Thermasol Fortis Outdoor Sauna | 3–4 Person, Aspen Interior, 9kW Harvia Heater, WiFi | Free Shipping
Thermasol Fortis Outdoor Sauna | 3–4 Person, Aspen Interior, 9kW Harvia Heater, WiFi | Free Shipping
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The Thermasol Fortis is a 3–4 person enclosed outdoor sauna built for permanent backyard installation. Its name means strength in Latin — an accurate description of both its construction and its thermal performance. The Fortis features a thermally modified aspen interior, a brushed Nordic spruce exterior, a 9kW Harvia heater with digital controls and optional WiFi integration, RGB LED lighting, and 8mm Parsol Dark Grey glass on an aluminum-framed door. At 90" × 87" × 89", it is sized for a couple or small group with room to move, and built for year-round use in any climate. $36,619 with free shipping.
Why Customers Choose This Product
- Thermally modified aspen interior — low-resin wood that stays smooth and splinter-free at sauna heat
- Thermally modified, brushed Nordic spruce exterior — no annual sealing or staining required
- 9kW Harvia heater with digital controls; an 8kW Virta WiFi configuration is available
- 8mm Parsol Dark Grey glass door in an aluminum frame with hidden hinges
- Multi-layer wood fiber insulation for year-round use, including cold winters
- Modern eave-style roofline with RGB LED lighting and remote included
Product Highlights
Thermally Modified Aspen Interior
The Fortis interior is lined with thermally modified aspen — a wood species selected specifically for sauna use. Aspen is naturally low in resin, so it does not release sticky residue or sharp odors when heated to sauna temperatures. Thermal modification increases the wood's dimensional stability, reducing expansion and contraction with heat and humidity cycling, and improves its resistance to moisture-related degradation — the interior stays smooth and splinter-free and holds a comfortable touch temperature even at high heat.
Thermally Modified Nordic Spruce Exterior
The exterior is clad in thermally modified and brushed Nordic spruce and pinewood. Nordic spruce is dense, straight-grained, and dimensionally stable; thermally modified, it becomes significantly more resistant to rot, mold, insects, and moisture absorption than untreated softwood. The brushed finish weathers to a consistent silver-grey patina over time, and the Fortis does not require annual sealing, staining, or treatment to maintain its outdoor durability.
9kW Harvia Heater with Digital Controls
The Harvia heater is the reference point for electric sauna heating — Finnish-made, with decades of sauna-specific engineering behind it. At 9kW, the Fortis heater brings a 3–4 person sauna to temperature efficiently and maintains it consistently, even in cold outdoor ambient temperatures, and digital controls allow precise temperature and session time management. The optional WiFi integration (the 8kW Virta model) adds remote control via smartphone — start preheating from inside the house and arrive to a ready sauna.
8mm Parsol Dark Grey Glass Door
The Fortis door is 8mm Parsol Dark Grey tempered glass in an aluminum frame with hidden hinges. At 8mm, the glass panel is substantially thicker than standard sauna door glazing, for better thermal retention at the door opening and a more substantial feel. The dark grey tint provides privacy from outside while maintaining a light-filled interior, and hidden hinges keep the door face clean.
Modern Eave-Style Roofline
The Fortis has a modern eave-style roofline — a clean architectural profile that positions it as a designed outdoor structure rather than a functional box. Functionally, the eave overhang protects the glass door and front face from direct rain accumulation, reducing water intrusion risk at the door seal, and the multi-layer insulated roof construction supports year-round thermal performance in cold climates.
RGB LED Lighting
The Fortis includes RGB LED lighting with remote control as a standard feature — not an add-on. Color and intensity can be adjusted during the session without leaving the bench, so the interior lighting can be set to suit the time of day and the mood of the session.
Multi-Layer Wood Fiber Insulation
The Fortis uses multi-layer wood fiber insulation throughout the wall and roof construction. Wood fiber insulation handles the humidity cycling of sauna use without degrading, compressing, or losing thermal performance the way conventional insulation materials can in high-humidity environments. Multi-layer construction also adds thermal mass: the sauna retains heat longer, uses less energy to maintain temperature during a session, and recovers temperature faster after the door opens.
Specifications
| Brand | Thermasol |
|---|---|
| Model | Fortis — Aspen |
| Capacity | 3–4 people |
| Exterior Dimensions | 90" × 87" × 89" (approx.) |
| Heater | 9kW Harvia (8kW Virta with WiFi available) |
| Interior Lumber | Thermally modified Aspen |
| Exterior Lumber | Thermally modified and brushed Nordic Spruce / Pinewood |
| Glass | 8mm Parsol Dark Grey Glass (PSDG) |
| Door Frame | Aluminum with hidden hinges |
| Insulation | Multi-layer wood fiber |
| Lighting | RGB LED lighting with remote |
| Controls | Digital; WiFi integration optional |
| Use | Outdoor, year-round |
| Price | $36,619 |
| Shipping | Free |
Ideal For
- Homeowners who want a premium 3–4 person outdoor sauna as a permanent backyard installation
- Those who prioritize material quality — thermally modified aspen interior, Nordic spruce exterior, Harvia heater — over entry-level sauna specifications
- Year-round outdoor sauna use in any climate, including cold winters where heater output and insulation quality matter
- Buyers who want the option of WiFi-enabled remote control for session preheating via smartphone
- Outdoor living spaces where the sauna's architectural profile and material quality should match the surrounding environment
Important Information
Electrical required: the 9kW Harvia heater requires a dedicated 240V outdoor-rated electrical circuit, typically 40–50A. Electrical installation must be performed by a licensed electrician and is not included — confirm exact electrical specifications before ordering so your electrician can prepare the circuit before delivery.
Heater configuration: WiFi control is optional — the WiFi-enabled configuration uses the 8kW Harvia Virta heater with smartphone control, while the standard configuration includes the 9kW Harvia heater with digital panel controls. Confirm your preferred heater configuration at the time of purchase.
Site and installation: a level, solid surface is required — a concrete pad or solid pavers — and site preparation is not included. The Fortis requires on-site assembly, and professional installation is recommended. A permanent outdoor structure with an electrical connection may require a building permit; verify local requirements before ordering.
Delivery: free shipping on this item. Orders are typically processed and shipped within 2–10 business days, with Economy (5–15 business days) or Standard (3–14 business days) delivery at checkout. A tracking number is emailed once shipped and is active within 24 hours. Orders are not shipped or delivered on weekends or holidays.
Frequently Asked Questions
What does "thermally modified" wood mean?
Thermal modification is a process that heats wood to high temperatures in a low-oxygen environment, permanently altering the wood's cellular structure. The result is wood that is significantly more resistant to moisture absorption, rot, mold, and dimensional movement than untreated wood of the same species — without the use of chemical treatments. For a sauna, thermally modified wood handles heat and humidity cycling without warping, and it does not off-gas chemicals when heated.
Why aspen for the interior?
Aspen is preferred for sauna interiors because it is naturally low in resin — it does not release sticky residue or strong odors when heated. It also stays at a comfortable touch temperature even at high sauna heat, which matters for bench contact during a session. Thermally modified aspen adds dimensional stability and moisture resistance on top of aspen's natural sauna-friendly properties.
Can the Fortis be used year-round in cold climates?
Yes — the multi-layer wood fiber insulation, 9kW Harvia heater output, and weather-resistant exterior construction are designed for year-round outdoor use. The Fortis will reach and maintain sauna temperatures even in cold ambient conditions. Supplemental weatherproofing of electrical connections may be advisable in very harsh winter climates — consult with your electrician during installation.
What electrical connection does the heater require?
The 9kW Harvia heater requires a dedicated 240V circuit with appropriate amperage — typically 40–50A. Electrical installation must be performed by a licensed electrician. Confirm exact electrical specifications with us before ordering so your electrician can prepare the circuit before delivery.
Is WiFi control standard or optional?
WiFi control is optional — the WiFi-enabled configuration uses the 8kW Harvia Virta heater with smartphone control. The standard configuration includes the 9kW Harvia heater with digital panel controls. Confirm your preferred heater configuration at the time of purchase.
The Thermasol Fortis is the outdoor sauna that earns its name — thermally modified materials built to last, a Harvia heater built to perform, and a design built to belong in a premium outdoor space. At $36,619 with free shipping, it is a permanent backyard sauna built for year-round use.
This product is a recreational sauna and is not a medical device. It is not intended to diagnose, treat, cure, or prevent any disease or health condition. Consult your physician before using a sauna if you are pregnant, have a medical condition, or take medication. Stop use and seek medical advice if you feel unwell at any point.
