Dramatically reducing the carbon footprint of residential new builds presents significant challenges for architects, designers and the entire industry. Revisiting basic design principles is one tool that will be useful.
DESIGNING AND constructing dwellings to meet low embodied and operational carbon budgets means an increased focus on material selection and basic design principles, particularly around the size, form, build complexity and orientation of the dwelling.
Carbon footprinting tools
Low-carbon residential design starts with an understanding of carbon footprinting tools and the inputs needed. The tool is used early in the design process to quantify total emissions compared to budget and identify where a reduction in emissions may be required and how this can be made. It can also directly compare the emissions of design alternatives. For more on carbon budgets, see Carbon budget in New Zealand housing (Build 182).
Using a tool can result in a final design with a carbon footprint that meets the required carbon emissions budget. It also provides a mechanism to compare any required material substitution during construction.
Carbon credentials of materials needed
For comparisons across similar products and systems, tools need to incorporate databases of current New Zealand relevant material information. This requires manufacturers to provide accurate data not only on product application and performance but also on embodied and operational carbon emissions where relevant.
Current practices of selecting materials based on client preference, aesthetic, durability, suitability and cost remain fundamental, but carbon emissions are also now a key part of this process.
Embodied emissions are a significant component of new builds and should be considered as such. However, operational emissions are equally important as they occur on an ongoing basis over the life of the building and can contribute a significant proportion of total emissions. This is where the real design challenges and opportunities lie, and a back-to-basics design approach can pay dividends.
Smaller is better
Dwelling size has become more critical. Larger homes incorporate more materials and therefore higher levels of embodied carbon – but they also require more heating, cooling and ventilation. They often have greater water consumption and more ongoing maintenance, all contributing to high levels of operational emissions. Consequently, a greater focus on floor area and building volume is required.
Consideration of not only the number of rooms but minimum fit-for-purpose room sizes, circulation and storage requirements and the layout of the home can all contribute to a reduction in operational emissions.
Larger homes also often incorporate more bathrooms, and considering emissions associated with water use is important. Reducing the number of bathrooms, specifying water-efficient fixtures and fittings and incorporating on-site rainwater harvesting and reuse all contribute to reduced water use.
Work with the building’s form
The form of the building is also a major consideration. Large areas of exterior glazing can contribute to an increase in solar gain in cooler months – reducing heating requirements. However, the opposite can occur in warmer months when the glazing can contribute to overheating of the interior and increase the need for cooling.
Looking at glazing-to-wall ratios and management of glazing through shade devices is fundamental. High exterior wall to floor area ratios can contribute to increased operational emissions. Larger areas of exterior walls with compliant but relatively low levels of insulation potentially contribute to lower overall thermal performance of the dwelling. Poor installation of wall insulation will also reduce performance and increase thermal bridging.
Interior volume is also a factor, with larger spaces requiring more heating and cooling. However, volume can also contribute to cooling, where higher-level opening glazing can be used to assist with increased air circulation and stack effect ventilation for a reduction in cooling requirements.
Orientation is crucial
Orientation of dwellings is critical to managing operational emissions. Correct orientation is often difficult to achieve in higher-density developments and with detached dwellings in lower-density subdivisions.
Attached dwellings often have only two exterior elevations, and these are not always oriented in a suitable direction. Glazing ratio and shading of glazing are key considerations when orientation increases the potential for overheating. Similarly, lack of exposure to sunlight increases the demand on heating.
Where possible, buildings should be oriented to achieve solar gain in the cool months and to make use of prevailing winds to assist ventilation and cooling in the warm months.
Exterior glazing can be located not only for solar gain but also to allow cross-ventilation of the interior. Considering the extent of west-facing glazing to reduce excessive gain and minimising south-facing glazing to reduce heat loss can contribute to a reduction in operational emissions.
Keep the design simple
Overly complex building designs can result in more material wastage and require greater maintenance. This means higher levels of both embodied and operational carbon.
While low-carbon residential design certainly poses some new challenges, a return to basic design principles can help significantly in overcoming these.
Register for the BRANZ Carbon Challenge: Science and solutions upcoming seminar at https://branz.arlo.co/
Articles are correct at the time of publication but may have since become outdated.