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Fire Retardant Impregnated Wood Products and the Question of Long-Term Fire Performance When Exposed To Outdoor Environments
By: Carolina Arvidsson and Konrad Wilkens, Lund University, Sweden.
Introduction
Wood is no longer just for cabins in the woods. It is climbing skywards in tall buildings like the 20-story Sara Kulturhus in Skellefteå Sweden [1] and planned to be entire neighborhoods like Stockholm Wood City [2]. The appeal is clear: wood is renewable, stores carbon, feels warm and promotes wellbeing more than concrete and steel [3,4]. However, wood and fire have a complex relationship. In modern cities, external parts of building i.e. façades, are potential fire highways, allowing flames to race from one apartment to the next, as seen in many recent façade fire events, Grenfell [5]and more recently at the Turkish ski resort Kartalkaya hotel fire 2025[6]. To keep wood façades both beautiful and compliant with fire codes and safety, many turn to treating them with fire-retardant chemicals. The idea is admirable: preserve the natural look, while reducing its susceptibility to fire. That has been, and still is, the concept. But does it hold up after two months, years or decades of real-world exposure to harsh climates and weathering?
Fire (or Flame) retarded (FR) wood, represent a group of materials/products that becoming more widely used in modern construction to reduce the fire risks associated with untreated wood, limiting potential flame spread and heat release, and contributing to compliance with countries building code requirements, e.g. via reaction-to-fire classifications under EN 13501[7]. Such treatments have expanded the use of wood in the built environment where regulatory frameworks have historically restricted its use due to its potential to burn.
However, what happens when you add some fire-retardant chemicals to your wood and then put them outside, exposed to the earth’s unpredictable weather and the onslaught of time? Well… we aren’t sure if we are being honest. Changes in the chemicals used, and the processing methods, make it very hard to generalize, but it has long been known, though maybe less widely acknowledged, that the fire performance of these materials is not necessarily stable over time, particularly when these materials are used in areas that are exposed to the external environment. Aging, moisture ingress, ultraviolet (UV) light exposure, cyclic wetting (rain)/drying and temperature changes, can all lead to the movement and partial or complete loss of the fire-retarding chemicals, through mechanisms like leaching and UV degradation. As a result, these materials and products that may have initially met fire performance requirements, after some time no longer behave as assumed in the fire safety design or regulatory approval. What then?
There are many technical and regulatory challenges associated with the long-term fire performance of these materials, here we are mainly focusing on external applications as these have the highest risks of exposure. What are the potential implications for fire risk, building code compliance and professional practice?
Fire Retardant Treatments for Wood: Principles, Problems, Testing and its Limitations
Fire retardant treatments for wood typically take one of two approaches: surface protection – using a coating with fire retarding performance – e.g. a paint or varnish, or via impregnation - using pressure and vacuum to embed the chemical agents into the wood product itself, leaving the end-product looking similar to how it was before treatment i.e. like natural wood. Within this article we focus on the latter method, which is desirable due to its natural wood finish. The chemical agents impregnated are intended to alter the thermal decomposition and combustion behaviour of the wood they are added too. How they work varies with the chemicals used – commonly for wood, the mechanisms include char promotion – converting more of the wood to char, rather than it becoming gaseous volatiles – that feed a flaming fire [8] and diluting combustible gases by producing more non-combustible gases [9]. These treatments can significantly improve important fire performance parameters, such as ignition time, heat release rate and flame spread propensity.
From a regulatory perspective, the performance of FR wood is usually demonstrated through reaction-to-fire testing, which in Europe is governed by the Euroclass system through EN 13501-1 [7], and for FR wood products generally this requires Single Burning Item (SBI) testing via EN 13823 [10] as a minimum (some countries may require further testing in specific use cases). Importantly, these tests are conducted on new or recently manufactured specimens, under controlled laboratory conditions. The implicit assumption here is that the tested performance is representative of the material or product throughout its service life. In the case of FR wood materials – for internal applications in dry, controlled environments, this assumption may be reasonable. For external applications, however, the validity of this assumption becomes far less certain.
Weathering, Aging and Loss of Fire-Retardant Chemicals
When FR wood is used outdoors, it is subject to a range of environmental stressors that can significantly affect its composition and performance, not only for fire. Moisture is one of the critical factors. Many of today’s commercially available fire-retardants use, in their base, hydroscopic chemicals (some add fixatives as a method to better hold these chemicals in the wood), and repeated exposure to rain, condensation or even high humidity can lead to migration and leaching of these chemicals from the wood matrix [11]. In addition, cyclic exposure, to wetting, drying, can cause microstructural changes in the wood, cracking and increased permeability [12]. UV radiation exposure degrades surface layers [13], while temperature fluctuations further drive diffusion processes within the material. Over time, the combination of these mechanisms can result in further chemical losses, particularly near the exposed surfaces, which will affect phenomena like ignition and surface flame spread more significantly.
Laboratory and field studies have exhibited that weathered FR wood can have markedly different fire behaviour compared to un-weathered specimens e.g.[14–23]. The late, Dr. Birgit Östman especially, had been repeatedly raising these issues surrounding FR wood products for decades, and have authored (with others) some of the only long-term studies available in the literature [24,25] on this topic. Recently activities at Lund University, including a number of master’s thesis projects (past and ongoing) [21,26–29] are continuing Birgit’s work investigating this problem using real-world samples, something that is severely lacking in the literature. All these studies, and other quality work from research institutes such as; Göttingen University [15,22,23], Luleå University of Technology[30], University of Tokyo [18–20] and others. All show similar outcomes to an extent – increasing heat release rates, changed ignition times, and more rapid flame spread have all been observed, after both natural and artificial weathering/aging, in some cases the performance of weathered FR wood approaches something more like untreated wood behaviour.
Implications for Facades and External Fire Spread
The issue of long-term fire performance is particularly critical for facades and external wall systems. In these applications, fire safety strategies, expect that materials/products chosen should contribute to limiting external fire spread, preventing vertical flame propagation, and limit fire growth that can compromise occupant and building safety, and firefighting operations. If FR wood loses its effectiveness over time, these assumptions may no longer be valid, meaning fire safety strategies and building compliance with regulations, which are built on these assumptions may also no longer be valid.
Building Codes, Compliance, Documentation and the Question of Service Life
From a regulatory perspective, the challenges associated with FR wood products are closely linked to how compliance is defined and demonstrated within existing regulatory frameworks. In Europe, reaction-to-fire performance is primarily regulated under the Construction Products Regulation (CPR) through harmonised standards and classification systems such as EN 13501-1 [7]. These frameworks are largely based on initial performance testing of products in a defined controlled condition at the time of placing on the market. Under the CPR, manufacturers are required to declare the performance of their products through a Declaration of Performance (DoP), supported by type testing and factory production control. However, the CPR does not explicitly require that declared fire performance be maintained for a defined service life. As a result, long-term degradation of fire performance due to weathering or aging is often outside the formal scope of regulatory assessment. The issue of durability is partially addressed in EN 16755[31], which introduced durability classes for FR wood products based on some leaching and weathering testing. While EN 16755 represents an important step forward, its application is currently not mandatory, and its results are more qualitative, as they provide limited insight into how performance evolves over time, just that a sample passes or fails a set criterion after a set amount of natural or artificial aging, and it cannot be used to say anything about a material or products service life or maintenance requirements.
Information on service life, expected durability of FR treatments, and required maintenance is frequently hard to find, completely absent or expressed in vague, non-committal terms such as “does not affect the service life of the wood”, “ages the same as untreated wood”, or “does not need any additional maintenance” and thus provides little practical guidance to designers, building owners or regulators and is counter to much of the evidence provided by the scientific literature. From a fire safety engineering perspective, this is problematic as this conflicting information hinders informed decision making – making it difficult to assess whether a proposed solution is robust over the intended life of the building, and places reliance on assumptions that do not seem to be supported by much quantitative evidence. Further, unlike active fire protection systems, FR wood products are rarely subject to routine inspection or maintenance regimes. Once installed, that’s it. Even if we wanted too, in practice, there is no straight forward way to verify the FR content, or fire performance of products once installed*. (*there are some projects that are looking at this issue e.g. [32,33].
Implications for Fire Safety Engineering Practice
For fire safety engineers, the challenges associated with FR wood materials and products highlight the importance of critically evaluating performance claims and being properly informed. Reliance on classification alone may be insufficient, particularly for external applications and long service lives. Where FR wood is proposed, engineers could consider questions such as:
- Has the material been tested after representative weathering or aging?
- Is there clear, creditable guidance and limitations with regards to maintenance and service life of the material or product?
- How sensitive is the overall fire safety strategy to the potential degradation of the material or products fire performance? (i.e. what if untreated wood were used?)
Based on our current understanding, it is likely that the engineering team may conclude that the answers to the first two questions are "No". Consequently, the design team should consider investigating the sensitivity of the facade material to the presence (or absence) of fire retardant (question 3). In many countries, this will demonstrate the clear need for a performance-based design approach. Addressing the first two questions better will require action between multiple stakeholders. Manufacturers/producers need to provide clear documentation, will explicit statements on durability, service life and maintenance requirements, and evidence as to how they were determined. Regulators should strengthen requirements for weathering and aging assessment, particularly for exposed or external applications. Within the fire safety engineering community, there is a need for diligence and critical assessment of materials, and for continued research into the long-term fire behaviour of materials such as FR wood, that are exposed to fluctuating environments, as improving our understanding of degradation mechanisms can support better products, guidance and more reliable design practices.
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