I've been obsessed with saunas since I tried one in Finland in 2019.
Not the gym sauna with the broken thermostat. Not the "infrared sauna" that's really just a heated box. A real Finnish sauna: wood-fired, properly hot (80-100°C), with that wave of steam when you throw water on the rocks.
When I got home, I priced out building one. The quotes made me laugh:
- Pre-fab sauna kit: $8,000-15,000 (plus installation)
- Custom-built sauna: $20,000-40,000
- Commercial barrel sauna: $6,000-12,000
For a small wooden room that gets hot? This seemed absurd.
Today I'm announcing the Saunosphere — our adaptation of the Thiosphere system for sauna use. Complete plans for building an authentic wood-fired sauna, using standard lumber, for under $5,000 in materials.
Why Another Sauna Design?
Fair question. The world has sauna designs. Why add another?
Most DIY sauna plans are terrible. I spent months reading DIY sauna blogs and forums. The information is scattered, contradictory, and often wrong. One guide tells you to use cedar; another says cedar will combust. One says vapor barriers are essential; another says they'll rot your walls.
The good plans cost $500-1,000 and still assume you have construction experience.
Commercial saunas are overpriced. A barrel sauna is fundamentally some bent wood staves held together with steel bands. The materials cost maybe $1,500. The retail price is $8,000+. You're paying for branding and convenience.
Existing designs ignore mobility. Most sauna plans assume you're building a permanent structure. What if you rent? What if you want to relocate it? What if you want to take it to a lake cabin for the summer?
The Saunosphere addresses all three problems: clear instructions, reasonable cost, and modular construction that allows relocation.
What Makes a Sauna a Sauna?
Before getting into the design, let's talk about what actually matters in a sauna.
Temperature Range: 70-100°C (158-212°F)
A real sauna gets hot. Not warm — hot. The kind of hot where you can't wear jewelry because it would burn you.
This requires:
- Sufficient heater capacity (typically 6-9 kW for a small sauna)
- Good insulation to retain heat
- Proper ventilation to circulate fresh air without losing temperature
Löyly: The Steam
When you throw water on the hot rocks, it instantly vaporizes, creating a wave of humid heat that's the heart of the Finnish sauna experience. This is called löyly (sounds like "LOW-loo").
Good löyly requires:
- Enough rocks (typically 20-30 kg) at high temperature
- Proper rock placement for even heating
- A heater designed for water throwing (many electric heaters warn against it)
Thermal Stratification
Hot air rises. In a well-designed sauna, there's a gradient from floor to ceiling — maybe 50°C at floor level, 90°C at bench level, 100°C+ at the ceiling. You control your experience by sitting higher or lower.
This requires:
- Adequate ceiling height (at least 2.2m)
- Benches at different levels
- Intake vents low, exhaust vents high
The Wood Experience
There's something irreplaceable about a wood-fired sauna. The smell of burning birch. The crackling of the fire. The ritual of tending the flames.
Electric saunas are easier. Wood-fired saunas are better.
The Saunosphere Design
The Saunosphere uses the same nested-sphere construction as the base Thiosphere, with modifications for sauna use:
Inner Shell: Heat-Resistant Materials
The inner shell that faces the sauna heat uses:
- Untreated softwood (spruce, pine, or aspen) for benches and walls
- Cement board behind the heater as a heat shield
- Proper clearances from the stove (varies by stove model, typically 300-500mm)
Insulation Layer
Between the inner and outer shells:
- Mineral wool insulation (rock wool or ceramic fiber)
- Vapor barrier on the warm side (aluminum-faced kraft paper)
- Air gap for drainage on the cold side
This achieves R-value of approximately 20, which is adequate for most climates. Colder regions may want additional exterior insulation.
Outer Shell: Weather Protection
The outer shell handles the elements:
- Same plywood/OSB construction as the standard Thiosphere
- Weather-resistant finish (paint or exterior stain)
- Roof cap for stovepipe penetration
Stove Integration
The design accommodates wood-fired stoves in the 15-20 kW range. We've tested with several common models:
- Harvia M3
- Kuuma 50
- TimberWolf 9
The stovepipe exits through a dedicated penetration with proper clearances and flashing.
The Build: What to Expect
Let me give you honest expectations about building a Saunosphere.
Time Investment
| Phase | Hours | Solo/Help |
|-------|-------|-----------|
| Material prep and cuts | 8-12 | Solo |
| Platform construction | 4-6 | Solo |
| Outer shell assembly | 8-12 | Help recommended |
| Insulation | 4-6 | Solo |
| Inner shell and benches | 8-12 | Solo |
| Stove installation | 4-6 | Solo |
| Finishing | 4-8 | Solo |
Total: 40-60 hours over 3-4 weekends.
This assumes you're reasonably handy. If you've never used a circular saw, add time for learning.
Skill Level
You need to be comfortable with:
- Measuring and cutting lumber accurately
- Using a circular saw or table saw
- Driving screws with a drill
- Following detailed instructions
You don't need to be a carpenter. If you can follow IKEA instructions, you can build this — it just takes longer.
Material Cost
| Category | Approximate Cost |
|----------|-----------------|
| Lumber (2x4, 2x6, plywood) | $1,200-1,800 |
| Insulation | $300-500 |
| Hardware (screws, bolts, brackets) | $200-300 |
| Interior finish materials | $300-500 |
| Stove and stovepipe | $800-1,500 |
| Rocks | $50-100 |
| Miscellaneous | $200-300 |
Total: $3,000-5,000
Compare to a commercial barrel sauna at $8,000-15,000, or a custom-built sauna at $20,000+.
Safety Notes
I need to be direct about safety because saunas involve fire, high temperatures, and potential for injury.
The Stove
Wood-fired stoves get extremely hot. The stove body can exceed 400°C (750°F). Contact burns are serious.
Requirements:
- Proper clearances from combustible materials (per stove manufacturer specs)
- Heat shields where required
- Non-combustible floor protection under the stove
- Fire extinguisher accessible from outside the sauna
Ventilation
Saunas need fresh air. A sealed sauna with a wood fire can produce carbon monoxide.
The Saunosphere design includes:
- Low intake vent (typically under the stove)
- High exhaust vent (near ceiling, opposite wall)
- Both vents adjustable but never fully closed during use
Structural
The structure needs to support snow loads, wind loads, and the weight of the stove and rocks (which can be 50+ kg).
The handbook includes structural calculations for common scenarios. If you're in an area with heavy snow or high winds, consult local building codes.
The Sauna Experience
Let me describe what it's like to use a Saunosphere, because the technical specs don't capture it.
Firing up: You arrive at your sauna an hour before you want to use it. Start a fire in the stove, using kindling and small logs. Within 15 minutes, the fire is established.
Warming: Over the next 45 minutes, the sauna heats. You can smell the wood smoke (the stovepipe takes it outside, but some aroma remains). The interior transitions from cool to warm to genuinely hot.
First session: You enter, sit on the bench, and immediately feel the heat pressing on your skin. Within minutes, you're sweating. You throw water on the rocks — the löyly hits you like a wave, almost too intense, then dissipates.
Recovery: You step outside. The cool air is a shock. Some people roll in snow, jump in a lake, or take a cold shower. You rest for a few minutes, letting your heart rate settle.
Repeat: Back in for another round. Maybe three or four sessions total, with breaks between. The whole ritual takes 1-2 hours.
After: You feel something hard to describe. Relaxed but alert. Clean in a way that goes beyond physical. It's why the Finns have a word — sisu — that's connected to sauna culture: a kind of resilient inner strength.
Development Status
The Saunosphere is currently in Phase 1 (CRAWL in our terminology). Here's what that means:
What's ready:
- Complete structural plans
- Thermal calculations and insulation specs
- Stove integration guidelines
- Bill of materials with cut lists
What's in testing:
- Long-term durability in various climates
- Different stove models and configurations
- Mobility testing (disassembly and reassembly)
What's coming:
- Electric heater variant for areas where wood-burning isn't practical
- Extended handbook with advanced features (cold plunge integration, changing room)
- Community build documentation
Getting the Saunosphere Handbook
The Saunosphere handbook is included in the Lifetime Edition ($79), which gives you access to all current and future handbooks.
If you just want the Saunosphere, individual handbooks are $29-55 each (pricing varies by complexity).
Both options include:
- Complete PDF plans
- Cut lists and materials
- Step-by-step assembly instructions
- Safety guidelines
- Community access for questions
Building in Community
We're specifically looking for early builders who want to document their projects.
If you build a Saunosphere and share your experience — photos, lessons learned, modifications you made — you help everyone who builds after you. The handbook will credit contributors, and the best documentation may be incorporated into future editions.
This is how open source works: we build on each other's experience.
The Invitation
A wood-fired sauna used to be something only wealthy people could afford, or something you built if you happened to be a skilled carpenter with Finnish heritage.
The Saunosphere changes that. Clear plans. Reasonable cost. Achievable for anyone willing to put in a few weekends of work.
If you've ever sat in a real sauna and thought "I want this in my life," now you can have it.
Let's build.
Get the Saunosphere Handbook — Complete plans for building your own wood-fired sauna.
Explore the Saunosphere — Photos, specs, and development status.
Join the community — Connect with other builders.