Do Outdoor Saunas Get Moldy?

Dec 27, 2025

Love your Outdoor Sauna but noticing a weird smell or mystery spots? Yep, outdoor saunas get moldy—thanks to moisture, poor ventilation, and the lovely UK weather. 😅 But don’t sweat it! Learn how to prevent mould, boost ventilation, manage humidity, and master your cleaning routine in a few easy steps.

The Science of Sauna Mould: Moisture, Heat, and Organic Matter

Outdoor saunas are a peaceful retreat—but they do come with a challenge: stopping mould from creeping in. The mix of warmth, moisture, and organic material makes them a perfect breeding ground. When sweat, steam, and weather combine, the risk goes up fast. That’s especially true in the UK's damp climate.

Controlling these factors with regular maintenance is essential.

Modern black cube outdoor sauna beside swimming pool with glass front.

Why Steam and Sweat Create a "Perfect Storm" for Fungi

Mould thrives in warm, humid, and enclosed spaces. That’s why saunas, especially outdoor ones, are so vulnerable. Temperatures between 10°C and 35°C are ideal for mould growth. Unfortunately, that includes much of the UK summer.

Add in human sweat and steam after use, and you’ve got a recipe for fungal fun—if left unchecked.

The Role of Stagnant Air: How Trapped Humidity Triggers Growth

Without proper ventilation, the air inside the sauna can become saturated with moisture. That lingering dampness creates the perfect environment for mould spores to thrive.

Poor airflow also leads to condensation on ceilings and walls. These wet areas become hotspots for mould to grow unnoticed. Stopping this starts with simple air movement.

Wood Selection: Why Cedar and Thermowood Resist Mould Better Than Pine

The wood you use matters. Some woods are naturally more resistant to decay. Cedar and Thermowood, for example, are excellent at repelling mould and moisture. They contain natural oils that help fight fungi and prevent rotting.

Pine, on the other hand, is more absorbent and can become a mould magnet if not properly treated.

Identifying the Warning Signs of Mildew and Wood Rot

Outdoor saunas can develop mould quickly if not properly maintained. Catching the signs early helps keep both your sauna—and your health—protected.

Visual cues, smells, and even mild health symptoms are all red flags.

Black Streaks vs. Natural Wood Ageing: How to Tell the Difference

Black spots or fuzzy patches on walls, benches, or ceilings are common signs of mould. These differ from the natural greying or silvering that happens as wood ages. If it looks patchy, streaky, or slimy—it’s likely mould, not age.

Check your sauna after each use, especially in corners where moisture collects.

The "Sniff Test": Detecting Musty Odours Before Visible Spores Appear

A musty or damp smell, even when the sauna looks clean, is a warning sign. If odours persist after airing out the space, there may be mould hidden in cracks or insulation. Trust your nose—it usually picks up on mould before your eyes do. Address it quickly to stop the spread.

Outdoor wooden sauna pod with glass door, benches, and heater on decking.

Prevention Strategies and Structural Solutions

The #1 Prevention Tool: Proper Ventilation and Airflow

One of the easiest and most effective ways to prevent mould in an outdoor sauna is through proper ventilation. Moisture can't escape without it.

With airflow, the heat remains enjoyable but less damp—ideal for health and hygiene.

The "Diagonal Path": Positioning Intake and Exhaust Vents for Cross-Flow

Installing intake and exhaust vents diagonally (on opposite corners) promotes cross-flow ventilation. This design ensures fresh air can enter while stale, moist air escapes efficiently. It’s a simple trick with a big impact.

The Post-Session Ritual: Leaving the Door Open for 30–60 Minutes

Once your sauna session is over, crack the door open. Letting air circulate for 30 to 60 minutes helps remove residual heat and moisture. It’s one of the easiest ways to keep mould at bay—just like airing out your gym shoes!

Mechanical Extraction: When to Use High-Temp Exhaust Fans

In humid regions or wetter seasons, passive airflow might not be enough. Using an exhaust fan rated for high temperatures can speed up drying. It’s especially useful during winter or in consistently rainy areas of the UK.

Construction Choices That Stop Mould Before It Starts

Prevention starts with smart design and material choices. When building or renovating a sauna, think beyond appearances.

A few tweaks can make a massive difference.

The Aluminium Vapour Barrier: Protecting Wall Cavities from Internal Steam

Installing an aluminium vapour barrier inside walls keeps steam from sneaking into the structure. Without it, insulation and timber framing can absorb moisture. This hidden dampness leads to long-term mould problems.

Sloped Flooring and Drainage: Preventing Standing Water Under Benches

Flooring should never be perfectly flat. A slight slope allows water to run off instead of pooling. Pair this with a drainage system—like a French drain—to whisk water away from bench legs and corners.

No puddles means no problem.

Wooden barrel sauna on raised deck surrounded by trees in autumn garden.

Safe Cleaning Practices to Maintain a Hygienic Interior

Cleaning your sauna doesn’t have to be complicated. But it does need to be consistent. The right methods protect both the wood and your lungs.

Removing Mould Safely: Why Vinegar and Hydrogen Peroxide Beat Bleach

Bleach might seem like a strong mould-killer, but it can damage wood and leave toxic residues behind. Instead, use natural options like white vinegar or hydrogen peroxide. They kill mould effectively without harming the wood or air quality.

Protecting the Wood: Using Paraffin Oil vs. Chemical Sealants

A protective barrier helps keep your sauna looking and feeling fresh. Paraffin oil is a safe, natural option for regular upkeep.

For more aggressive protection, chemical sealants are available—but always choose ones safe for sauna use.

The Importance of Towels: Preventing Sweat Saturation in Bench Fibres

Encourage everyone to sit on towels. It’s a simple habit that absorbs sweat before it reaches the wood.

This prevents saturation, bacteria build-up, and reduces the chances of mould forming in hidden crevices.