The Green Wall Question

Architectural practices, keen to utilise the many benefits of a green wall on their project are faced with a challenge: Add a green wall and you add the increased risk of fire spreading across the outer surface of your building. So how does that work with compliance, regulations and guidance documents? What about the additional design considerations for tall buildings?

Green Walls, also known as Living Walls, are finding their way onto larger and taller buildings. However, this trend might seem contradictory considering the increased scrutiny of external cladding products.

The issue remains unavoidable: Plants are combustible. The frequent media coverage of wildfires, and their devastating consequences, would reinforce the notion of rapid-fire propagation in plants that is then very challenging to control.

Designers should therefore carefully assess whether the pursuit of the numerous benefits associated with greening, is overshadowing the additional inherent risks related to potential fire propagation, inherent to Living Walls. With added significance in the case of tall buildings, a comprehensive understanding of the risks is essential.

So how then are green walls being permitted for use when other façade products that would very likely perform better in reaction to fire testing, are prohibited for use over certain heights and in particular locations?

An examination of the fire certification and test reports available from the main system suppliers uncovers a narrative that is not as widely recognised as it ought to be.

It reveals that green walls cannot achieve a full reaction to fire classification of Class A to E when tested in accordance with BS EN 13501-01, that would enable designers and review bodies to determine if a product meets the current guidance as shown in Approved Document B, with regards to height and building use.

Yet, surprisingly, some prominent system suppliers are boldly and unashamedly claiming that their system holds a fully tested classification, (typically class B), despite this being unattainable. Unfortunately, this claim is being mistakenly accepted as factual.

The Fire Engineering Assessments (FEA) provided as evidential support by suppliers, reveal that certain aspects of the formal testing criteria, such as conditioning and crucial dimensional consistency are disregarded. Instead, green walls are soaked and completely saturated before undergoing the key laboratory fire testing which has a significant impact on the results.

This “Ad-hoc” testing approach, as described in detail in each FEA, is by itself questionable, and the “wet and test” method to produce a so called “likely” reaction to fire rating is strictly prohibited for other façade products.

In addition to this, the current FEA approach to fire performance for living walls generates supplementary requirements (as well as additional costs) for both the initial design, and then throughout the life of any living wall installation.

Any installation that is approved for use on tall buildings, based upon the “likely” classification opinion of an FEA document, must also be: (a) Managed, (b) Maintained, and (c) Continually Monitored to remain exactly in the “Perfect” condition, mirroring what was tested in the lab. Otherwise, the Fire Engineering Assessment is deemed invalid, and the reaction to fire performance of the installed living wall is unknown.

Despite persistent inquiries and attending supplier CPD sessions, little evidence has been found to show that all of the stringent and fundamental criteria outlined in the FEA’s is being given the required high priority or being effectively and correctly communicated to designers by the system suppliers.

Furthermore, recently published videos showcasing extended fire testing of living wall systems have unsurprisingly shown that plants burn well. Even for green and healthy vegetation, as the heat from the fire source dries parts of the wall above, the spread can be very rapid. This has strongly highlighted significant shortcomings in the existing testing procedures. These videos would suggest that the third-party Fire Engineering Assessments, used to permit Living wall use on tall buildings, rely on limited data from small scale tests, which do not necessarily reflect real-world installations.

While the importance of maintenance is championed, suppliers are not addressing the key requirement of accurate monitoring of moisture levels within the installed plant modules. This monitoring obligation is being explained as “you could do it”, rather than “You must do it”. The stipulation of constant monitoring of moisture levels is critical to fire spread, but this is not typically being implemented on each and every installation, as is required by the FEA.

Side by side testing carried out by Prof Wojciech Węgrzyński, Jakub Bielawski and the team @ ITB shows that there is a significant increase in fire spread across living walls with a marginally lower moisture content or for a wall rapidly dried and fuelled by the wind.

This confirms that a detailed and specific green wall testing regime is very much needed.
The videos indicate that the flames quickly spread across the face for each of the samples tested, with the differences being limited to time of flame spread. These are much more realistic conditions for green walls when used externally and at higher levels. Within minutes of the tests starting, there was significant fire spread as shown in the images below.

This raises additional doubts about the practice of omitting crucial aspects of BS EN 13501-1 testing and saturating a wall to attain a so called “Class B” rating which has clearly opened the door (and the market) for use of green walls on tall buildings. The recent Luton MSCP fire has also raised doubts about the common practice of green wall installations on car parks via an apparent Class B and table 12. Compared to other façade products achieving a full class B rating, living walls have been shown to be significantly inferior in preventing fire spread. It is evident: Green walls have the potential to ignite and burn effectively.

As the reaction to fire properties of the Living wall system can rapidly change as demonstrated in the larger scale tests of both wet and dry walls, the stated “Likely class B” designation for these systems would appear to be dubious.

The lack of adequate monitoring:

Construction products given a fire classification rating are expected to perform as tested in a fire even if fully dry. As living walls will perform differently dependent upon the moisture content, the FEA will stipulate that they must be “constantly monitored” and irrigated as required to prevent them from falling below the minimum permitted level. The minimum stated levels for saturated Ad-hoc tested walls varies dependant on the growing medium used with some systems being as low as 40% minimum moisture content (soil) and some being closer to 70% of volume (mineral wool).

The lab method of doing this (measuring by weight of modules before and after saturation) is not practically possible for almost all project installations, so therefore some other method is required to confirm the actual percentage level. It is worth noting that test labs simply set the minimum levels, but no guidance is given as to how this should be achieved.

The current monitoring method that the main system suppliers suggest is computerised management of flow rates from the irrigation system, and this is now typically standardised across most of the available marketed systems. This method does not confirm the exact moisture level of the substrate itself or give accurate readings of moisture content percentage levels (%MC). Flow rates can be finely tuned with published daily data readings being produced for the volume of water being pumped to the wall and this can be remotely adjusted if required. While this form of monitoring is essential in plant health, it falls short in meeting the requirements of the Fire Engineering Assessments that set a minimum level.

This is because all current monitoring happens primarily ‘Kit side’ as per location (B) on the below diagram which represents a typical installation. Flow rate monitoring happens on the irrigation feed pipes for the system, usually located in the plant room or on the supply lines.

While a potential issue with a leaking, damaged pipe or blockage may be flagged up by a pressure drop indication when using this method, there is not an accurate method being applied to most installations to constantly monitor the wall itself, within the plant modules. This should happen at location (A) to confirm an actual moisture figure, (%MC) which is a clearly determined stipulation of the validity of the supporting FEA document.

As the fire engineering assessments set out a minimum level in percentage terms that the installed wall must achieve (as per testing), again logic would suggest that to understand if this minimum level is being met for each project, the actual moisture level (%MC) in the wall must therefore be known. It is typically not.

Advanced moisture sensor technology is commonly employed in commercial agriculture and vertical farming to manage plants. However, through research and conversations with suppliers, this technology is generally not being implemented in most living wall installations. When the moisture percentage is not monitored accurately, the probable fire reaction classification for the wall also remains uncertain and potentially non-compliant.

For designers, it is important to understand that the reaction to fire of a living wall system can be impacted by local environmental conditions such as dry weather and wind. As the moisture levels are reduced, so does any protection that this gives against fire spread.

This may not always be obvious to spot.

Take the Living Wall Challenge: Just how wet is your wall?

If you have been involved in a project with a living wall that has been approved for use on a tall building via this non-standard Ad-Hoc testing (as detailed within each systems FEA), then the likelihood is that this is fully dependent upon the moisture content remaining above a stated minimum level.

If so, ask what the moisture content (% level) of the living wall is today? This should be known by whoever is managing the wall and should be relatively easy to find as it is the critical element of the Fire Engineering Assessment.

Is this level above the minimal levels in the systems FEA? These documents should also be readily available.

If this information is not readily available or known for your project, the FEA for the system is likely invalid and the reaction to fire classification for the installation is therefore not known.

Myth busting:

There have been various ideas suggested to address the logical fire concerns that come when putting known combustible materials onto the outside of buildings.

Some have these appear to have ‘stuck’ and have been incorporated within recently installed green walls.

But do they satisfy the current regulations and guidance or are they just a good idea?

“We can hook up the Living Wall irrigation system to the fire alarm”.
The idea here is that in the event of a fire, the buildings fire alarm activating would send a signal to the irrigation system. This would then irrigate or “Drench” the wall which would then reduce its likelihood to catch fire. The obvious question here is: where is the testing to prove that this works? Any building, or part of, that is reliant on a sprinkler or other fire suppression system is required to be designed by qualified and experienced specialists and be certified and commissioned. Green wall irrigation systems are pressurised drip systems to evenly distribute water and nutrients to the plants in a controlled manner. They are not proper fire suppression systems. While there is a likely benefit, this is not a requirement of the regulations for external facades, nor is it an alternative pathway to allow products that fall short of the test standards to be used. Also, if the wall meets the required reaction to fire performance in the first place, why would this even be suggested by suppliers? The suggestion to do this in fact further highlights that the inherent risk with living walls becoming a source for fire spread, is a major problem.

“We have done this on dozens of projects, of similar height and size to yours”.
While this once held much weight, the industry must now be fully aware that just because something was done (or permitted) on a previous project, this is no guarantee that those involved in the approval process got everything right, especially with regards to fire.

It is imperative for all stakeholders in the design process to thoroughly scrutinize and question both living walls and the associated documents submitted for approval, ensuring a comprehensive understanding of the potential risks.

Suppliers should refrain from overstating the “Likely” classification achieved through the Fire Engineering Assessment and ad-hoc testing process in both discussions and publications.

Designers must fully understand the risks and FEA stipulations to be able to make informed design decisions. They must comprehend the implications of deviating from standard fire testing procedures of products and the complete reliance of moisture within the system to limit fire spread.

“A healthy wall is a safe wall”.
This simplified claim would suggest that maintenance contracts are therefore the silver bullet for fire safety.

While regular maintenance is essential for plant health, it alone falls short of showing regulatory compliance. Current maintenance contracts include live flow rate monitoring of the irrigation system (during work hours) and consists of a monthly or a bi-monthly visit to do a full visual inspection of the installation and irrigation equipment. This tends to be quite thorough and includes the replacement of any unhealthy or dead plants as and where necessary. It helps, but still falls short of carrying out the FEA stipulation of “constant monitoring of the moisture levels” to confirm the likely reaction to fire performance of the system. Variations in weather and other factors can influence the health of the wall, even over a short timescale, which is why maintenance is needed in the first place as some parts of the wall may not thrive. As demonstrated by full scale testing, even healthy-looking walls, when subject to a realistic fire load, will quickly dry out, enabling fire to spread rapidly over the surface. The more moisture, the slower the spread as the plants will take longer to dry but overall, spread is only slowed rather than “resisting fire spread” which is the overarching intention of B4.

This simplified assertion being put forth by suppliers that “a healthy Living wall is a safe Living wall” currently lacks substantial evidential support. This notion parallels the “it does not burn, it just chars” claim made in relation to insulation products, now proven to be misleading. For designers tasked with navigating the complexities of these systems and fire safety, caution must be taken with such claims.

The problem with failure.

Green walls can fail and inevitably we will see increased examples of this as more are installed. So where does that leave us?

Living wall system suppliers are rightly passionate about their installations looking good and will endeavour to secure the maintenance contract.

Unfortunately, they are also in a position where they cannot guarantee that their maintenance contract will be renewed or that a client will not “turn off the tap” for some unknown reason.

This creates a unique and bizarre situation with regards to regulatory guidance for Living walls.

The reality is shown below in a recent image of a Living wall placed side by side with an earlier image showing it in a healthy condition.

Once the jewel of this mixed-use and multi award winning community project, for whatever unknown reasons within the last 12 months, the irrigation system appears to be switched off and no maintenance has been taking place.

While it is undeniably disheartening to witness these outcomes, this project along with some other prominent and more widely publicized cases, do serve as a reminder that although failed Living Walls may be relatively uncommon, there remains the potential for Living Walls to deteriorate and dry out, factors beyond the control of system suppliers.

At times, either the entirety of a Living wall or a specific localized sections may not thrive, and the result will be a highly flammable outer surface.

This situation presents a challenging dilemma concerning regulatory guidance, necessitating further clarification to determine responsibly for allowing parts of a building to deviate from the requirements set out in Approved Document B.