Dehumidifiers preserve history beneath the streets of Tallinn

The Bastion Tunnels, hidden beneath the UNESCO-listed Tallinn Old Town, are a core part of the Kiek in de Kök Fortifications Museum. Built in the 17th and 18th centuries as strategic defence works, these limestone passages have served as ammunition stores, WWII bomb shelters, and even underground hubs for the city’s punk movement.

When the City of Tallinn decided to open these historic corridors to the public in 2004, they faced a massive environmental hurdle: water.

After being sealed in the 1930s, parts of the tunnel network sat underwater for decades.

A massive renovation was carried out to preserve the deteriorating historical monument and make it safe for visitors. Even after initial drainage, the underground environment remained hostile to both visitors and the structural integrity of the museum.

Dehumidification was therefore necessary both to preserve the structure and to ensure a better indoor climate for visitors.

Centuries of stagnant humidity

The bastion passages are built relatively deep into the earth embankment and do not have modern waterproofing. The Old Town is located quite close to the sea, and although the tunnels are slightly above sea level, the groundwater level there is naturally high. When the tunnels lost their military purpose and were no longer maintained, groundwater began to seep into them. In addition, rainwater and meltwater infiltrated through the soil into the bastion embankments, and without the drainage system, this water would eventually also accumulate in the tunnels.

The challenge was threefold:

• Extreme humidity: Before the dehumidifiers were installed, the floors, walls, and ceilings were damp, and the relative humidity could reach 90–100 %.

• Cold surfaces: Low average temperatures caused constant condensation on the ancient stone walls and ceilings.

• Visitor impact: The moisture exhaled by thousands of tourists threatened to further destabilise the climate.

Introducing robust and user-friendly humidity control

The renovation of the tunnels took place over several years. During the first phase, desiccant dehumidifiers were installed to address the humidity issues. This initially seemed a sensible choice, as desiccant units operate reliably at low temperatures and do not require water drainage. A rotating wheel containing desiccant material absorbs moisture from the air stream. However, the moist air must then be exhausted to the outside atmosphere via ducting, which proved difficult to manage from the underground tunnels.

In the later stage of the renovation, it was decided in cooperation with the Tallinn-based HVAC and climate specialist EB Therm to install room-based condensation dehumidifiers, which do not require the construction of air ducts and are more energy efficient. EB Therm selected the CDF series from Dantherm, engineered for tough environments and capable of performing well at low temperatures.

In total, seven wall-mounted dehumidifiers were installed in strategic locations and supplemented by two mobile units, which can be positioned as required.

The choice of the CDF series was driven by several key technical factors:

• Operating range: Designed to function between 3–32 °C and 40-100 % relative humidity, it was perfectly suited for the chilly subterranean climate.

• Capacity: Each unit is capable of removing approximately 40 litres of water from the air every 24 hours.

• Aesthetics: The slim, modern design allowed for installation in narrow passages without obstructing the historical view.

• Low noise: With a sound pressure of only 50 dB(A) at 1 metre, the CDF dehumidifier does not interrupt the visitors’ experience.

A stable environment for exploration

The installation has successfully transformed the tunnels from a damp ruin into a world-class museum.

• Humidity control: The environment now maintains a stable relative humidity below 75–80 %. This is low enough to protect the building structure, but also high enough to provide the authentic experience of being underground.

• Operational reliability: Despite the rough, high-moisture conditions, the equipment remains highly durable. 

• Enhanced experience: The sleek units blend into the background, allowing the museum to focus on storytelling rather than climate management.

"EB Therm has been marketing Dantherm dehumidifiers, ventilation, and heating equipment since 1995. For the Bastion Tunnels project, we chose the CDF series, because it was a very robust and reliable dehumidifier, which was easy to fit into the building." Toivo Kopli, EB THERM Ltd.

The combination of reliable dehumidifiers and the expertise of EB THERM have contributed to the preservation of an important piece of Estonia’s heritage.

The energy-efficient and quiet CDF range of powerful dehumidifiers is ideal for preserving and protecting valuable assets from humidity damage. They are suited for humidity control in garages, warehouses, museums, churches, archives and waterworks. 

For full technical details, go to the product pages here:

• CDF 40
• CDF 50
• CDF 70

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