Does My Sauna Need Insulation?

It's one of the most common questions we hear at Nightjar: "Do I really need insulation in my sauna?" The short answer is yes—most saunas absolutely need proper insulation. But like everything we do, the full answer is a bit more nuanced and worth understanding if you're serious about building a sauna that will serve you well for decades to come.

The Finnish Exception

If you've spent any time researching saunas, you've probably heard someone say, "The Finns have been building saunas for centuries without insulation, so why would I need it?" It's a fair point, and there's some truth to it. Many older Finnish saunas—particularly rural smoke saunas and traditional wood-fired buildings—were constructed with little to no insulation. I've been in some of these older Finnish saunas where you can literally see daylight through gaps in the wall boards. Cracks, spaces between logs, minimal concern for air-sealing—and yet, these saunas get absolutely scorching hot.

So what gives? How can a sauna with visible gaps in the walls possibly work?

The answer lies in one crucial element: a massive wood-burning stove with serious rock mass. These traditional Finnish saunas weren't relying on insulation to hold heat—they were relying on sheer thermal output. When you have a substantial wood stove burning and hundreds of pounds of rocks radiating stored heat, you can overcome almost any amount of heat loss through the walls. The stove isn't just heating the space; it's overpowering it.

Our trailer sauna operates on this same principle. We didn't use any insulation in that build because the wood stove and rock mass generate more than enough heat to maintain temperature. In a scenario like that, quality wood construction and a reflective foil barrier are sufficient. You're not trying to conserve energy or prevent heat loss—you're generating so much heat that it simply doesn't matter.

But Not All Saunas Are Equal

Here's the critical distinction: most modern saunas aren't built around massive wood-burning stoves. If you're installing an electric sauna or even a smaller wood-fired unit, you don't have that overwhelming thermal capacity to work with. Your heater is sized appropriately for the space—it's not designed to overcome significant heat loss through poorly insulated walls.

For the vast majority of saunas, especially electric saunas and smaller wood-fired units, insulation isn't optional. It's essential.

Why Proper Insulation Matters

A well-insulated sauna does several critical things. First and foremost, it maintains consistent temperatures throughout your session. There's nothing worse than hot spots and cold corners in a sauna—it disrupts the entire experience. Proper insulation creates that enveloping, even heat that makes a great sauna feel like a warm embrace.

Insulation also prevents drafts, which can break the meditative quality of a good sauna session. When your sauna holds its heat properly, it stays hot longer after you're done firing it, giving you flexibility in your routine and reducing energy consumption. For electric saunas especially, good insulation means your heater doesn't have to work as hard to maintain temperature, which extends the life of your equipment and keeps your energy bills reasonable.

Beyond performance, there's a structural consideration. Proper insulation, installed correctly, protects your sauna from moisture issues that can lead to mold or degradation over time. This is where building it right from the beginning pays dividends for years to come.

The Right Way to Insulate

Not all insulation is created equal, and in a sauna environment, you can't cut corners. At Nightjar, we use rockwool insulation exclusively, and there are compelling reasons for this choice. Rockwool won't break down when exposed to moisture, which is obviously crucial in a sauna environment. Equally important, it doesn't off-gas or release any chemicals when heated—something you definitely don't want happening while you're trying to relax and improve your health.

But insulation is only part of the equation. The way you construct your sauna walls matters just as much as the materials you choose. The correct build sequence is: cedar furring strips, then a foil vapor barrier, then your rockwool insulation, with an air gap between the foil and the hot room. This system ensures that moisture from your sauna sessions doesn't penetrate into the wall cavity where it could cause problems. The foil reflects radiant heat back into the sauna while the air gap allows any incidental moisture to dissipate safely.

The Cost of Cutting Corners

We see too many saunas out there that either skip insulation entirely or use inappropriate materials. Sometimes it's a DIY build where someone didn't know better. Sometimes it's a manufacturer trying to save on costs. Either way, the result is the same: a sauna that doesn't perform well, doesn't last, and ultimately disappoints.

When you're investing in a sauna—whether you're building it yourself or having one built for you—you're making a commitment to your health, your wellness routine, and ideally to something that will serve your family for generations. That's certainly how we think about it at Nightjar. Every sauna we build is crafted to last, not just for you, but for your children and perhaps even their children. We're creating heirlooms, not appliances.

Building to Last

Quality craftsmanship means understanding why each component matters and refusing to compromise on the fundamentals. Insulation might not be the most exciting part of a sauna build—it's hidden behind beautiful cedar walls after all—but it's foundational to everything that makes a sauna work properly.

If you're planning a sauna project, whether it's with us or on your own, take the time to do the insulation right. Use rockwool. Build your walls with proper vapor barriers and air gaps. Think about how every detail contributes to the overall experience and longevity of what you're creating.

Because at the end of the day, a well-built sauna isn't just a wooden box with a heater. It's a sanctuary, a place for restoration, and when done right, a beautiful piece of functional art that will reward you with countless sessions of warmth and wellness for years to come.

Have questions about building your own sauna or interested in having Nightjar build a sauna for your space? We'd love to hear from you.