The challenge
To reach our climate goals, we need to build houses with smaller carbon footprints. The Building for Climate Change (BfCC) programme of Aotearoa New Zealand’s Ministry for Business, Innovation and Employment (MBIE) calls for caps on both operational and embodied carbon. MBIE wanted to know if building lower-carbon homes would cost more. While buildings with better thermal design often have lower whole-of-life costs, this project focused on the upfront capital costs only.
MBIE commissioned thinkstep-anz, engineering consultancy Mott MacDonald, and quantity surveyors Prendos to investigate if it is possible to achieve BfCC’s proposed carbon savings without increasing the upfront cost of construction.
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What we did
We used a single reference building as the basis for analysis. The building selected from the LCAQuick reference library is a single-level home that features four bedrooms, two bathrooms, two living areas, one kitchen, one laundry, and a two-car internal garage.
We then modelled operational energy/carbon and embodied carbon for the reference building with different levels of thermal performance and different material combinations. All modelling was done at the system level (floors, roofs, walls, windows) rather than the material level.
Overall, this study considered 22,032 unique combinations of building elements, comprising:
- 6 roof options,
- 17 envelope wall options,
- 6 internal wall options,
- 12 floor options, and
- 3 window/door options.
The study also looked at three regional scenarios (Auckland, Wellington, Christchurch) for the operational energy/carbon modelling.
What we found
New Zealand’s residential buildings already have a relatively low carbon footprint due to the use of timber framing.
This study shows that timber products have low upfront and embodied carbon, even when we exclude stored biogenic carbon. New Zealand’s widespread use of timber framing means that the potential to reduce embodied carbon is more limited than if other materials were used. Reductions of carbon footprint per square metre are therefore likely to be small, unless removals of carbon dioxide from the atmosphere (e.g., by trees that are converted into timber products and then used in buildings) are allowed in the calculations.
Without smart design, BfCC may increase the initial cost of construction by 5-10%.
Achieving these caps while also reducing embodied carbon (but without optimising the design) would likely result in construction costs increasing by roughly 5-10% per square metre of floor area.
However, there is significant variation in upfront costs for the same level of decarbonisation. By choosing the best solution, significant savings can be made for no additional upfront cost as long as the
Smart design can deliver lower carbon buildings at no additional cost.
However, because the reference building selected for this study was one of the cheapest options available, only 2.8% of all options considered delivered a lower upfront carbon footprint for a lower upfront cost. Only 1.0% delivered a lower whole-of-life embodied carbon footprint (excluding biogenic carbon and recycling credits) for a lower upfront cost. This means that the building industry has already access to both low carbon and low cost, but only deliberate optimisation is likely to achieve both outcomes.
There can be trade-offs between operational efficiency and embodied carbon.
We found that moving from a concrete slab to a suspended timber floor was one of the most effective strategies to reduce embodied carbon in the reference building. However, the lower thermal mass led to decreased thermal performance for the reference building considered.
BfCC’s final operational efficiency cap will likely require changes in building design that achieve better thermal performance.
None of the scenarios modelled in this study achieved the proposed final cap for thermal performance or services efficiency. To reach these, optimised building design for better thermal performance and higher efficiency heat pump hot water systems are needed.