Why Nightjar Outdoor Saunas Have Open Floors (And Why You Don't Need To Mind The Gap)

We've built over 50 saunas now, and one of the most common questions I get is about floor design. "Should I seal my sauna floor? Won't heat escape through the gaps?"

Let me explain why we build outdoor saunas with open floors—and why you should actually be thankful for it.

The Real Problem: Mold and Bacteria

Saunas are incredibly wet environments. Löyly, sweat, rinsing, splashing—water is everywhere, constantly. That's not a problem. That's how saunas work.

But where that water goes? That's everything.

A sealed floor traps moisture. Water sits underneath with nowhere to go. In Michigan's humid summers and freeze-thaw winters, you're creating perfect conditions for mold and bacteria growth. The EPA's guidelines on moisture and mold prevention make it clear: the key to controlling mold is controlling moisture. Research on building science and moisture dynamics shows that trapped moisture in building materials creates ideal conditions for microbial growth—exactly what you get with a sealed sauna floor.

Open wood decking with small gaps between boards solves this completely. Water drains through immediately. It evaporates or runs off below. The wood dries out fast. No standing water. No trapped moisture. No mold or bacteria problems.

Most people who have a Nightjar sauna use it every single day. These saunas are built like small houses—proper structure, elevated on joists—so they can last 20-30 years without rotting out from the bottom. An open floor with proper drainage is key to that longevity.

"But Won't All That Heat Escape?"

This is the concern I hear most: "Won't those gaps in the floor make the sauna cold? Won't it take forever to heat up?"

Here's the question sealed floor guys can never answer: How is cold, heavy air supposed to magically travel UP through those deck boards into your hot sauna? Basic thermodynamics tells us that "natural convection is driven by buoyant forces: hot air rises because density decreases as temperature increases." Cold air is denser—it sinks, it doesn't rise. Without forced airflow or negative pressure in the sauna, cold air from below stays below. That's just physics.

I can go into a deep dive on thermodynamics if you really want me to, but here's what matters: most studies on sauna heat loss don't account for the thermal mass of the stones. That's the critical piece.

The Power of Thermal Mass

Here's what most studies on sauna heat loss completely miss: the thermal mass of the stones.

Our 9 kW stoves hold 120 kg of rocks—that's a massive thermal battery. Those stones get superheated by the electric elements, with the stones reaching temperatures over 400°F. They store enormous amounts of energy and radiate that heat continuously throughout your sauna session.

When you have that much thermal mass, a few hairline drainage gaps in the floor don't even register. The stove completely overpowers any minor loss. The rocks are doing the real work—creating that deep, penetrating heat and explosive steam when you throw löyly.

This is why it's so important to have a large thermal mass of stones in your sauna. Those superheated rocks make any concerns about floor gaps completely irrelevant. The energy stored in 120 kg of rocks at 400°F+ is orders of magnitude greater than any heat that could possibly escape through small drainage gaps.

Keep It Simple, Keep It Clean

Here's the bigger picture: make your sauna easy to keep clean with less to go wrong. Open deck floors are simple. Water drains. Wood dries. No complex waterproofing systems. No drains to maintain. No hidden moisture problems waiting to show up years later.

If you're still concerned about open floors making your sauna cold, I get it. It sounds counterintuitive. But there are over 50 Nightjar saunas out there right now with open deck floors, and soon there will be even more! At the sauna club we will have the opportunity for you to check out and even sweat in a Nightjar Sauna. 

Come try one. Feel the difference between theory and reality. The gaps aren't pulling heat out of your sauna. They're keeping it dry and healthy.

Indoor vs. Outdoor: Different Rules

Quick note: if you're building an indoor sauna, you absolutely need a sealed floor with a proper drain. Water can't be draining into your house structure. Indoor sauna building codes and drainage requirements are very specific about this—and for good reason.

But for outdoor saunas? Open deck-style floors with drainage gaps are the way to go. Mold and bacteria prevention is the priority. The temperature concern? I've tested it. The difference is negligible.

Don't be afraid of those floor gaps—they're not your problem. Make sure you have proper inlet and outlet ventilation. That's what makes a sauna feel alive.

Stop overthinking the floor gaps. They're keeping your sauna healthy, and they're not making you cold.

Stay hot,
David