Should a Sauna Roof Be Insulated?

We came across this question in a sauna discussion group in social media recently:

“I wondered if anyone has experienced if it is best for the sauna roof to have insulation? We have just placed an order with [company] for [model] outdoor sauna with electric heater, and they advise that insulation is not that important for a 5 m² sauna. Any thoughts would be appreciated!”

The answer from our side is clear: Yes, a sauna roof absolutely needs to be insulated.

And just to clarify: While the sauna mentioned in the discussion was not a Hetki Sauna, our saunas are also outdoor models roughly that size. Which makes the question directly relevant, and our answer so clear.

Insulation plays a critical role in how a sauna performs. Without it, heat escapes quickly, the heater works harder, and the whole experience becomes less enjoyable.

Getting insulation right makes all the difference, not only for comfort and usability, but also for the durability and longevity of your investment. And when it comes to heat insulation, the roof is where it matters the most – since heat always rises, it is also the most vulnerable part for heat loss.

Comfort, Efficiency and Savings

Proper insulation keeps the warmth inside your sauna, allowing it to heat up faster and maintain a steady temperature throughout your session. This means gentler löyly, less strain on the heater, and an overall more balanced sauna experience.

At the same time, insulation improves efficiency. The heater doesn’t have to work as hard, which reduces energy use, lowers running costs, and can even extend the heater’s lifespan. In colder climates, thicker insulation ensures your sauna remains comfortable and energy-efficient all year round.

Over time, the benefits are clear: better enjoyment, lower costs, and longer durability.

Protection, Durability and Vapor Barriers

Insulation is not only about heat. Properly built roof structures also protect the sauna from moisture damage. A traditional Finnish sauna is not a dry sauna, as is a common misconception. Water is poured on the hot stones, creating löyly, that humid steam and heat that is both wonderful and essential to the sauna experience.

Combined with a vapor barrier and correct detailing, proper insulation keeps condensation out of the roof structure and prevents long-term issues like mold or rot. Another key factor is to leave a controlled air gap between the insulation and the inner ceiling, and between the insulation and the outer roof, because that further reduces the risk of moisture buildup.

This is important for outdoor saunas too, but it is often even more critical for indoor saunas, where moisture problems can affect not only the sauna itself but also the surrounding building structures.

A vapor barrier is an essential part of this. It stops moisture from seeping into the insulation and prevents mold, mildew, or structural damage. It also helps keep the insulation dry and ensures that the heat stays inside.

Aluminum foil is a common choice, as it not only blocks moisture but also reflects heat back into the sauna, improving efficiency and comfort. The key is in the detailing: all seams, joints, and penetrations need to be carefully sealed.

A well-insulated and vapor-protected sauna offers both reliability and resilience, giving you a space that stays warm, efficient, and healthy for many years.

Design and Construction Considerations

If you are planning to build or buy a sauna yourself, our sincere advice is this: Always ensure proper roof insulation. And no matter the design, use materials that can handle high heat and moisture so your outdoor sauna will stand the test of time.

Different sauna designs have unique insulation needs. Although good roof insulation is widely recognized as important, too often outdoor saunas, for example popular barrel saunas, are built without it.

When it comes to sauna insulation materials, two are especially worth mentioning: mineral wool and modern foam boards, such as PIR (polyisocyanurate).

Mineral wool, typically rock wool, offers strong resistance to both heat and moisture, making it a reliable option for sauna walls and ceilings. It is durable, easy to install, and provides consistent protection against heat loss.

PIR foam boards, on the other hand, deliver higher insulation values, meaning excellent thermal performance with thinner material. They are straightforward to work with, maintain their properties well over time, and are particularly effective in both walls and ceilings.

For your safety, choose only sauna-specific insulation materials that can handle heat without releasing harmful or toxic fumes. Equally important, the insulation should not present a fire hazard, so always make sure that only approved materials are used in your sauna.

How We Build the Roof Insulation at Hetki

Our roofs are carefully built to combine efficiency, safety, durability, and a truly authentic Finnish sauna experience.

We rely on Finnish-made PIR boards, engineered for the demanding heat and moisture conditions of saunas. These thin yet highly efficient PIR boards make the most of available space while ensuring maximum energy efficiency. The sauna heats up quickly and retains its warmth. The 50 mm PIR board we use has a declared thermal conductivity λ of 0.022 W/mK (R ≈ 2.27 m²K/W, R-12.9), whereas the same thickness of rock wool typically has λ around 0.037 W/mK (R ≈ 1.35 m²K/W, R-7.7), meaning our PIR board is almost twice as insulating as rock wool.

The boards combine both insulation and vapor barrier in one layer. Their vapor resistance comes from the aluminum laminate surface on both sides. Every joint is carefully sealed with heat-resistant polyurethane foam and aluminum tape, ensuring a completely tight structure.

The boards are designed for high-performance use: their operating temperature range is up to +120 °C (248 °F), and they can even withstand brief peaks of more than +200 °C (392 °F). This makes them perfectly safe for use in the hot and humid environment of a sauna.

The insulation itself is obviously hidden from view. On the inside, it is covered with beautiful ceiling panels of your chosen wood species. On the outside, it is protected by robust stainless-steel roofing. You don’t see it, but you feel the difference every time. At Hetki, made in Finland, we build for both beauty and endurance, making sure every log sauna is properly insulated and delivered ready-to-use, so that all that’s left for you is to enjoy.

Built for Joy, Built to Last

Cutting corners saves money, which we believe must be the reason too many outdoor saunas are built without or with insufficient roof insulation. But heat loss, structural issues, higher energy bills – and a poorer sauna experience – soon outweigh any initial price savings.

Proper insulation is key to getting the most out of your sauna investment. It ensures durability, energy efficiency, and years of reliable use.

In sauna building, every detail counts, and roof insulation is no exception.