Partisans Grotto Sauna
Jonathan Friedman Custom brackets hold the 117 interior wooden panels, creating a seamless appearance.


The outer island in Ontario’s Georgian Bay is mostly undeveloped where a private owner commissioned Toronto-based Partisans to design an artificial grotto. The 800-square-foot sauna appears to be carved from the shore and has a “very simple geometry, rising and being born of this rock,” says Partisans co-founder Alexander Josephson.

The firm worked with Mississauga, Ontario–based contractor EllisDon to survey the site using a Leica 3D-scanning device, collecting 90 million data points. The team manipulated the scan with Autodesk ReCap and modeled the structure with Rhinoceros and Grasshopper.

Partisans Grotto Sauna
Jonathan Friedman Partisans designed the private sauna to blend with the rocky shoreline.


Instead of western red cedar, a popular choice for saunas due to its warm hue, the team used custom-cut and dried northern white cedar because of its quality, aroma, and abundance in the area. “It also has some knots in it, so there’s a bit of character to the wood,” Josephson says.

Inside the sauna, 117 curvilinear wooden panels line the walls. Toronto-based Millworks Custom Manufacturing (MCM) CNC-milled the panels, which each have a distinct shape. Cut from raw wood blocks up to 4 feet by 8 feet, the 7.5-inch-deep panels range from 10 inches square to 48 inches by 114 inches; the larger pieces were joined after milling. Custom 8-inch-by-4-inch-square, 11-gauge galvanized steel brackets inconspicuously hold the panels together, creating a smooth, sinuous space.

Partisans Grotto Sauna
Wall section
Partisans Grotto Sauna
Jonathan Friedman The sauna features two porthole windows.

Air circulation and energy transfer within the sauna were significant design concerns because annual temperatures in Ontario range between minus 35 C and 35 C (minus 31 F and 95 F). From inside to outside, the cedar paneling, a 2- to 4-inch-wide plenum, a sauna foil vapor barrier, plywood sheathing, a steel cage structure with wood stud framing, closed-cell spray foam insulation, and a water-resistive barrier work together to control heat distribution and minimize heat loss. The team also had to balance between providing enough fresh air to feed the sauna’s two electric ovens and not inadvertently cooling the space, Josephson says. The two porthole windows and one skylight all rely on sealed powdercoated aluminum frames and high-performance glass to help maintain a consistent interior temperature.

On the exterior, the sauna’s walls and roof are clad in approximately 280 square-cut cedar planks up to 15 feet long. Their distinct carbon color comes from shou sugi ban, a Japanese wood-charring process, which Josephson says “was analogous to what was going on inside the space, which is obviously a hot room.” The planks are sealed for weather protection and attach to cleating on the structure’s exterior using adhesive resin, as well as brad nails and screws up to 3 inches long. A screen prevents insects from nesting behind the planks.

Partisans Grotto Sauna
Jonathan Friedman The sauna exterior is clad with cedar planks, finished using shou sugi ban, a Japanese wood-charring process.


Ontario-based Jordan Group Construction used dynamite with blasting caps to remove a few cubic meters of rock to prepare the rugged site for the sauna’s high-performance concrete foundation, which is engineered to withstand drifting ice floes in the winter. Reinforced footings, 12 inches wide, are pinned and grouted to the rock using 24-inch-long rods. Depending on the slope of the rock, the concrete infill between the footings is 12 inches to 24 inches deep.

Given the environmentally sensitive nature of the site, MCM assembled and tested the sauna structure in its own loading dock in Toronto before transporting the entirely built unit on a flatbed truck, which hauled it three hours north. The sauna was then loaded onto a barge in Parry Sound, Ontario, where it traveled five hours to the remote island. A crane lifted the sauna from the barge and placed it onto the foundation.

Partisans Grotto Sauna
Courtesy Partisans On-site construction of the grotto sauna took just 12 days.


The Jordan Group secured the sauna to the foundation with grout and ¾-inch-diameter, 9-inch-long bolts. On-site construction took just 12 days, which Josephson attributes to the thorough digital scanning and modeling work at the project’s outset. “We planned the whole thing very accurately,” he says.

Partisans Grotto Sauna
Partisans Site plan

Partisans Grotto Sauna
Courtesy Partisans The panels were milled from raw wood blocks, up to 4 feet by 8 feet.