Want to lower emissions? Look to modular homes

In the housing sector we hear a lot about the benefits of modern methods of construction – including that homes constructed off-site using modular build technologies have greenhouse gas emission reduction benefits.

At Altair, our clients utilise off-site manufactured housing or “OSM”, a type of modern method of construction, to develop more homes quickly. But the OSM supply chain is developing, and the business case for cost savings is weak. So many of our clients are looking at OSM’s added value, including its sustainability benefits. But why is OSM development more environmentally friendly than traditional development?

This article breaks down the main ways OSM helps affordable housing developers build low-carbon homes and shows that sustainability is not just about carbon emissions “in use”. The key is looking wider: to the materials, workforce activities and characteristics of manufactured versus bespoke housing products.   

What is OSM?

OSM for housebuilding is a Modern Method of Construction (MMC). The term describes a construction technology that uses fabrication of elements prior to installation on the site of intended use.

At Altair, we think of OSM along a spectrum, seen below.

Figure 1: Spectrum of OSM housing

On the left side of the spectrum is construction with most components erected on-site. Traditional “brick and block” construction sits at this end. On the right side of the spectrum is construction with most components erected off-site. Modular or “3D volumetric” housebuilding sits at this end.

The spectrum approach allows us to analyse the relationship between OSM and greenhouse gas emissions. As a rule of thumb, the more components manufactured off site (e.g. OSM), the less carbon emitted. Here are five reasons why:

1. Manufacturing reduces waste

In traditional, bespoke construction, contractors may order and transport more materials like wood, cement and bricks than required or used. Off-cuts and scraps are recycled or go to landfill.

OSM modules, comparably, are standardised and allow for precise materials orders. Once in the factory, large machines cut materials like cross-laminated timber with scientific accuracy so off-cuts are minimised. This is in the business interest of manufacturers, since it reduces materials costs.  

Some OSM manufacturers do not have the standardisation or machinery to achieve this benefit fully, so check with your supplier about how they manage wastage and scraps.

Figure 2: On-Site Construction can lead to waste from unused materials or scraps

2. Less time on-site, less carbon emitted

OSM offer benefits related to the speed of construction and less time spent on-site. Manufacturing principles speed up the process by standardising the factory line. Factories also help prevent delays due to weather and may enable 24-hour or shift working. Condensing the time spent on-site reduces the number of trips by workers to site by car or van. Less trips by car or van reduce congestion and air pollution in the local area.

OSM uses standardised processes with all materials located in the factory. This keeps the majority of activity in the factory and reduces the need for ad-hoc visits to the builder’s merchant.

There is a possibility however that workers transitioning from commuting to urban build sites to mainly rural factories may increase worker travel. It is therefore important to check with suppliers where their factory is and how workers commute.

4.  Materials used have less embodied carbon

While not explicitly a benefit of OSM, OSM technologies tend to use materials which have less embodied carbon, like timber frame and wood laminate materials. Embodied carbon is the carbon footprint of a material. It considers how many greenhouse gases are released throughout the supply chain and is often measured from cradle to site. Some also consider the longer-term impacts of materials, like heaviness, lifespan and how easily they can be disused and discarded.

Concrete for example, has a high embodied carbon due to the energy it takes to produce, transport and erect it.

Figure 3: Swan’s NU build Factory in Basildon, Essex uses cross-laminated timber. 

Timber, however, is renewable and lightweight, and can be easily recycled. Research suggests buildings with moderate to high timber use have lower embodied carbon than their traditional, brick-built counterparts.

Not all OSM uses timber. Some manufacturers also utilise steel technologies, which have a much higher embodied carbon by weight than concrete. However, other considerations should be made. Steel may have a much longer lifespan than timber and steel manufacturers are designing frames to be reconfigured or recycled after the “first” lifespan of the building.

Another crucial element is design. The building’s design, including how much timber or steel it uses, greatly impacts embodied carbon. Check with your supplier about how their designs maximise use of low-carbon materials.

5. Less heat required 

OSM homes benefit from air tightness due to precise manufacturing and assembly of modules using digital testing of air leakage.

The manufacturing principles allow for continuous refinement of the assembly process, and so with each factory run of a specified design the manufacturer can make adjustments to improve joinery and air tightness. In turn this will support a fabric first approach, helping to reduce the bolt on technologies required to drive further emission reductions in new homes. In the long term, asset management may find properties less resource intensive from a cost and time perspective.

Alternatively, traditional construction does not benefit from this standardisation and the data that comes with factory-built systems. Instead, every home constructed on-site is unique, and gaps caused by human assembly may result in higher heating requirements resulting in increased building emissions and bills to residents over time. In a society where fuel poverty is an ever-growing issue, this is not a desired outcome.

OSM reduces carbon emissions

With big targets to deliver more homes, more quickly, OSM is a growing development approach with clear benefits in helping organisations contribute to carbon emission reduction targets.

With experience in traditional and modern methods of construction for building new homes, Altair helps organisations review their development programmes to identify which sites are most suited to OSM. Once identified, we also help organisations determine the business case for OSM by assessing impact on financials, timings and carbon emissions. Contact us today for more information about how we can support your organisation.

Written by Cassidy Curls, Consultant at Altair.