Prefab is a building methodology where you build the modular parts off-site and assemble them at the final destination. 80% drop in waste production can be seen in prefab, as this is the most eco friendly method with its use of standardised building material dimensions compared to traditional construction.
Paul Adams from Fairweather Homes is my extremely talented (and patient!!) architect. Paul is also an expert in prefab. This means that my house will be built off site. In some circles prefab has a reputation for being lower quality and not as aesthetically pleasing but I’m hoping my build will help combat that perception. I’ve included some images of Paul’s projects which I think are beautiful and I’m going to do my best to explain why the quality is actually better with prefab for an eco friendly house.
There are many reasons! In the McGraw-Hill Construction SmartMarket Report of US firms who use prefab for eco friendly houses, they found that the build time decreased for the majority of participants surveyed, with the majority seeing a decrease of 4 weeks or more.
Time is money for your eco friendly house! If you have to move out and pay rent, or you have to pay a builder to be onsite for an extra month, that’s a lot of extra money down the drain. In fact, 42% of survey respondents saw a reduction in the budget of 6% or more.
These cost reductions are mostly due to the reduction of builders’ time on site. If you live in a high-density area like me, with very restricted access for builders pushing up time spent, that can really add up.
The other point to make is that the costs are fixed. If I were to do a “normal” build, there’s no guarantee that the quote would match the price that I end up paying given the frequency with which costs blow out during the construction process. There’s also the quality of the build of eco friendly houses. With prefab, things fit together properly because they’re constructed in a factory ready for assembly on site.
When it comes to waste, prefab is definitely a winner compared to traditional methods. 76% of the firms surveyed in the McGraw-Hill report said that prefab decreases the amount of their construction site waste, with 41% reporting decreases of 5% or more.
I have specified that I want the build to be primarily wood and to have no concrete. My selection criteria for build materials is based on a hierarchy that includes:
Paul’s prefabricated building elements are constructed using lightweight timber-framed floors, walls and roof which is pre-clad, insulated and pre-lined where possible. These are conventional construction materials pre-assembled in a factory to achieve efficient construction time on site.
The structural timbers – natural solid framing timbers, LVL engineered timbers and plywood bracing materials – are sourced from Australian and New Zealand plantation timbers. With this supply chain they are able to seek chain of custody verification of the source of these timbers through Forest Stewardship Council or other certification systems. While these materials would all be considered new, they are from a sustainably managed renewable resource which also sequesters carbon into the building fabric.
For external cladding the preference is for timber-based product and I will use Shadowclad or Weathertex products for the same reason as above, i.e., it’s a locally produced timber product with durability characteristics. These products can also be painted to suit style and colour scheme.
We would also look at recycled/reclaimed timbers for feature timber cladding elements. On boundary walls we’ll need to look at a product that is compliant for fire-rated walls. There are a number of possibilities but they generally tend to be cement-based. The preference would be to utilise a Magnesium Oxide board (MgO)o rHebel panel.
Windows can be from a variety of sources. Paul encourages double-glazed, timber-framed windows due to the inherent thermal performance and ability to build them from a renewable resource. Other approaches may be to rescue new windows from factories which are surplus stock, built in error, returned etc. Bespoke timber windows can also be sourced using reclaimed and recycled timber.
For internal wall and ceiling linings plasterboard is to be reduced or excluded from the build. This gives the opportunity to look at lining with other products that are either reclaimed or timber based such as limed plywood, reclaimed timber lining boards etc.
Due to the site conditions and requirement to have a suspended floor for sub-floor water storage and management, Paul will be supplying a long-span timber subfloor with a plywood lining. This will be a suitable substrate for a natural/recycled timber or other floor lining.
The house won’t have any heating or cooling installed, so our clever building design needs to use the sun to heat the house up in winter and keep the sun out during summer. I’ll be adding more information once I get the thermal dynamics report back from the thermal assessors. These are our plans so far:
The right orientation depends on the climate you live in. For climates that require heating in winter, it’s best to have your living areas facing north. I’ll only be discussing my solution and the climate I live in at the moment, but if you’d like to learn about optimal orientation in other climates, this government site is really useful, especially the diagram on the right. I’ve circled the orientation that is most like my house. Unfortunately the layout of my property is such that the first half of the house is comprised of bedrooms and the living areas face southwest.
We are keeping the bedrooms where they are so I can’t do much about that. But Paul has changed the design of the back end of the house so that the living areas are separated from the bedrooms by a courtyard. He has also raised the ceiling on the living area and included a north-facing window. You can see in Paul’s first drawing below that the living areas will be protected from the sun during summer by eaves which cover the windows.
This involves keeping the summer sun out and winter sun in. We are achieving that through building design, placement of windows and eves, and thermal mass.