Outdoor Sauna Cost of Ownership - Aluminum vs Cedar vs Thermowood
The Three Materials
Western Red Cedar
What it is: A softwood native to the Pacific Northwest (Thuja plicata). The default exterior material for barrel saunas, cabin saunas, and many traditional outdoor builds. Used by Almost Heaven, many Redwood Outdoors models, and numerous independent builders.
Why it's popular: Naturally aromatic. High concentration of thujaplicins (natural fungicides) gives it class-leading rot resistance among softwoods. Low density means it stays cool to the touch inside a heated sauna. Beautiful warm reddish-brown color when fresh. Widely available and well-understood by builders.
Janka hardness: 350 lbf — one of the softest commercial woods, which means it dents and scratches more easily than hardwoods but also means excellent thermal properties (poor heat conductor, comfortable to sit on).
Thermowood (Thermally Modified Timber)
What it is: Wood (typically spruce, pine, or ash) that has been kiln-treated at 374–428°F (190–220°C) in a low-oxygen environment. The heat permanently alters the wood's cell structure — reducing moisture absorption, improving dimensional stability, and darkening the color. Used by Redwood Outdoors (select models), SaunaLife, and several Nordic manufacturers.
Why it's popular: Better moisture resistance than untreated wood. Less expansion/contraction with humidity changes. Naturally darker, more uniform color. The thermal modification process replaces chemical treatment — no preservatives or pressure treatment needed. Widely used in Scandinavian outdoor construction.
Janka hardness: Varies by base species. Thermowood spruce: ~350 lbf (similar to cedar). Thermowood ash: ~1,000+ lbf (significantly harder). Most sauna thermowood uses spruce or pine.
Aerospace-Grade Aluminum
What it is: Aluminum alloy panels (typically 5000- or 6000-series) with a powder-coated or anodized finish. Non-porous, non-organic, UV-stable. Used by Sun Home on the Luminar outdoor sauna line, paired with a stainless steel roof. The interior remains Canadian western red cedar — the aluminum is exterior-only.
Why it's used: Zero moisture absorption, zero UV degradation, zero rot potential, zero biological vulnerability (insects, mildew, fungi). Requires no cover, staining, sealing, or treatment of any kind. The material is functionally permanent in outdoor conditions. Dezeen featured the Luminar's aluminum design as an example of contemporary sauna architecture.
Weight/structural note: Aluminum is lighter than an equivalent volume of hardwood but heavier than cedar. The Luminar's aluminum shell adds structural rigidity — the panels are load-bearing, unlike wood staves on a barrel sauna that rely on steel bands for compression.
Material Properties: Side by Side
| Property | Western Red Cedar | Thermowood | Aerospace Aluminum |
|---|---|---|---|
| UV resistance | Low — grays within 6–12 months without treatment | Moderate — grays more slowly than cedar, still needs UV oil | High — powder coat or anodized finish is UV-stable indefinitely |
| Moisture absorption | Moderate — absorbs and releases moisture with each rain cycle | Low — thermal modification reduces absorption by 40–60% | Zero — non-porous surface |
| Dimensional stability | Fair — expands and contracts noticeably with humidity | Good — 50–70% less movement than untreated wood | Excellent — no expansion/contraction from moisture |
| Rot resistance | Good (natural thujaplicins) — best among untreated softwoods | Very good — reduced moisture content starves rot organisms | N/A — metal cannot rot |
| Insect resistance | Moderate — thujaplicins deter some insects, not all | Improved — reduced sugars and moisture make wood less attractive | Complete — no biological vulnerability |
| Freeze-thaw resistance | Poor — absorbed moisture expands when frozen, causing checking | Fair — reduced moisture means less freeze damage | Excellent — no moisture to freeze |
| Salt air / coastal | Poor — salt accelerates wood degradation | Fair — better than cedar, still a wood product | Good — aerospace alloys resist salt corrosion |
| Impact resistance | Low (350 lbf Janka) — dents easily | Low–moderate (varies by species) | Moderate — resists dents better than softwood, can scratch |
| Fire resistance | Low — wood is combustible | Low — thermally modified wood is still combustible | High — aluminum is non-combustible (melts at 1,220°F) |
| Thermal conductivity | Low (0.11 W/m·K) — stays cool to touch | Low (similar to cedar) | High (205 W/m·K) — conducts heat rapidly; requires insulation layer |
| Acoustic properties | Warm, absorptive — dampens sound | Similar to cedar | Reflective — can feel hollow without insulation |
| Repairability | Easy — sand, fill, re-stain individual boards | Easy — same techniques as cedar | Harder — dented or scratched panels require specialist refinishing |
How Each Material Ages Outdoors
Cedar: Year 1 through Year 5
Month 1–6: Fresh cedar has a warm reddish-brown color and distinctive aromatic scent. The surface is smooth, the grain is tight, and the wood looks premium. This is what buyers fall in love with.
Month 6–18 (untreated): UV exposure begins breaking down surface lignin. The color shifts from reddish-brown to a silvery gray, starting on the most sun-exposed surfaces. This is not rot — it is cosmetic UV degradation. Some owners prefer the gray patina. Others find it looks neglected.
Year 1–3 (maintained): With UV-protective stain or oil applied every 1–2 years, cedar maintains its warm color and smooth surface. The stain repels moisture and slows UV damage. Between treatments, you may notice slight fading on south-facing surfaces.
Year 3–5: Checking (hairline surface cracks) begins to appear on untreated or under-maintained cedar, especially in harsh climates. On barrel saunas, the top staves — which take the most rain and UV — age fastest. Steel bands may rust on cheaper models. With diligent maintenance (staining + cover), cedar can look excellent at year 5. Without it, the sauna will show visible weathering.
Thermowood: Year 1 through Year 5
Month 1–6: Thermowood starts darker than cedar — a uniform chocolate-brown to dark brown, depending on the base species and modification temperature. The color is consistent and modern-looking. No strong scent (the thermal process drives off most volatile compounds).
Month 6–24 (untreated): Thermowood grays more slowly than cedar because its modified cell structure contains less lignin available for UV breakdown. But it still grays — typically a uniform silver-gray rather than the patchy graying of cedar. UV oil slows this significantly.
Year 2–5: Thermowood's dimensional stability advantage shows over time — less checking, less warping, less movement at joints compared to cedar at the same age. Moisture-related damage is reduced. The main vulnerability is the same as cedar: UV degradation on untreated surfaces. With periodic UV oil (every 2–3 years), thermowood holds its color and structural integrity well through year 5.
One trade-off: Thermowood is more brittle than untreated wood. The thermal modification process reduces the wood's flexibility, making it more prone to splitting if struck or stressed at a joint. This is rarely a problem in normal use but matters if the sauna is moved or if hardware is overtightened during assembly.
Aluminum: Year 1 through Year 5
Month 1–60: The aluminum exterior looks the same. The powder-coated or anodized finish does not gray, check, warp, rot, absorb moisture, or degrade under UV. Dirt and pollen can accumulate on the surface — a garden hose or damp cloth is the only maintenance. The matte black finish on the Luminar shows fingerprints and water spots more visibly than wood, but these wipe clean.
The trade-off: Aluminum does not develop character. A 5-year-old cedar sauna that has been well-maintained has a warm, lived-in patina that tells a story. A 5-year-old aluminum sauna looks exactly like it did on day 1. Whether that is a feature or a flaw depends entirely on your aesthetic preference.
Which Material Performs Best in Your Climate?
| Climate | Best material | Why |
|---|---|---|
| Desert / high UV (Arizona, SoCal) | Aluminum | UV is the primary threat — aluminum is unaffected; cedar grays fastest in high UV |
| Pacific Northwest / heavy rain | Aluminum or thermowood | Moisture cycling is constant — cedar needs frequent treatment; thermowood handles it better; aluminum is immune |
| Northeast / freeze-thaw | Aluminum | Freeze-thaw causes the most damage to cedar; thermowood fares better; aluminum is unaffected |
| Coastal / salt air | Aluminum | Salt accelerates wood degradation regardless of species; aerospace aluminum resists corrosion |
| Humid Southeast | Thermowood or aluminum | High humidity + heat favors mold/mildew on cedar; thermowood's reduced moisture content helps; aluminum is immune |
| Mild / dry (Mediterranean) | Any — cedar works fine here | Low rainfall, low UV intensity, mild temperatures — all three materials perform well with minimal maintenance |
| Snow belt (Minnesota, Montana) | Aluminum or thermowood | Snow accumulation + freeze-thaw is hard on cedar barrel tops; thermowood's stability helps; aluminum sheds snow off stainless roof |
Aesthetics: Three Different Design Languages
Cedar reads as warm, natural, and rustic. A cedar barrel sauna in a backyard evokes the cabin aesthetic — it looks like it belongs near a lake, a forest, or a garden. Fresh cedar is reddish-brown; maintained cedar stays warm-toned; weathered cedar turns silver-gray. The natural grain, knots, and color variation give each sauna a unique character. This is the aesthetic most people picture when they think "outdoor sauna."
Thermowood reads as modern-natural. The uniform dark brown color and consistent grain are cleaner and more architectural than cedar — less rustic, more designed. Thermowood cubes (like the SaunaLife CL5G) with glass walls bridge traditional and contemporary. The aesthetic works well in modern outdoor spaces that want warmth without the cabin look.
Aluminum reads as contemporary architecture. The Luminar's matte black panels, black-tinted window walls, and LED accent lighting look more like an outdoor pavilion than a traditional sauna. Dezeen featured the design alongside contemporary architecture. This aesthetic polarizes — buyers either love the modern look or want nothing to do with it. There is no middle ground.
Which Material Is Right for You?
| If you are… | Best material | Why |
|---|---|---|
| A traditionalist who wants steam and löyly | Cedar or thermowood | Traditional saunas use wood construction; aluminum saunas are infrared-only (no steam) |
| A low-maintenance buyer in a harsh climate | Aluminum | Zero exterior maintenance regardless of weather, UV, moisture, salt |
| Budget-conscious (under $6,000) | Cedar | Cedar barrel saunas start at ~$3,500 — the most affordable outdoor option |
| Designing a modern outdoor space | Aluminum or thermowood | Both offer cleaner, more architectural lines than traditional cedar barrels |
| Living in a mild, dry climate | Cedar (maintenance is minimal here) | Low rainfall and UV means cedar lasts well with light maintenance |
| Living on the coast or near saltwater | Aluminum | Salt air accelerates all wood degradation; aluminum is corrosion-resistant |
| Planning to move within 5 years | Cedar barrel (portable) or aluminum (durable) | Barrel saunas disassemble; aluminum maintains resale condition; thermowood is less portable |
| Someone who enjoys caring for wood | Cedar | Staining, oiling, and watching the patina develop is part of the ownership experience |
| Wanting the longest possible exterior lifespan | Aluminum | Non-organic, non-degrading — lifespan limited by hardware and mechanical components, not the material |
| Prioritizing natural scent and warmth | Cedar (exterior and interior) or thermowood exterior + cedar interior | Cedar's aroma is distinctive; aluminum exteriors have cedar interiors but the exterior is industrial |
What About Hemlock and Pine?
Some budget outdoor saunas use hemlock or pine exteriors. Both are significantly less durable than cedar, thermowood, or aluminum outdoors:
Hemlock: Less naturally rot-resistant than cedar (no thujaplicins). Grays faster, absorbs more moisture, and requires more frequent treatment. Common in budget indoor saunas but underperforms outdoors. Adequate in mild, covered applications; risky in exposed outdoor placement without aggressive maintenance.
Pine (untreated): High moisture absorption, low rot resistance, prone to sap bleed in heat. Not recommended for uncovered outdoor sauna construction. Pressure-treated pine has better moisture resistance but introduces chemicals that are inappropriate for sauna interiors and controversial for exteriors.
If an outdoor sauna uses hemlock or untreated pine as its exterior material, expect a shorter exterior lifespan and more frequent maintenance than cedar, thermowood, or aluminum.
A Note on Interior vs. Exterior Materials
This article is about exterior materials — the surfaces exposed to weather. Interior materials face a different set of demands (heat resistance, comfort, moisture management, scent) and are evaluated differently.
The Luminar uses aluminum on the exterior and Canadian western red cedar on the interior — putting each material where it performs best. Cedar's low thermal conductivity, natural aroma, and comfortable touch make it the preferred interior material. Aluminum's weather immunity and zero maintenance make it the preferred exterior. This dual-material approach is specific to Sun Home; most traditional saunas use the same wood for both interior and exterior.
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FAQs
What is the best wood for an outdoor sauna?
Western red cedar is the most popular and offers the best natural rot resistance among common softwoods. Thermowood (thermally modified timber) offers better moisture resistance and dimensional stability at a similar or slightly higher price. For zero-maintenance durability, aluminum replaces wood entirely. In mild climates, cedar is excellent. In harsh climates, thermowood or aluminum performs better long-term.
How long does a cedar outdoor sauna last?
With proper maintenance (staining every 1–2 years, cover use, periodic inspection), a cedar outdoor sauna can last 15–25+ years. Without maintenance, the exterior will show significant weathering within 3–5 years and structural degradation within 8–12 years, depending on climate. Interior cedar lasts much longer because it is protected from weather.
Is thermowood better than cedar for outdoor saunas?
For moisture resistance and dimensional stability, yes — thermowood absorbs 40–60% less moisture and moves 50–70% less with humidity changes. For natural rot resistance and aroma, cedar has the edge (natural thujaplicins). For initial cost, they are comparable. Thermowood requires less frequent treatment (every 2–3 years vs. 1–2 for cedar), but still needs UV protection. Both require a cover outdoors.
Does an aluminum sauna feel different inside?
No — the interior of the Luminar is Canadian western red cedar, identical to what you would find inside a traditional cedar sauna. The aluminum is exterior-only. Inside, the warmth, scent, and feel are the same. The difference is what you see and touch on the outside: matte black aluminum panels and dark-tinted glass instead of natural wood grain.
Can you paint or stain aluminum?
Technically yes, but it is not recommended or necessary. The factory powder-coat or anodized finish on aerospace aluminum is significantly more durable than any aftermarket paint. Repainting would void the manufacturer's exterior finish warranty and is unnecessary — the original finish does not degrade under UV or moisture.
What happens if aluminum gets scratched?
Surface scratches on powder-coated aluminum are cosmetic and do not affect structural integrity or weather resistance. Minor scratches can be touched up with manufacturer-matched paint. Deep scratches that expose bare aluminum may develop surface oxidation (a white powdery film), but aluminum oxide is self-sealing — unlike iron rust, it does not spread or deepen. Scratches are harder to repair than on wood, where sanding and re-staining fix most surface damage.
Which material has the best resale value?
No reliable resale data exists for outdoor saunas specifically. In general, condition at resale is the best proxy for retained value. Aluminum maintains its original appearance indefinitely. Well-maintained cedar retains visual appeal. Weathered, unmaintained cedar or thermowood loses value faster. Barrel saunas are more portable (disassemble for transport), which can help resale in some markets.
