How Do You Care for an Outdoor Sauna During The Rainy Season?

An outdoor sauna built from Canadian Thermowood (the heat-treated hemlock Redwood Outdoors uses across its line) is built to handle rain. The heat-treatment process restructures the wood at a cellular level, reducing moisture absorption and increasing resistance to rot and fungi. That said, weeks of steady rain put extra wear on any outdoor structure. A few targeted habits during the wet season keep your sauna dry, fresh inside, and performing the way it should.

Key Takeaways

1. Thermowood's heat-treatment process permanently lowers the wood's ability to absorb moisture, making Redwood Outdoors saunas more dimensionally stable in wet climates than untreated lumber.
2. Drying the interior after every session is the single most effective way to keep the wood in top condition. Run the heater for an additional 60-minute cycle with the door closed after each session to dehumidify the wood. If conditions are dry outside afterward, open the door briefly to finish airing out.
3. Redwood Outdoors saunas ship with pre-cut ventilation ports designed to cycle air several times per hour during a session, which also speeds post-session drying.
4. The ground beneath the sauna matters as much as the roof above it. Well-drained gravel or crushed stone prevents moisture from wicking upward.
5. Running the heater for an empty 60-minute cycle once or twice a week during periods of low use keeps the interior dry and fresh.

Rain doesn't threaten a well-maintained outdoor sauna. What threatens it is trapped moisture that never gets a chance to leave. During the rainy season, whether that's the Pacific Northwest's long autumn drizzle, the Gulf Coast's summer downpours, or anywhere else with weeks of wet weather, the goal is simple: let water that lands on the sauna shed off, and make sure the cabin dries out between sessions.

Here's what to check, what to do, and how often to do it.

Why does Thermowood handle rain better than untreated wood?

Redwood Outdoors builds every sauna from Canadian Thermowood, a heat-treated hemlock. The treatment follows the three-phase process developed by the Finnish ThermoWood Association: first, heat and steam dry the wood to near zero percent moisture; then the temperature rises to 414°F and holds for two to three hours in a low-oxygen steam environment; finally, water spray cools the wood and brings its moisture content back up to a workable 4 to 7%. That process permanently changes the wood's cellular structure.

The practical result: Thermowood absorbs less water, swells and shrinks less with humidity swings, and resists rot and fungi far better than raw lumber. The benefits include dimensional stability, rot and fungi resistance, increased longevity in all climates, and improved insulation. No exterior treatment is strictly required, though it's an option if you want to maintain the wood's original color in a high-UV environment.

Since Thermowood already resists moisture at the cellular level, just a few simple habits during wet season will keep things in great shape.

What's the most important rainy season habit?

Ventilation. Specifically, drying the interior of your sauna after every session.

When your session ends, run the heater for an additional 60 minutes with the door closed. The residual heat bakes moisture out of the benches and walls. After that cycle, if the rain has stopped, open the door and let the cabin air out until surfaces feel dry to the touch. If it's still raining, keep the door closed. That heater cycle is usually enough to dry things out, and opening the door during active rain would invite humid air back in. Either way, close the door once the cabin is dry. A closed door between uses keeps rain and ambient humidity from drifting back in.

Redwood Outdoors saunas come with pre-cut ventilation ports that keep air moving during use. During the rainy season, keep those vents clear of debris: leaves, cobwebs, and anything that blocks airflow.

If you're not using the sauna regularly, run the heater for 30 to 60 minutes once or twice a week to dry the interior. Alternatively, place a small dehumidifier or fan inside and run it periodically. Empty the dehumidifier's water tank regularly, so it keeps working.

What should you check on the exterior before the rainy season starts?

A quick exterior walk-around takes about 15 minutes and keeps everything ready for the wet months ahead.

Area	What to check	What to do
Roof and shingles	Loose, lifted, or missing shingles; visible gaps	Resecure or replace damaged shingles before the rain arrives (replacement roll roofing is available from Redwood Outdoors)
Exterior stain or sealant	Fading, peeling, or bare spots (if you've stained)	Reapply before wet weather; stain adheres best to dry wood in mild temperatures
Door seal and threshold	Gaps, warping, or light visible around the closed door	Adjust hinges; replace seal material if worn
Ventilation ports	Blockages from leaves, dirt, and insect nests	Clear each vent so airflow isn't restricted
Electrical junction boxes	Cracked or unsealed weatherproof boxes (Wi-Fi heater models)	Reseal or replace covers to keep water away from wiring
Band tension (barrel saunas)	Bands loosened as wood adjusted to climate	Tighten per the owner's manual; Thermowood moves less, but check anyway

Does the ground underneath the sauna matter during rain?

Yes, more than most people expect. If your sauna already sits on compacted gravel or crushed stone, you're in good shape. If it sits on bare soil or grass, rainy season is a good time to check whether water is draining away properly. Keeping plant growth trimmed beneath and around the sauna also helps.

It's also worth clearing any leaves, branches, or debris that have collected underneath over the summer. That buildup holds moisture against the base and slows airflow where you need it most.

During the rainy season specifically: check that water flows away from the base, not toward it. If you notice pooling, you may need to re-grade the area slightly or extend the gravel bed.

How do you keep the interior dry and fresh during wet weather?

The key is making sure moisture doesn't stay on the wood for too long. A sauna that dries out after every session stays clean and fresh for years. Here are the habits that make the difference.

If you're using your sauna regularly, the heat from each session already dries things out. A few extra habits help during the wetter months.

Habits that work:

Dry the cabin after every session. Run the heater, then ventilate. Wipe benches with a clean, dry towel if they're visibly damp.

Towel off before entering. If you're coming from a cold plunge or shower, a quick towel-off before stepping in is enough. Just avoid dripping onto the benches.

Don't throw water on walls or benches. Water goes on the stones only.

Skip wet swimwear inside the sauna. Wet fabric pressed against wood pushes water into the grain.

Treat benches with paraffin oil. It adds a water-repellent layer that makes cleanup easier and slows sweat absorption. Wood surfaces must be completely dry before you apply it.

If you ever notice discoloration or a musty smell, clean the affected area with a sauna-safe cleaner and a soft brush, brushing with the grain. Rinse with water, not a pressure washer, then dry the area completely. This is uncommon for saunas in regular use, since the heat cycle itself keeps the interior well above conditions where moisture-related issues develop.

What kind of interior wood care helps during prolonged rain?

Beyond the post-session drying routine, a seasonal treatment goes a long way. Redwood Outdoors recommends a sauna wax for interior surfaces. It helps maintain the wood's natural finish, prevents cracking, and extends the sauna's life without sealing the wood shut. Thermowood needs to breathe, so never use paint, varnish, or a film-forming sealant on the interior. Those trap moisture in rather than letting it cycle out.

A once-a-year deep clean with a sauna-specific cleaner (applied with a soft brush, always with the grain) removes residue that regular wipe-downs miss. Rinse with a hose (low pressure, not a power washer), then run the heater to dry everything thoroughly before closing up.

Fine-grit sandpaper handles stubborn stains on benches. Sand lightly, reapply paraffin oil or sauna wax to the sanded area, and let it dry fully.

FAQs

Will rain damage my Redwood Outdoors sauna?

Normal rain won't harm a Thermowood sauna. Redwood Outdoors' owner's manual states that some water seeping in during windy rain is normal and "will not harm your sauna or diminish its lifespan." The risk comes from moisture that's trapped inside without a chance to dry, so ventilate your sauna after every session and keep the vents clear.

Can I pressure wash my outdoor sauna?

No. The Redwood Outdoors care guide warns that pressure washing "may permanently damage the wood surface and structures." Use a low-pressure rinse from a garden hose instead, and let the exterior air dry or run the heater to dry the interior.

Should I cover my sauna during the rainy season?

A fitted, breathable cover can reduce the amount of water hitting the exterior, but it isn't necessary for a Thermowood sauna in most climates. If you do use a cover, make sure it allows airflow. A non-breathable tarp can trap humidity against the wood and create the exact moisture problem you're trying to avoid.

How do I know if my sauna has a moisture problem?

A persistent musty smell, visible discoloration, or soft spots in the grain are signs. These are uncommon in saunas that get regular use, but if you notice them, a deep clean and a few extra heater cycles usually resolve it. Contact Redwood Outdoors' customer service team if the issue persists.

Is it safe to use my outdoor sauna in the rain?

Yes. Redwood Outdoors saunas are designed for year-round outdoor use. The heater and electrical components are rated for the environment, and the Thermowood structure handles rain and humidity by design. Running a session during rain actually helps: the heat dries the interior while you're using it.

The rainy season doesn't put your sauna on pause. If anything, those dark, wet evenings are when a hot sauna really hits the spot. Treat it well during the wet months, and it'll be ready every time you are. For cold weather prep, see our guide to winterizing your sauna and cold plunge.

Explore the full Redwood Outdoors sauna collection to find the right fit for your climate, or browse sauna accessories for care products like sauna wax and paraffin oil.