An eco-friendly prefabricated tiny home is a great way to literally reduce the footprint of your home but, as I’ve explained elsewhere in this series, a tiny home is not necessarily more eco-friendly than a standard sized home.
How about prefabricated buildings in general? Are they inherently lower impact and a better option for the eco-friendly homebuilder? Let’s look at the environmental impact of conventional construction compared to prefabricated homes.
The health and environmental impact of residential buildings
Before digging into construction techniques themselves, it bears examining why it matters how we build buildings.
The structures in which we live, work, and play collectively account for 40% of energy use in the United States, according to the Department of Energy. Buildings also represent the largest single end-use of energy and emitter of greenhouse gases (GHGs).
In addition to the energy involved in construction and during the use of buildings, to determine how environmentally friendly and healthy a building is, we need to think about the kinds of materials and chemicals used in its construction. Building materials can significantly affect the quality of the air we breathe indoors, so much so that toxic adhesives, polishes, stains, insulation foam spray, and fire retardants can cause respiratory difficulties in some occupants. In addition, poor building design can mean a leaky envelope, damp, and resulting mold and mildew. These can also cause serious acute and chronic health problems.
In a small residential space, such as a tiny home or microhome, flaws, oversights, and a lack of care over building design can quickly cause problems. However, prefabrication and modular home construction can sometimes support a tighter building envelope and the use of healthier materials because everything can be made in a climate-controlled factory rather than having materials being exposed to the weather for days, weeks, or even months at a construction site.
Life Cycle Assessments for conventional and prefabricated homes
To help us figure out what’s what when it comes to construction, homes, and the environment, researchers use something called a Life Cycle Assessment (LCA). The LCA of a home will include energy used for heating, cooling, lighting, equipment, and appliances as well as for construction and the production of building materials.
Currently, the biggest impact a home has on the environment is during the time it is occupied, rather than its actual construction. As homes incorporate more renewable energy technology and become more energy efficient in general, however, construction will represent a growing proportion of a building’s environmental impact.
In a 2010 study, Gustavsson and Joelsson estimated that improvements in design and the adoption of renewable technologies could mean that material production and construction account for 60% of a building’s life cycle energy consumption. This recognition would then push developers to further reduce the impact of construction and material production.
Prefab eco-impact may be easier to estimate
In some ways, it is simpler to estimate the energy costs and environmental impact of prefabricated construction because more of the steps of building a prefabricated home are repeated across several home designs and in a controlled, systematic fashion. In a site-built, conventionally constructed home, costs are often less predictable and unique to the home design, building site, contractor, and laborers.
One comprehensive analysis looked at the environmental impact of a typical residential home using data from several modular construction companies and conventional homebuilders. The authors found that the average impacts were far less for modular construction in every category, including material production and transport, off‐site and on‐site energy use, worker transport, and waste management. That said, there was “significant variation among the individual projects and companies”, suggesting that there are ways to reduce the environmental impact of both conventional site-built homes and modular homes (R).
Modular building is particularly good for reducing the environmental impact of construction. This prefabrication process can be used to build single-family homes, multifamily housing, hotels, dormitories, and commercial and retail structures. In modular construction, discrete section of a building are created off site and are transported to site for assembly into the final building. In some cases, an entire modular home may be constructed for delivery to site, whereupon the only remaining tasks are to hook up the home to services (or not, if the home is to be off-grid).
In modular construction, most interior and exterior finishes are applied in the factory. Modules are typically 80-90% complete upon arrival at site. This means that the modules can be created while the site is being prepared, making for excellent efficiency within a project. In some cases, this may reduce construction times by 25-50% (R).
Problems of conventional construction
What are we talking about, then, when considering environmental problems of conventional construction techniques? Well, some of the key environmental impacts come from:
- Construction equipment
- Waste production
- Water consumption
Heavy use of diesel construction equipment, the transportation of heavy materials, such as concrete, to a building site, and construction of a structural frame on site are some of the biggest contributors to energy consumption in conventional construction.
On-site heating costs are another factor affecting the environmental impact of construction. Building a house on-site in colder temperatures can increase energy usage quite significantly over prefab builds that have a shorter timeframe and are carried out in climate controlled factories that are energy efficient. Research suggests that in the mid-Atlantic region, for example, heating is required on-site for at least half of a construction project. Sometimes, this heat will need to be continuous, i.e. not just when workers are on-site working, in order to protect interior finishes and other materials before the building envelope is sealed.
Advantages of prefabrication construction techniques
In off-site construction, less solid waste is produced, there’s a lower need for transportation energy (less commuting and trips to suppliers for laborers, for instance), reduced material consumption, and fewer greenhouse gas emissions.
In one case study of two homes in Michigan (a 1,456 ft2 modular home and conventionally, site-built home), two and half times less solid waste was generated using modular construction off-site, compared to on-site construction (R). The same study also noted that the “total life cycle energy consumption for modular home is 5% less than the conventional site home”. And they found that the “total global warming potential for the modular home is 5% less than the conventional site built home”.
In another study, a factory built home required more materials compared to a site-built home but performed better energy-wise and the energy and carbon debts associated with the extra materials were ‘paid off’ within six years (R).
Compared to conventional builds, a prefabricated design can also help reduce confusion on the construction site. With the design ‘frozen’ at an early stage of the project and many elements of the design having already been tested dozens, if not hundreds, of times, laborers are better positioned to do the work safely, quickly, and accurately. The result is a higher-quality building with greater integrity of design, form, and function.
Construction waste takes many forms. It can include building materials and waste created when preparing a site for construction. Examples of construction waste include:
- Insulation materials
- Electrical wiring
- Tree stumps, branches, etc.
- Dredging materials
A large proportion of construction waste for on-site builds comprises materials that were damaged or unused during construction. Untreated wood delivered too early to a site may become infested, warped, or otherwise damaged, for instance, while cement and other materials may get wet and become hardened and unusable. In a conventional build, an estimated 10-15% of materials are wasted, according to observational research, despite industry claiming a much lower percentage (2.5-5%) (R). Either way, these numbers suggest opportunities to reduce construction waste.
Construction waste materials may contain hazardous substances such as lead, asbestos, propellants, flame retardant chemicals, and other toxic chemicals. Plasterboard is especially hazardous as it breaks down in landfill to release the toxic gas hydrogen sulfide.
Because of the greater degree of control in prefabricated building projects, it is easier for companies to separate out waste materials ad divert these successfully from landfill. Wood off-cuts, for instance, can be reused in other areas of a build or in another project at the factory, with such use planned for in advance. This can also help lower the costs of multiple projects.
Building waste must be disposed of in accordance with local regulations. Because there are costs involved in the proper disposal of waste building materials, some construction companies choose not to follow these regulations and illegally dump materials. This can lead to pollution of waterways, soil, and air, cause fire hazards and other health and safety problems for humans and the environment. Thankfully, close regulation and fines that can rise to tens of thousands of dollars for improper disposal of construction materials, especially those containing asbestos, have helped curb this practice in many places.
How to reduce construction waste
In prefabricated and modular building, waste can be reduced through the use of demountable and reusable framing systems. This helps reduce reliance on plywood and dimensional lumber for framing.
In all construction projects, improvements in design and manufacture can enable materials to be transported with minimal packaging. That packaging, as well as other materials, can be carefully sourced to ensure it is recycled, reusable, and recyclable.
With a prefabricated home, the design has typically been tweaked and refined to create a formula that results in much less material waste than in conventional construction. It is much easier to calculate how much material is needed in advance, and to make the best use of those materials so as to minimize waste. Indeed, knowing the kinds of off-cuts you’ll have means you can actively repurpose those materials for us in other areas of the design, rather than buying stock lumber and other materials new for each section.
During renovations, materials such as doors and windows can be donated, sold, or diverted to other projects.
In some ways, prefabrication can seem to be less efficient, materials wise, than on-site construction. For example, marriage walls are required in modular construction so that modules can be safely transported to site from the factory. These marriage walls (or ceilings), which join rooms together can add around 25% more wood mass to a building.
Despite possibly using more materials in the final building itself, prefabrication and modular building may use less material, including waste, throughout a building project and use this material more efficiently. This is because most modular building companies build several homes simultaneously, allowing them to order materials in a cost-efficient fashion. And, because off-site builds can make use of digital fabrication equipment, cutting is more accurate than on-site cuts, meaning fewer off-cuts and less waste. Modular and prefab companies also tend to wait to begin construction until all materials are at the factory, which helps reduce over-ordering and keeps delivery and transportation energy use to a minimum.
In conventional construction, issues with storing materials on site (such as theft, spoilage, and vandalism) mean that homebuilders usually order materials in stages and for one building at a time. On-site builders also make more frequent forays to local building supply stores to acquire materials, further adding to energy use through transportation. This ‘order as you go’ approach isn’t as efficient, energy wise or cost wise, and can greatly increase material waste and the timeframe of a project. Indeed, some researchers report that on-site builders have the habit of ordering a surplus of 5-15% to account for materials wasted during the build (R).
In some ways, however, an ‘order as you go’ approach may be the smartest option for some site builds. That’s because scheduling delays are common with on-site construction, and these delays could mean that materials ordered too soon end up being wasted.
Scheduling delays can also contribute to increased transport costs associated with a conventional site-built home. Indeed, some researchers suggest adding 50% to estimated worker miles for on-site builds because of such delays and for repetitive transportation during construction (such as multiple trips by site inspectors).
In a prefabricated build, the mileage and costs of transporting modular or prefab pieces to the site is likely higher than the initial cost estimates for a conventional build, but cost estimates are more likely to be accurate for an off-site build and can take advantage of certain efficiencies. In a conventional build, workers need to commute to site, as do individual contractors, and inefficiencies in construction mean multiple trips to hardware and supply stores may be necessary throughout the project.
Toxic air emissions and greenhouse gases
Construction is a huge contributor to toxic air emissions and often involves the use of toxic materials. In one study, construction accounted for 57% of toxic air emissions and 51% of hazardous waste generated in a building’s life cycle (R). Conversely, the construction phase of a building’s life cycle only accounts for around 2% of energy consumption and 1% of greenhouse gas emissions (R). The contribution to emissions of carbon monoxide (7%), nitrogen oxides (8%) and particulate matter (8%) are, however, higher for construction.
In an analysis of three modular and five on-site construction companies, on average, greenhouse gas emissions from conventional construction were about 40% higher than for modular construction (R). This figure may be even higher across the industry, however, given that one of the modular companies in this analysis had significantly higher emissions than all other companies (site builds included) because of its rural location and increased daily commute for workers and because they heated their factory with fuel oil.
In terms of absolute differences in greenhouse gas emissions for these companies, on-site construction produced nearly six metric tons more carbon dioxide equivalents (CO2-eq) per 2,000 sq ft home compared to modular construction. Reducing unnecessary worker trips, implementing a ‘no idling’ policy for machinery, and using efficient methods of temporary heating can all help lower the environmental impact of on-site construction.
Prefab, Modular, and Industrialized Building Systems
In Malaysia, government and industry are working together to reduce the environmental impact of construction. One of the major tools they are using is IBS, or Industrialized Building Systems. IBS is a construction process that use techniques, products, components or building systems which involve prefabricated components and on-site installation.
Some examples of IBS include assembling precast elements such as floor slabs, in-filled walls, bathrooms and staircases in the factory, thereby reducing labor on-site and increasing productivity. Putting these pieces in place on site involves three separate trades (steel bending, formwork fabrication and concreting) and is both labor intensive and inefficient, while also increasing energy use, greenhouse gas emissions, and transport costs.
IBS can also reduce waste production. In one study, total waste generated by a conventional building project was 54.6 tons 100 m-2 compared to just 1.5 tons 100 m-2 for a similar prefabricated/IBS project. In addition, 94% of waste generated at the IBS site was reused and recycled, compared to 73% at the conventional build site (R).
Durability is another factor to consider when assessing the environmental impact of a home. As with all products, the more durable your home, the more eco-friendly it typically is, because you don’t have to rebuild or renovate and sink yet more materials and energy into maintaining your home.
Prefabricated homes, especially modular homes, tend to be more durable than conventionally built homes because they incorporate things such as marriage walls and are built to endure transportation to site. Prefabricated and modular homes are also often durable in another way: they can be designed with flexible living arrangements in mind.
For example, FabCab, a prefabrication design company, typically designs flexibility into a home, meaning that its layout can be modified easily to fit whatever changes in your life. This might mean adding or removing room divisions as your family grows or as children leave home. Or, it may mean building all on one level so that you can age in place and not need to go through costly renovations or a complete rebuild or relocation.
The bottom line
In summary, a home built using prefabrication techniques, and that is well designed and energy efficient, is arguably the most eco-friendly housing option in the long-run. In most cases, prefabrication is much less wasteful than conventional construction, but you still have to be careful to choose a company that uses green-building techniques and avoids the use of toxic chemicals and materials that contribute to global warming.