March 1, 2023

Does offsite mean sustainable?

by Mark Thorley
Example H2


Modern methods of construction have long been discussed as a forward-thinking solution to the building sector’s problems, but are reported benefits around sustainability, including reduced waste and fewer transport emissions credible and are there any drawbacks?

With the climate crisis now front and centre in many peoples’ agendas, the spotlight is on the built environment and construction, which together are responsible for 39% of all carbon emissions in the world, according to United Nations data. Operational emissions (energy used to heat, cool and light buildings) account for 28% of those emissions and embodied carbon, the ‘upfront’ emission associated with materials and construction processes throughout the building lifecycle, account for the remaining 11%.

Efforts to address these challenges will be crucial if the UK is to achieve its legally-binding commitment to net zero carbon emissions by 2050, and among the solutions on the table, MMC is gaining traction for its reported ability to cut both embodied and operational carbon. Crown Commercial Service, the biggest public procurement organisation in the UK, is backing off-site approaches, explaining the benefits for sustainability in a recent white paper. “There is an increasing acceptance of the importance of eco-friendly solutions and the need to embrace MMC,” it said in a recent blog, adding: “Far from restricting economic opportunity, modular buildings have the potential to be an exciting and expanding area of growth, as well as offering a host of benefits to end users and a more sustainable way to build.”

Another organisation backing the approach is NHBC Foundation, whose MMC study from 2018, found that 53% of developers considered sustainability a key factor driving the uptake of MMC. Efforts to drive down the use of virgin materials and cut the volume of waste generated by construction and demolition will be critical in the years ahead, especially considering that around half of the urban environment needed by 2050 is not yet built.

Factory settings

The controlled environment of a factory has been shown to optimise production to minimise waste. A study by sustainable construction and regeneration specialists KLH Sustainability concluded that modular approaches result in a 45% reduction in material use and over 50% reduction in waste generation when compared to traditional. Opting for offsite solutions can result in fewer deliveries to site. A report by the Waste & Resources Action Programme found an average of 83% less site traffic on offsite developments compared to onsite, cutting carbon associated with transport. However, some industry critics dispute this type of claim (see below).

Moving into operation, prefabrication has been shown to improve the performance of buildings, thanks to factors such as finer tolerances than conventional construction, resulting in more consistent levels of airtightness and U-values. For example, housing developer Keepmoat Homes has estimated that homes built with MMC require 20–30% less energy to heat. Increasing scrutiny of building whole life carbon has led to demands that buildings are designed with reuse at end-of-life in mind to further increase resource efficiency.

Clients and main contractors may look in future towards off-site systems designed for deconstruction and disassembly, including kit-of-parts systems that can be easily taken apart and re-appropriated on a newbuilding. Regeneration company BeFirst, owned by Barking & Dagenham Council, is showing the direction of travel. A design guide it recently produced with Mæ Architects calls on architects seeking work on its five-year home delivery programme to ‘strive’ for standardisation and MMC, expressing a preference for “designing for disassembly, with parts which can be re-used.”  


Although MMC is widely viewed as more environmentally friendly than traditional approaches to construction, some industry critics claim that certain aspects are either unsustainable or require more refinement to limit impacts on the climate. Effective recycling of building materials at end of life depends on the ability to separate products recovered from refurbishment or demolition projects into distinct, contaminant-free waste streams. This ensures that materials sent back into manufacture are reliable and that new products meet technical performance criteria. But offsite systems made in factories often incorporate a larger number of composite elements, which by their physical nature are harder to break down at end of life, potentially making them more prone to being treated as waste.

A spokesperson at a prominent organisation targeting sustainability in the built environment commented: "When materials are bonded together it is really difficult to do anything with them. There are studies that show offsite reduces waste during the construction phase, but if you talk to some demolition contractors, when they come to take down these buildings, recovery rates have been as low as 60%, compared to 90-95% on a regular building.” Certain other aspects of the pre-fabricated approach may elevate carbon emissions during construction. On a traditional build, products and raw materials are simply shipped to site, but offsite may require an intermediate step whereby items are shipped from a production facility to another factory for assembly before being sent to site, increasing carbon emissions associated with transport.

A lean Just-in-Time ordering system can mean more frequent deliveries and when trucks haul large finished volumetric sections, even though the number of deliveries is reduced, a large proportion of the load is essentially air, raising questions over actual efficiency. More industry research is required to understand the journey of an offsite product to capture its true carbon impact. Sustainable design also encompasses thermal and acoustic comfort, a particular concern in a warming climate. Offsite’s focus on airtightness and mechanical ventilation may accelerate overheating problems if factors such as the local climate, site orientation and occupier behaviour are not properly considered during design. New UK Building Regulations for overheating and ventilation, due to come into force in June, highlight the importance of the issue and measures needed to address it.

With policy makers, clients and other actors in the sector increasingly focused on zero carbon and the climate emergency, we can expect the sustainability credentials of MMC to come under even closer scrutiny in the critical months and years ahead.