Sweat Decks on wood selection is worth evaluating through the homeowner’s real week, not a perfect catalog photo. The best setup is the one that gets used, stays safe, and does not become a maintenance headache.
Cover image suggestion: Close-up of two side-by-side wood sample boards, one western red cedar with pronounced grain and warm color, one thermo-treated hemlock with a darker uniform tone, both freshly cut, photographed under natural daylight on a workbench.
Meta description: Cedar and hemlock are the two most commonly specified woods for outdoor sauna and pavilion construction. The performance differences are real and affect both upfront cost and 20-year maintenance. Here is what to look at.
Last October, a builder named Greg in Bend, Oregon, texted me a photo of two wood samples sitting on the tailgate of his truck. One was a clear vertical grain western red cedar board, the other a piece of thermally modified hemlock, both cut that morning. “Client wants the cheaper one,” he wrote. “But she also wants it to last 30 years. Tell me I’m not lying to her if I say yes.” I told him he wasn’t lying. But the full answer took more than a text.
That exchange is the conversation happening in every custom sauna shop and backyard build forum right now. For decades, cedar was the only serious answer for outdoor wood structures. It isn’t anymore, and the reasons are worth understanding properly, not just from marketing sheets but from the wood science and the field performance data.
Most of what follows comes from conversations with two custom sauna builders, a wood science researcher at the FPInnovations group in Quebec, and the published literature on accelerated weathering testing.
Cedar: The Incumbent for Good Reason
Western red cedar (Thuja plicata) is native to the Pacific Northwest and has been used in outdoor applications for thousands of years. There’s a reason it became the default.
Three properties matter for sauna and pavilion work. First, natural rot resistance. Cedar contains thujaplicins and other extractives that actively suppress fungal growth. Untreated cedar in ground contact typically lasts 15 to 25 years before significant degradation. Above ground and properly detailed, it can go 30 to 50 years.
Second, it’s light. About 23 pounds per cubic foot dry. That low density gives it good thermal insulating properties (useful inside a hot sauna, where you don’t want bench surfaces that scald) and makes structures easier to ship and handle on site.
Third, dimensional stability. Cedar shrinks and swells less than most North American softwoods as moisture fluctuates. Joints stay tighter. Doors keep closing. The building holds its geometry better than pine, untreated hemlock, or Douglas fir over the same period.
Here’s the thing, though. Quality clear cedar has gotten brutally expensive. The 2024 Random Lengths price index showed clear vertical grain western red cedar trading at roughly 4 to 6 times the price of equivalent dimensional pine. That premium reflects slow growth, constrained supply, and decades of demand pressure. Old-growth cedar harvest has been contentious for years, and even second-growth replanting cycles are slower than faster-growing alternatives. Some specifiers in 2026 avoid cedar entirely on environmental grounds, cost aside.
Hemlock’s Reinvention
If you’d suggested hemlock for an outdoor sauna build fifteen years ago, any decent builder would have talked you out of it. Eastern hemlock (Tsuga canadensis) and western hemlock (Tsuga heterophylla) have low natural rot resistance, tend to twist and check during drying, and were historically relegated to industrial framing where appearance didn’t matter.
What changed everything is thermal modification.
The process was developed primarily in Finland in the 1990s. Wood gets heated to 180 to 230 degrees Celsius in a low-oxygen environment for several hours. That drives off most of the hemicellulose content (the polymer fungi feed on), crystallizes the cellulose, and dramatically reduces moisture uptake.
Think of it like tempering steel. You’re using controlled stress to change the material’s fundamental behavior.
The result: a wood with roughly 80 to 90 percent of cedar’s rot resistance at 50 to 70 percent of the cost, improved dimensional stability comparable to cedar, and a darker, more uniform appearance that some buyers actively prefer.
The catch is that thermal modification reduces wood strength by 10 to 30 percent depending on temperature and duration. For sauna cladding, bench tops, and most pavilion applications, this is irrelevant. For structural members under real load, it matters. Good builders compensate by adjusting spans and member dimensions.
For homeowners weighing the two species for a backyard sauna or pavilion, Sweat Decks on wood selection walks through the species-by-application matrix in more detail.
What 12 Years of Weathering Data Actually Shows
A 2019 study from the Building Research Institute Finland tracked thermally modified hemlock and untreated western red cedar in outdoor cladding applications over a 12-year exposure period in southern Finland. The two materials performed within a few percentage points of each other on both rot resistance and dimensional stability metrics. The thermo hemlock weathered to a more uniform gray; the cedar developed a more variegated, mottled patina.
Sauna interiors are a harder test because the moisture cycling is more aggressive. A sauna interior swings from 30 percent relative humidity at room temperature to 95 percent or higher during use, then back down. Thermally modified hemlock handles this well because the modification process cut its moisture uptake. Cedar handles it well because of its natural dimensional stability. Different mechanisms, similar outcomes.
Bench surfaces are where the two actually diverge in a way you can feel. Cedar bench tops feel warmer to bare skin at sauna temperatures because of cedar’s lower thermal conductivity. Thermo hemlock bench tops feel slightly cooler initially but equilibrate to body temperature fast. The difference is real. It’s also minor enough that most users stop noticing after a few sessions.
On aesthetics: cedar has more visible grain variation and develops a richer patina over time. Thermo hemlock runs more uniform in color and ages to a consistent darker brown-gray. Neither is objectively better. It’s a styling call, like choosing between quartzite and soapstone for a kitchen counter.
The Budget Math, Spelled Out
Pricing the same cabin sauna in both species makes the gap tangible. A custom 6-by-8 cabin sauna in clear western red cedar runs roughly $18,000 to $25,000 for the structure, before electrical, foundation, and labor. The same cabin in thermally modified hemlock runs $11,000 to $17,000.
That entire cost difference sits in the wood. Hardware, glass, doors, heater: all roughly the same regardless of species. For builders working volume, the species choice shifts project margins significantly. For homeowners pricing a single install, it can be the difference between a project that fits the budget and one that doesn’t.
The 20-year maintenance picture is roughly equivalent for both. Exterior surfaces benefit from a UV-protective sealant every 3 to 5 years. Interior surfaces need no sealing. Both species gray if left unsealed, which some owners like and some don’t. Neither species demands meaningfully more upkeep than the other.
Picking the Right Wood for the Right Job
My honest read, after years of watching both species in the field:
If you’re building a custom heirloom sauna and budget is secondary, cedar is still a fine default. Fifty-plus years of exterior track record is hard to argue with.
If you want a quality build at a sane price, thermally modified hemlock is now a legitimate choice, and frequently the better value. The performance gap has closed enough that saving 30 to 40 percent on materials doesn’t mean a meaningful durability compromise.
If the primary touchpoint is heated interior cladding and benches, both species work well. Cedar people love cedar. Hemlock people love hemlock. Either performs at sauna temperatures.
If you’re building in a high-rainfall coastal climate or a hot, humid region, cedar’s rot resistance margin becomes more relevant. It still holds a small edge on that specific metric.
If environmental sourcing matters to you, thermally modified hemlock often scores better because it comes from faster-growing North American hemlock plantations rather than slower-cycle cedar harvest. This varies regionally, and it’s worth checking with your specific supplier.
The Boring Truth
The cedar-versus-hemlock conversation has shifted from a clear cedar win to a genuine tradeoff discussion over the past decade. Thermal modification changed the math. Both species perform well for backyard saunas and outdoor pavilions. The choice should come down to budget, aesthetic preference, supplier reliability, and environmental priorities, not some imagined performance hierarchy.
For builders and dealers: knowing this comparison cold is now baseline competence. Your customers have done the research. The vendor who can speak fluently to both species earns trust faster than the one who only sells what’s in the warehouse.
And Greg’s client in Bend? She went with the thermo hemlock. Saved about $7,000 on materials. Six months in, she hasn’t complained once.
Frequently Asked Questions
Is thermally modified hemlock as durable as western red cedar for outdoor saunas? In field testing, including a 12-year exposure study by the Building Research Institute Finland, thermally modified hemlock performed within a few percentage points of untreated western red cedar on rot resistance and dimensional stability. For most residential sauna and pavilion applications, the durability difference is negligible.
Why is cedar so much more expensive than hemlock? Clear vertical grain western red cedar trades at roughly 4 to 6 times the price of dimensional pine (2024 Random Lengths data). The premium reflects the species’ slow growth rate and a constrained supply chain. Thermally modified hemlock costs 50 to 70 percent of an equivalent cedar specification.
Does thermal modification weaken the wood? Yes, by 10 to 30 percent depending on process temperature and duration. This is not a concern for cladding, bench tops, or typical pavilion framing, but it matters for structural members carrying significant loads. Builders should adjust spans and dimensions accordingly.
Which wood feels better on sauna benches? Cedar has lower thermal conductivity, so bench surfaces feel slightly warmer to bare skin at sauna temperatures. Thermo hemlock feels marginally cooler on initial contact but warms to body temperature quickly. Most users find the difference minor after repeated use.
Is hemlock more environmentally friendly than cedar? In many cases, yes. Hemlock comes from faster-growing North American plantations with shorter replanting cycles than cedar. Old-growth cedar harvest has been controversial for decades. That said, sourcing practices vary by supplier, so it’s worth asking for specifics.
Can I leave either wood unsealed outdoors? Both species will weather to a gray patina if left unsealed. Neither will fail structurally without sealant in above-ground applications. A UV-protective sealant applied every 3 to 5 years preserves the original wood color if that matters to you.
Which wood is better for a hot, humid climate? Cedar retains a small edge in rot resistance in high-moisture environments, such as coastal or subtropical climates. If your build site sees heavy, sustained rainfall or high ambient humidity, cedar may be worth the premium for the additional margin of safety.
By
