Understanding Sustainable Materials
If sustainable building were easy, everyone would do it as a matter of course. A genuine commitment to the principles of sustainability is a challenge SGB believes is worth rising to the challenge of.
The first challenge is an issue of definitions. There is no such thing as an inherently sustainable product, or practise. Everything is complicated. The difference between ‘embodied carbon’ and ‘operational carbon’ is a good place to start. Embodied carbon is the emissions that have been created in the production of a certain material. An aluminium window has far high embodied carbon than a timber framed window. Operational carbon is how the material will reduce emissions over the course of its life. In this case, the aluminium window frame retains heat better than the timber, and therefore has better operational carbon.
The calculation then must be applied to the maintenance of the material, and finally its disposal. Timber requires regular maintenance and will probably end up in landfill. Aluminium requires minimal maintenance and is 100% recyclable.
We then must consider the emissions created by differing building practises. A low emissions product may no longer be a low contributor if it has had to be shipped from overseas. Newer, lesser known products will likely be a more challenging install for builders, this can result in wastage. Take for example the often touted ‘rammed earth’. This is a very cool method of building, essentially compacting layers of earth inside form work to build walls. The result ticks both the operational and embodied carbon boxes. The downside is it is a technical, complex style of building that most carpenters have never attempted. This will translate into carbon emissions. It may take a week to frame a house with timber. It may take two months to do it with rammed earth. That means roughly eight times the return trips to and from site for workers. Small mistakes in the complicated process may result in ongoing maintenance and all the associated cost and emissions.
Instead of printing a back catalogue of spurious or tokenistic greenwash, SGB is honest about the fact that reducing emissions in a meaningful sense requires hard work and lots of thinking. We aim to embed these considerations into the culture of the company. We aim to work with designers and architects who use sustainable materials and will always promote them to clients. Our part comes into play regarding sustainable practise, in other words, we build efficiently. We aim to minimise all unnecessary time on the road. This can be achieved through carpooling, organising bulk material pick ups and drop offs, building in designated sections (such as all weatherboards installed at once, then all linings etc) and using our versatile team flexibly according to site needs. For instance, if a job were 50 kilometres away, and would take one carpenter four days, it is more environmentally conscious to send one vehicle, with four carpenters for one day. Onsite, carpenters are encouraged to work with modular shapes where possible to minimise offcuts, to keep track of materials in our storage yard for use, and to recycle wherever possible.
All of that said… what does a sustainable house in New Zealand actually look like?
Realistically it will be timber framed. Softwoods such as pine are good, sustainable options to build with, so long as they are sourced responsibly. We look for the Forestry Stewardship Council accreditation for this, which our suppliers provide. Timber has a lower carbon footprint to steel or concrete across all metrics, arguably apart from maintenance. Where possible, we would suggest LVL structural timber beams over steel.
About 90% of roofs in New Zealand are existing concrete tile, or long run sheet metal. Sheet metal has a lower embodied carbon footprint than something like an asphalt shingle roof, or a torch on membrane roof. They are also simple, scalable and tried and tested. A good team can reroof a house in a day. Sheet metal is compact and commonplace, so a delivery truck will likely do multiple site drop offs each time it is on the road. For instance, our supplier is based in Levin, and organises all their Wellington drop offs on a Wednesday morning. Sheet metal is reliable and will keep water from leaking into the home. Insulation will be kept dry and the house will retain its heat.
Windows are important, as they are the primary way that heat is lost from a building. Timber is a better thermal block than aluminium, but aluminium allows for all the modern efficiencies to be achieved. The most thermally efficient window will be double glazed with a gas such as argon, or even a vacuum, between the two panes. This is not possible with timber framing. Therefore, the best windows environmentally, are modern, double glazed, thermally broken aluminium framed windows. The higher embodied carbon is offset by the operational carbon. This is not to say however that a well-maintained timber window is bad, especially if it has draft stoppers, a pelmet or curtains. The restoration of old windows has almost no embodied carbon and can make them much more efficient.
We choose to use Earthwool insulation wherever possible as we believe it has the best environmental credentials on the New Zealand market. Insulation is key to creating good operational carbon efficiencies. Even the best product on the market will be ineffective if installed incorrectly. Heat will find any route through insulation to escape. In renovations, it is common to discover incorrectly installed insulation, rendering it largely pointless. To remedy this, hire trained tradesmen who will install insulation correctly, including things like air seals around windows and expanding foam in the cavities on the corners of framed walls.
Plasterboard (often known by the brand name GIB) is used almost exclusively as internal wall linings in NZ. It raises relatively few environmental red flags, but also is not a perfect product. The drying and calcination processed used on the gypsum use a lot of electricity and natural gases. Gib has one of the worst wastage rates of any material used onsite, with recent estimates placing it as high as 30%. Modern builds are beginning to see more recycled sheet products coming into circulation such as strand board and even reconstituted plastic sheet products.
External cladding benefits from being kept simple. A single material choice means a single measure, a smaller margin of error and less material wastage. A house with say, weatherboards, metal corrugate and cedar is more complicated to build and maintain. If products are being brought in from afar, there must be more material bought than necessarily needed, in case of running out. Generally speaking - the more customised the material, the greater the embodied carbon. In simple terms, timber has the lowest embodied carbon costs of the cladding materials, followed by concrete and steel, with aluminium having the highest. With good building practise any of these can be used to create optimal operation carbon foot-prints.
The foundations and flooring of a building again are subject to variables. Cement, which forms a key ingredient of concrete, scores poorly in terms of embodied carbon. There are however alternative cements such as Ashcrete or Hempcrete, although these are not in wide circulation. Concrete has probably the worst wastage margins out of any site material as well, its messy and a great deal of a mix will stay in the pumping equipment. A concrete slab however is arguably more efficient to build, and more thermally efficient than a suspended timber floor. Concrete works well at scale. If a developer is building a dozen townhouses, they can pour the slabs simultaneously, minimising wastage. When correctly orientated, concrete also benefits from having a high thermal mass, that is, it retains the heat from the sun better than a timber subfloor.
Finished floors have a few clear winners in terms of environmental impact. Locally sourced, woollen carpets are superior to synthetic carpets in every way. Obviously, recycled materials are the best option environmentally, but are not always practical. For a floor however, recycled hardwoods have zero embodied carbon and look absolutely beautiful. They can be sealed using water-based products and are a favourite of SGB.
Thermally Broken, double glazed aluminium stacking doors.
New insulation installed into a 1939 state house
