Modular building has been described as ‘the creation of discrete volumetric sections of a building’, which are then transported to a site 80%-90% complete and assembled into a complete building upon arrival. The modules are fabricated offsite in a factory where there is no exposure to the weather. The construction time of the building is as a result, said to be reduced by 30%-50%.
The conventional means of on-site construction however differs from modular building in that the materials are directly delivered to the site and a ‘cut-as-you-go’ approach is used to put together a building. A modular building we should however note is not of lesser quality to a site-built home since the same materials and design are used. What simply differs is the process they each go through to arrive at the same result.
Despite promising opportunities to gain energy and material efficiency, an area that has not been fully discussed is that of the environmental tradeoffs between prefab construction and on-site construction. The diagram below charts the processes and material flows that occur during the construction phase of a building and questions of their impact on the environment begin to arise.
From the diagram above, the life cycle assessment tool becomes useful in showing us how either construction techniques differ in the production and transportation of building materials, the construction process in itself, whether workers are transported to the factory or job site, and the way waste is handled.
Waste management is one of the critical issues that arises from this discussion and it is evident that either means of construction handles it differently. Due to shipping restrictions on public roads for modular units, redundant walls and floors (also referred to as marriage walls) are needed to maintain the structural integrity of the modules. These extra walls are said to represent roughly 25% of additional materials to construction. Nonetheless, modular building does a great job at ensuring that the exact quantity of material is ordered for production and ordering is done in bulk, which means less wastage is occurring. On the other spectrum, on-site construction adopts a ‘order-as-you-go strategy which oftentimes leads to over-ordering with excess material mostly left unused and extra trips being made to make the purchases.
Energy use is another big category that highlights the different impacts that prefab and on-site construction have on the environment, and the diagram below charts the flows that occur.
Upon hearing that fabrication of a building is being done offsite in a factory in some location, it is quite easy to assume that prefab building will require a lot of energy. Energy is after all needed to run the equipment for material fabrication, provide lighting, heating and cooling for the factory. As earlier discussed, extra fuel is needed to provide transportation for the modules to their final site. On-site construction on the other hand, requires energy to power the tools and equipment like cranes, fuel to run generators, and occasional heating to protect interior finishes of the building. Energy is also required to run the on-site offices of the contractors and idling of these machines contributes to the greater energy consumption on site. The big energy gobbler with on-site construction mainly arrises from the issue of worker transport to site. With workers living near and beyond, it is inherent that a lot of fuel will be required to transport them to and from the site each day of the construction schedule.
Taking into consideration all these factors, it has been established that impacts from modular construction are on average lower than on-site construction. The conventional means of construction results in 40% higher Green House Gas emissions than modular construction, and is as a result of the energy use on-site and worker transport to site (direct combustion of fossil fuels). Nonetheless, there are some strategies that can be employed to even make both modular and on-site construction more efficient.