In The Shape of Shelter I explained why the Thiosphere uses a truncated icosahedron — a football shape built from flat panels — instead of a geodesic dome or a rectangular box. The geometry solves problems of waterproofing, modularity, and buildability.
But when you fill that geometry with heat, something else happens. Something that makes it a fundamentally better sauna than any box or barrel you can buy.
The Problem with Rectangular Saunas
A conventional sauna is a rectangular room. Four walls, a flat ceiling, a flat floor. You heat the air inside. The hot air rises to the ceiling. The cool air sinks to the floor.
This creates a brutal temperature gradient. The air at ceiling height might be 100°C. The air at ankle level might be 50°C. Your head is baking while your feet are cold. This is not a flaw in the heater — it is a direct consequence of the geometry. Rectangular rooms with flat ceilings create stable thermal stratification because there is nothing to disrupt the layering.
Every sauna builder knows this. The standard solution is tiered benches — sit higher to be hotter. But this is a workaround, not a fix. The air is still stratified. The lower bench is still significantly cooler than the upper. And the heat concentrated at the ceiling is doing nothing useful — it is heating the roof, not the bathers.
Barrel saunas have a slightly different problem. The curved ceiling does circulate air somewhat better than a flat one, but the shape is still fundamentally a cylinder — open at both ends, with benches running lengthwise. The air still stratifies vertically. And barrel saunas have their own issues: the curved walls make bench installation awkward, and the stave construction creates hundreds of potential leak points.
What the Double Shell Does
The Saunosphere is not one shell. It is two — an outer weather shell and an inner thermal shell with a continuous gap between them.
In a conventional sauna, heat that rises to the ceiling has nowhere productive to go. It hits the flat surface, spreads sideways, and gradually conducts through the ceiling material to the outside. Wasted.
In a Saunosphere, the inner shell has a vent at the peak — the hottest point. That vent opens into the gap between the inner and outer shells. The hottest air in the sauna — the air you are currently wasting at the ceiling — gets pulled into the shell gap by its own buoyancy.
Once in the gap, that hot air has only one path: down. The gap channels it along the outer shell toward the base of the structure. At the bottom, it re-enters the inner shell through vents at floor level.
The result is a convection loop. The hottest air leaves the ceiling, travels down the outside of the inner shell, and returns at the floor. The temperature gradient that plagues every rectangular sauna is dramatically reduced — not by a fan, not by a duct system, but by the geometry itself.
Your feet get warm.
The Thermal Envelope
A sphere has the lowest surface-area-to-volume ratio of any shape. The truncated icosahedron approximates a sphere closely enough to capture most of this benefit. Compared to a rectangular sauna enclosing the same volume:
- Roughly 15-20% less exterior surface area
- 15-20% less material to insulate
- 15-20% less heat loss through the walls
This matters in a sauna because you are maintaining a 70-80°C temperature differential between inside and outside. Every square foot of wall surface is conducting heat outward. Fewer square feet means less energy to maintain temperature, faster heat-up times, and less wood or electricity consumed per session.
The double shell adds another layer to this. The gap between shells is not empty — it is filled with slowly moving air from the convection loop. That moving air is itself an insulating layer, significantly slowing conductive heat loss through the outer shell. The outer shell stays cool to the touch even when the inner shell is at full sauna temperature.
Wood-Fired and the Chimney Panel
The Saunosphere is designed primarily for wood-fired heat. A 15-20 kW sauna stove with a proper rock mass sits on a fireproof base inside the inner shell. The chimney exits through a dedicated chimney panel — one of the 22 standard module types, fitted with a fire-rated thimble and flashing.
Because the chimney panel is a standard module, it can be positioned anywhere on the upper hemisphere. This matters more than it sounds. In a rectangular sauna, the chimney position is constrained by the roof pitch and the bench layout. In a Saunosphere, you choose the chimney position based on where you want the stove relative to the benches, the door, and the ventilation. The geometry accommodates your layout rather than dictating it.
The stove placement also interacts with the convection loop. Position the stove near one side and the convection pull is stronger on the opposite side, creating a gentle directional circulation. The sauna has a "flow" that you can tune by stove position.
Sound and the Double Wall
Saunas are social. People talk, laugh, pour water on rocks with a satisfying hiss. This is wonderful inside the sauna.
Your neighbors might feel differently.
The double-shell construction creates a significant acoustic break. The inner shell vibrates with interior sound. The air gap between shells absorbs much of that vibration before it reaches the outer shell. The effect is comparable to double-stud wall construction in recording studios — two independent structures with an air gap provide far more sound isolation than a single thick wall.
This is not a designed feature — it is a free consequence of the double-shell geometry. But it matters if you are building a wood-fired sauna twenty feet from your property line.
Mobility
Most Saunospheres are designed for trailer mounting. The footprint — roughly 10 by 12 feet including the trailer frame — is towable behind a standard pickup truck.
This is unusual for a sauna. Conventional saunas are permanent structures bolted to foundations. Moving one means demolishing it.
A trailer-mounted Saunosphere can be repositioned in your yard as seasons change — sun-facing in winter, shade-seeking in summer. It can travel with you if you move. It can be rented or lent. It avoids permanent foundation permits in most jurisdictions.
The structural geometry helps here too. The roughly spherical shape distributes road vibration evenly across the shell rather than concentrating it at corners as a rectangular structure would. The panel connections flex slightly under road loads without loosening — they are designed for thermal expansion and contraction, which produces similar stresses.
The Löyly Question
For sauna purists, the ultimate test is löyly — the quality of the steam that rises when you pour water on the rocks. Good löyly is soft, enveloping, evenly distributed. Bad löyly is harsh, patchy, and gone in seconds.
Löyly quality depends on three things: rock mass, rock temperature, and air circulation. The first two are about the stove. The third is about the room.
In a rectangular sauna, steam rises from the rocks, hits the flat ceiling, and spreads laterally. If you are sitting directly above the stove, the löyly is intense. Two meters away, it is noticeably weaker. The flat ceiling creates an uneven steam distribution.
The Saunosphere inner shell is curved. Steam rising from the rocks contacts a surface that redirects it back toward the center of the space rather than channeling it sideways. The steam distribution is more even across the interior. And the convection loop means steam-laden air circulates rather than collecting at the ceiling and stagnating.
This is the kind of difference that is hard to quantify on paper but immediately obvious when you sit in the space. The steam wraps around you instead of hovering above you.
What It Costs
Materials for a Saunosphere run $4,000-$7,000 depending on the stove choice, the insulation level, and the interior finish. A quality wood-fired sauna stove with adequate rock mass is $800-$2,000. The rest is lumber, insulation, and hardware from a building supplier.
Compare this to $8,000-$15,000 for a pre-fab sauna kit that arrives in a box and requires professional assembly. Or $6,000-$12,000 for a barrel sauna with inferior thermal performance and a geometry that fights the physics of heat distribution.
Build time is 40-60 hours — three to four weekends with a helper.
The geometry is not just structurally better. It is economically better because it uses standard materials efficiently, and thermally better because it works with the physics of heat rather than against it.
Configure a Saunosphere — place modules, choose panels, get a bill of materials
Read the Full Specs — temperatures, materials, build timeline
The Shape of Shelter — the geometry behind every Thiosphere variant
Join the Community — sauna builders and heat nerds welcome