Reports raise concerns about volumetric modular construction’s fire safety risks

Two recently published reports have drawn attention to volumetric modular construction and the importance of undertaking further research into the method’s fire safety performance.
The first report, a joint research project between Harlow Consulting and Edinburgh Napier University, had been commissioned by the previous Conservative administration after the then government reaffirmed its commitment to build 300,0000 homes a year in England using Modern Methods of Construction (MMC) by the mid-2020s.
However, this policy commitment also acknowledged that the existing regulatory framework only related to traditional construction and therefore issues with MMC’s building quality, performance and safety could arise.
As a result, the Harlow Consulting/Edinburgh Napier University report, which was finalised in July 2022, but only published on 26 November, set out to investigate the potential performance and safety risks posed by one type of MMC – offsite manufactured volumetric [modular] construction (VMC) – and identify potential options to mitigate them.
As the Volumetric Modular Construction research report explains, volumetric [modular] construction is a type of offsite construction. The method involves the manufacture of fully enclosed, six-sided building modules – although they can be five-sided without a floor or roof – in a factory that are then assembled onsite, typically by stacking and/or joining them together. The advantage of this method is that once site installation is complete, most buildings will only require limited additional work before they are ready to use.
While the report notes there is ‘insufficient evidence to suggest whether a fire is more or less likely in a modular building compared with a traditionally constructed building’, it does point out that should a serious fire break out in a modular building, the consequences are likely to be more severe if combustible elements have been used in the voids and cavities through which fire and smoke can travel quickly. However, these risks can be mitigated within the design, manufacture and installation.
For this reason, the report says it is essential that fire performance and the requirements particular to modular construction are well understood from the outset and factored into the design.
"Risks can be introduced if there is no fit-for-purpose fire strategy, in the event of lack of continuity of structural and general fire engineering expertise throughout the project and where standard fire testing does not work effectively for volumetric products and systems," it adds.
Volumetric Modular Construction research contains a section on the fire safety risks posed by modular buildings, which reflect industry concerns shared in interviews.
Significantly, the report explains that firefighters may not understand how a modular building could behave in the event of a fire, which could lead to life safety issues.
"If fire gets into a void without effective compartmentalisation, the fire can continue to spread while firefighters are focused on rescue,’ it notes.
‘This risk can be exacerbated if combustible materials are used in the voids. Fire compartmentation could also be undermined by drilling the modules to install additional services…"
The joint report points to gaps in the regulatory framework in relation to volumetric construction and says that further research is required to gain a ‘clearer understanding of how materials (and combinations of materials/components) behave in modular buildings in the event of a fire’.
Significantly, the researchers warn that standard fire testing for volumetric [modular] construction is not fit for purpose.
"Evidence from fire consultants and the firefighting community suggests suitable fire tests do not exist for all forms of volumetric construction," adds the report, which highlights concerns about testing single units, rather than units operating together as a system, notably in high-rise buildings.
"It is not clear from existing tests how protection will be maintained – i.e., if there is failure in one module, how this affects the others. Walls could be tested in isolation – potentially presenting an unrealistic scenario as walls typically have sockets, light switches, and pictures."
Another reason for undertaking further research into how materials – and combinations – behave in modular buildings should a fire break out, is to feed the findings into the ‘development of a standard fire testing methodology for volumetric construction,’ says the report. Currently, there isn’t one that is directly applicable.
Many of the issues covered in Volumetric Modular Construction research are also present in a second report, which was compiled and published by the Health and Safety Executive (HSE)/Building Safety Regulator (BSR) in early December.
This second report, which identifies incidents related to this construction method drawn from a literature review, data search and a stakeholder focus group up to October 2023, further underlines the importance of undertaking more research to better ascertain the possible building safety risks related to VMC.
Although RR1203: The impact of permanent volumetric modular construction (PVMC) on building safety acknowledges that the evidence ‘does not support any conclusions about the relative safety of PVMC versus other construction methods’, the HSE/BSR publication does cite a wide range of evidenced examples of building safety issues that have the potential to impact on fire safety.
These include missing or incorrectly installed cavity barriers; defects found in fire doors and door frames; penetrations through compartment walls that are not correctly fire stopped; compartmentation compromised during modifications to fire safety barriers; and hidden voids, for example in walls, ceilings and floors.
The report, however, does make it clear that some of the building safety issues identified above could also relate to other (non-VMC) constructions. In addition, it says there is no ‘evidence to suggest whether the frequency of the issues could be higher or the consequences of the issues more severe in relation to VMC compared to other types of construction.’
In its Potential Pathways Forward section, RR1203 outlines some suggestions for mitigating the potential risks that could be associated with VMC such as ‘undertaking fire and structural testing of modules in a way that better represents the typical contexts of these modules’ because this would ‘help identify the consequences of any building safety issues in comparison to traditional builds.’
The report adds: "Testing should include timber PVMC to determine the nature of potential risks of VMC overlapping with the potential building safety risks of timber structures. Such tests might have implications for existing buildings and may inform any urgent concerns about PVMC."