalls. “But community is such an integral part of sauna culture, and we wanted a more accessible and public-facing product,” adds Christopher. As demand grew and the two developed their product range, having a mobile sauna that could travel to wild locations, where people come together to connect with nature, was front of mind.

In San Francisco, one of the founders of the first floating sauna in the area, Fjord, makes a hobby of trailering a mobile unit to different swimming spots; in the U.K., horse box conversions and DIY trailers appear on beaches and lakeshores every weekend. The appeal is obvious: take the elemental pleasure of heat and cold to where people already gather. But most transportable units are (to put it generously), utilitarian and boxy, designed for the road, not for architectural grace.

Out of the Valley set out to change that with the Aquila. “We wanted to design a transportable structure without compromising spatial quality, aesthetics, or environmental responsibility,” McKelvie explains. “The question was how to create something elegant and in keeping with our values, that could still fold down to highway height and width.”

The answer is a 20-foot sauna that travels like a compact trailer yet transforms on-site into a sculptural pavilion. Its folding Cor-Ten steel roof collapses for transport, then lifts when stationary to create generous eaves that protect guests from rain or snow, and frame views of sea, forest, or moor. A fold-down deck lowers, and has drop-down steps and adjustable legs for uneven ground. Sheep’s wool insulation regulates humidity and improves energy performance, while a modular construction is meant to ensure longevity and easy repair.