The numbers are in (3)

Part 3 of a 5-part series in which we quantify the value the Flexi House system can provide Kiwis looking to build. This blog explores the embodied carbon of a 185m2 Flexi House and how we’re doing more for the planet.

At Flexi House, we’re helping Kiwis break free from the bare minimum by building climate-positive homes that are better for the planet.

How? By specifying materials that sequester more carbon than they emit.

The Flexi House envelope was selected to be part of an Embodied Carbon study conducted via the University of Auckland.

Civil and Environmental Engineering Honours student Cameron Taylor studied the construction and envelope buildup of five different homes and concluded that the materials used in the construction of a 185m2 Flexi House (House 5 in fig. 1) would remove 4.9T of carbon dioxide equivalent (CO2eq) from the atmosphere. For comparison, a 185m2 new build in New Zealand built on a concrete slab emits 35-60TCO2eq.

Figure 1: Global Warming Potential of five building designs (Source: Taylor et al, 2021)

The next best construction build-up in the study after the Flexi House was a 230m2 house with a timber subfloor, cemented piles, timber framing, plywood cladding and a metal roof. This house clocked in at 8.1TCO2eq, so building a 185m2 Flexi House instead would leave the planet 13TCO2eq better off.

Hang on a minute, what’s Embodied Carbon?

There are two types of carbon to consider in the construction industry - embodied carbon and operational carbon. 

Embodied carbon is the carbon of the physical materials used to build the building. As trees sequester carbon from the atmosphere during photosynthesis, using renewable and organic materials such as timber reduces the embodied carbon of a building.

Operational carbon is the carbon produced by a building in order for it to function - the carbon cost of heating and cooling the home, using electricity and water. The more energy-efficient a home is, the less operational carbon it emits. 

Figure 2: Life Cycle Stages (Source: One Click LCA)

In the University of Auckland study, Taylor et al explored embodied carbon as it relates to Life Cycle Stages (fig. 2). It excludes Life Cycle Analysis stages B1 - Use and B5 - Refurbishment. It also excludes B6 - Operational energy use and B7 - Operational water use which relate to the operational emissions of a building.

Taylor’s report said of Flexi House, “The total Global Warming Potential emissions produced in [Flexi’s] building envelope are -4900 kg CO2eq.”

How can this number be negative? “The high use of timber in the design […] fully [offsets] the positive emissions.” While Flexi reduced the timber fraction in the walls by removing unnecessary nogs and dwangs, the extra timber accounts for the macrocarpa cladding as well as the plywood wall and ceiling linings, and cork flooring. All three of these are significantly more environmentally friendly than other cladding, lining and flooring options such as fibre-cement weatherboards, gypsum board, and vinyl respectively.

Figure 3: Macrocarpa cladding contributed to the significant amount of timber used to construct the Flexi House.

The report also considered the end-of-life of materials, which decreased the environmental impact of certain materials. “The corrugated metal roof and steel foundation piles [...] sum to contribute +5,500 kg CO2eq yet are the most significant materials contributing to a decrease in emissions within the D Environmental Benefits life cycle stage due to the ease of recycling/re-use of metal materials.”

Figure 4: The Surefoot Micropiles used for the Flexi House foundations are made from steel but can be removed and redeployed, which offsets their Global Warming Potential.

This provides some insight into how the Flexi House is able to provide a net carbon offset - through the circular economy principles drawn upon in the development of the system. Flexi’s panels are designed for disassembly and redeployment (reuse) so that a home can grow and change with your needs. They can also be deconstructed and recycled at end-of-life with the aim of reducing waste to landfill. 

Figure 5: Flexi panels are designed with circular economy principles in mind.

To get a clearer understanding of our environmental impact, our next step is to dive deeper into how offsite manufacturing as opposed to traditional onsite construction affects the Global Warming Potential, as well as looking at the operational carbon emitted in the day-to-day running of a Flexi House. Flexi’s carbon story is by no means complete, and we look forward to bringing you along on the journey with us!

The bigger picture

The difference between Flexi House and other homes that are thought to be sustainable is that many only address operational carbon by targeting energy efficiency. This is a valid approach as operational carbon makes up the majority of carbon emissions produced by the built environment, but it fails to address the bigger picture. Embodied carbon must also be considered if we are to reach and maintain net zero by 2050. 

For example, typical residential construction in New Zealand consists of a concrete slab as the foundation system which CAN act as a thermal mass and improve energy efficiency (if insulated properly), but concrete is a massive contributor to embodied carbon. 

Seeing as New Zealand’s electricity grid is about 82% renewable, it would take a home with a concrete slab a long time to pay itself off from a carbon footprint perspective. 

Timber subfloors (like Flexi’s) can achieve a higher R-value rating than a concrete slab with a significantly lower environmental impact, making it not only a better option from an embodied carbon perspective but also from an operational carbon one.

Figure 6: Flexi House uses a timber subfloor as opposed to a concrete slab to reduce embodied carbon and achieve better thermal performance.

How can I reduce the carbon emissions of my new home?

It is critical to address both embodied carbon and operational carbon in order to build a truly sustainable home. 

Select low-carbon materials and construction methods to reduce embodied carbon. A timber subfloor rather than a concrete slab is a good place to start and has a high impact on reducing the overall embodied carbon of the home. 

Opt for high-performance build-ups and construction techniques that utilise natural materials in order to reduce your need for heating and cooling without skyrocketing embodied carbon. It’s also important to consider all of your operational emissions, not just those from heating and cooling. Emissions from water use can be reduced by implementing a rainwater harvesting system and having low-flow taps. Other energy use emissions can be reduced by opting for energy-efficient appliances.

At Flexi, we have addressed both embodied and operational carbon emissions. We have removed concrete, cement-based claddings and plasterboard from our standard specification, offering our timber subfloor panels, timber cladding and plywood linings as sustainable alternatives.

We’re educating our clients about the benefits of these alternatives beyond their environmental impact as well - they also enable our built-for-change methodology allowing your home to grow and change with your needs. These choices are a critical part of enabling renovation without waste in true circular economy fashion.

Figure 7: Plywood wall and ceiling linings and cork plank flooring helped to significantly reduce the Global Warming Potential of the Flexi House.

If you’re looking to build with the planet in mind, Flexi House is here to help. Our free Kickoff Design Package is designed to get you a floor plan and a price so you can start making decisions about whether our low embodied and operational carbon construction system is right for you.

If you haven’t already, simply fill out our quick Kickoff form and book a time to jump on a call with us. We look forward to hearing about what you want to build and how we can help you into a truly sustainable home.