Water Mist in Historic Heritage. Pros and Cons

Fireriskheritage.net has already covered the use of water mist systems in historic and heritage buildings. The recent reopening of Notre Dame de Paris has shown the fire safety consultants’ choice to install a water mist system to protect the “Forest“. The Forest was (and is now) the complex wooden structure of the roof, that needed more than 1.000 oaks to be rebuilt and is now protected by a water mist system.

In this regard, a brief summary of the pros and cons of using these powerful but complex systems may be useful.

Water mist fire protection systems have several advantages specific to historic buildings, focusing on minimizing damage and preserving structural integrity. Here are the key benefits:

Reduced Water Damage

• Water mist systems use up to 85% less water than traditional sprinkler systems.
• This significant reduction in water consumption translates into decreased water damage in the event of a fire.
• Protecting delicate furnishings, artwork and architectural elements within historic buildings is a top priority, and water mist systems offer an approach that minimizes the risk of collateral water damage.
• Water mist systems are particularly suited to protecting areas such as libraries, archives, historic buildings and museums, where the preservation of sensitive materials is critical.
• Studies have shown that water mist systems can control damage to sensitive objects, using only 30% of the water required by a traditional sprinkler system.

Greater design flexibility and discreet integration

• “Smart” design: The customizable nature of water mist systems allows for more discreet integration into historic buildings. Unlike sprinkler systems that often require bulky piping and visible infrastructure, water mist systems can be designed with smaller piping and compact components, minimizing visual impact on historic aesthetics.
• Adaptability to complex spaces: Water mist systems can be adapted to complex and obstructed spaces common in historic buildings, providing effective protection where traditional systems may be inadequate. Their ability to reach shielded areas by dispersing fine droplets makes them ideal for protecting structures with high ceilings, exposed beams and other architectural elements that may impede traditional sprinkler systems.
• Retrofit Installations: Water mist systems are suitable for retrofit installations in historic buildings where structural changes must be kept to a minimum. Compact components and design flexibility allow for easier integration with existing systems and reduce the need for invasive work that could compromise the structural integrity of the historic building.

Long-Term Preservation and Maintenance

Despite ongoing concerns regarding the long-term maintenance of water mist systems, particularly concerning the risk of corrosion, empirical evidence demonstrates that systems employing stainless steel piping exhibit satisfactory condition retention even after a fifteen-year operational period.

Water mist systems necessitate routine maintenance akin to conventional sprinkler systems, encompassing inspection, testing, and periodic replacement of stored water.

A commitment to meticulous maintenance can effectively mitigate corrosion risks and guarantee the sustained operation of the system within historic buildings.

Efficient testing procedures: Restricting water discharge during water mist system testing minimises disruption and potential damage to protected areas within historic buildings.

Considerations for Water Mist Fire Protection Systems

Water mist fire protection systems present several challenges, particularly when applied to historic structures. While they offer advantages such as reduced water consumption and enhanced design flexibility compared to conventional systems, their distinctive characteristics raise concerns in sensitive environments, such as historic buildings in the following areas:

• Corrosion Potential: Water mist systems rely on high-pressure nozzles with small orifices to generate fine droplets. This, combined with potential corrosive products within the piping, can lead to clogs and blockages. In historic structures, where piping systems may be older and susceptible to corrosion, this risk is amplified.
• Maintenance and Inspection: While maintenance of water mist systems is comparable to that of traditional sprinkler systems, long-term maintenance data is still limited. The lack of a solid historical knowledge base on the effectiveness and long-term maintenance of these systems in historic buildings can make it difficult to manage them and assess potential risks.
• Design Complexity: Designing a water mist system for a historic structure requires careful consideration of multiple factors such as ceiling height, shaft size, ventilation, construction materials, and type of fire load. The complexity of these calculations and the lack of a standardized design approach make the design process more onerous and potentially more expensive.
• Limited Historical Documentation: Approval of water mist systems for sensitive applications such as historic buildings is often hampered by the lack of a solid historical data base on their effectiveness and safety in real fires. The lack of empirical evidence and precedent can lead to a reluctance by approval authorities to grant permission for installation in historic settings.

A brief costs comparison Water mist/sprikler

The installation and maintenance costs for water mist fire suppression systems are typically highly unbalanced in favour of installation, as these systems are significantly more capital-intensive to install than to maintain. Based on industry and governmental cost analyses, the installation-to-maintenance cost ratio generally ranges between 15:1 and 25:1 throughout the system’s lifespan. Below, as an illustrative example, a typical cost structure for commercial systems is presented:

Analytical examples provide additional elements to evaluate the optimal equilibrium between technical requirements and financial limitations:

• UK retrofit case studies (Gov.Scot, 2024) show installation of mist systems in converted heritage hotels costs 30–40% more than sprinkler systems (BS 8489 vs BS 12845), with annual maintenance around £2,000–3,000—yielding a roughly 22:1 ratio.
• U.S. hospital retrofit studies (Penn State, NFPA 750 case) found annual maintenance rates of $1,500 versus capital installation differences exceeding $30,000, again a 20:1 installation/maintenance ratio.
• Market analyses confirm that high initial capital costs remain the main barrier to adoption, while maintenance costs—though higher than sprinkler systems—represent only a small fraction (< 5 %) of total ownership costs.

Water mist systems require a larger upfront investment due to high-pressure components and proprietary technology, but routine upkeep is comparatively modest. Over a 10–15 year service life, installation costs represent 90–95 % of total expenditure, with maintenance forming only 5–10 %, giving a typical ratio of approximately 20:1 between installation and maintenance.

In conclusion: when fire risk analysis suggests to install water mist?

For cultural heritage buildings, anticipated water mist systems incur a cost that is approximately 1.3 to 1.4 times greater compared to conventional sprinklers, representing a premium of 30 to 40%. Nevertheless, when juxtaposed with the already elevated costs associated with heritage building installations (ranging from $10 per square foot to $7 per square foot, in contrast to the $2 to $7 per square foot incurred for ordinary buildings), the resultant cost disparity becomes substantial.

Consequently, the decision to implement a water mist fire protection system within a historic structure necessitates a meticulous evaluation of its potential advantages and disadvantages. The intricate nature of the system, the restricted historical documentation, and the concomitant financial outlay demand a comprehensive analysis of the specific circumstances and close collaboration with experts in fire safety and preservation. It is paramount to meticulously consider all critical factors and assess whether the benefits surpass the potential challenges before proceeding with the installation of a water mist system in a historic edifice.

Risk analysis may recommend the use of water mist fire suppression systems over less expensive systems in a number of situations, particularly when the benefits in terms of damage reduction and environmental impact outweigh the higher initial costs, like the most of historic buildings and sites of high cultural value.

In these contexts, the minimisation of damage caused by water is of primary importance. Water mist systems, thanks to their significantly reduced water consumption (up to 85% less than traditional systems), offer effective protection while minimising collateral damage to the building and its contents. Conservation of delicate artefacts, works of art and architectural elements justifies the investment in a more expensive but less invasive system.

In summary, the risk analysis may lead to recommending the use of water mist systems when the protection of high-value buildings or artefacts, minimization of collateral damage, design flexibility and environmental impact are priorities over initial costs.