FMI Zero Carbon houses: fantastical or feasible?

National carbon goal

New Zealand is on a journey to net zero carbon1 by 2050. Reaching this goal, if we do, will be our national contribution to the global effort under the Paris Agreement of maintaining the world’s temperature rise to no more than 1.50C above pre-industrial levels.

Net zero carbon at a country level means achieving a balance between the volume of carbon (carbon dioxide and equivalent greenhouse gases) emitted from activities across industry sectors and society, and the volume of carbon taken out of the national system either by sequestration in carbon sinks like forests or—as a stopgap—buying carbon credits from another country. Our FMI Zero initiatives contribute in their different ways to reducing the volume of carbon emissions from new houses and commercial buildings, and thus reduce the sinks and credits required.

Definitions

Agreeing definitions and measurements is an important part of assessing starting points and charting progress from them. The contribution our local building and construction industry makes to New Zealand’s carbon emissions today ranges widely, depending on three considerations:

• Are all three of a building’s emission types included?

Embodied carbon comes from the manufacture and transport of materials that form the building and can occur right across the building’s life cycle. Operational carbon occurs only during the use stage of a building’s life from the energy and other resources like water used to operate the building. Refrigerant carbon in homes typically comes from refrigerator and air conditioning systems which contain gases far more warming to the atmosphere than carbon dioxide

• Where along the chain of building activities is the carbon deemed to be emitted? A production view accounts for emissions at the point they are emitted, such as a factory. A consumption view accounts for emissions in goods and services at the point they are finally used, typically further along the value chain

• How have activities in the industry or social sector been allocated? In its consultation document on New

Zealand’s Emissions Reduction Plan2, the Ministry for the Environment (MfE) allocated the operational energy to run homes and commercial buildings to

1 Excluding biogenic methane from agriculture and organic waste 2 “Transitioning to a low-emissions and climate-resilient future”, MfE, October 2021

the Energy & Industry sector rather than to Building & Construction.

Combining a production approach with embodied and refrigerant emissions puts 4%3 of this country’s carbon at the building industry’s door. Expanding to all emission types and a consumption view, the share rises to 20% of this country’s carbon. Globally, the latter number has been estimated at almost 40%4 .

It would be fair to say that developing and harmonising measurement approaches to different aspects of carbon emissions is still a work in progress both internationally and nationally. This is certainly true in New Zealand where the wider building industry is new to concepts

Embodied carbon: a life cycle

of carbon emission management and measurement. To date these have been familiar to a handful of interested practitioners of high energy efficiency construction with an emphasis on operational rather than embodied carbon emissions.

What do the terms we use in our FMI Zero Carbon initiative mean? The schematic on pages 4-5 steps through our definitions for FMI Zero Energy, FMI Zero Carbon and FMI Zero Waste. FMI Zero Carbon, which we explore in this article, refers to a house designed and built to minimise its lifetime embodied carbon emissions and, further, to fully offset those emissions with renewable energy generated onsite that is surplus to the occupants’ operational demands across the seasons.

The notion of cradle to grave (and beyond) is core to international thinking on managing embodied carbon out of buildings. This is because, unlike operational carbon which can be reduced during a building’s lifetime, embodied carbon is locked in as soon as a building is constructed. In the initial years of a building’s life, embodied carbon also represents a significant share of carbon emissions. BRANZ has calculated that in the first thirty years of a newly built detached house’s life, embodied emissions contribute 45%6 of its total emissions; the share falls to a little over a quarter by the end of its ninety-year life – see Figure 1.

FIGURE 1:

The contribution of materials, energy use and water consumption to a new house’s greenhouse gas emissions over 90 and 30 years (source: BRANZ)

Successive life stages of a building have been defined in European standard EN 15978:20117 and adopted by the Ministry of Business, Industry and Enterprise’s (MBIE) in their Building for Climate Change programme8 as shown in Figure 2.

3 Ibid 4 Whole-of Life Embodied Carbon Emissions Reduction Framework”, MBIE, August 2020 5 “2018 Global Status Report” for Global Alliance of Building and Construction 6 “A carbon budget for New Zealand houses”, BRANZ Research Now: Zero-carbon built environment #1, September 2020 7 “Introduction to LCA of Buildings”, Danish Transport and Construction Agency, 2016. 8 Whole-of Life Embodied Carbon Emissions Reduction Framework, August 2020

FIGURE 2:

Module framework for life cycle assessment of buildings

To date, more attention has been given to the challenge of cutting operational carbon (B6 and B7 in Figure 2) than embodied carbon. The result has been more advanced design and measurement systems for operational carbon, including Passive House standards for energy efficiency and indoor comfort. As building practices for energy efficiency improve and are used more widely, and as renewable energy powers a greater proportion of building operations, however, the prize for reducing operational carbon will likely fall. The outlook is for embodied carbon and its successive lifecycle stages to become the bigger target for designers and engineers.

The strategies to cut embodied carbon are already well understood internationally. Drawing once more on MBIE’s work, Figure 3 groups these under three main objectives.

FIGURE 3:

Objectives and strategies to reduce whole-of-life embodied carbon

FMI Zero’s initiative for FMI Zero Waste fits under ‘Material Efficiency’. This objective is also where our efforts to ensure durability of our window and door frames sits—see our article on Page 40-41 on how we are building longevity into our insulated glass units.

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Could our FMI Zero Energy house be FMI Zero Carbon too?

Target

BRANZ’s Carbon Budget project9 estimated the volume of carbon emissions that a newly built detached house could emit while still moving towards New Zealand’s net-zero goal for 2050. The answer was an ‘allowable’ total of 35 tonnes of embodied and operational emissions over a 90-year house life. The researchers acknowledged this limit was likely to change as data capture and methods evolved.

They tested 10 real life newly-built houses and concluded that all exceeded the 35-tonne measure by several multiples, including builds whose building envelope surpassed the (old) H1 clause of the building code standards relating to energy efficiency. They noted,

Higher-performance houses are not necessarily low-carbon houses

We felt there was a warning in those words for our FMI Zero Energy house, which is a 120m2 three-bedroom, single storey home, styled to suit Kiwi tastes. Our early focus was solely on the annual energy balance of this house in different local climates, and we gave little explicit consideration to the embodied carbon of the building materials chosen. Similarly, we ignored some Passive principles like window configuration, focusing instead on achieving high energy efficiency in the building envelope and allowing for renewable energy generation from solar panels on the roof.

If our FMI Zero Energy house was to pass as an FMI Zero Carbon house, it would have to do it by generating enough energy off its roof in the building’s lifetime to cover both the operational needs of the occupants and to offset all the embodied carbon including in the solar panels and fittings.

Test limitations and parameters

We have said that definitions and measurements are a work in progress. The sustainable building scientists who we asked to test our FMI Zero Energy house for its net carbon emissions across its 90-year life could sensibly assess only a subset of stages. Specifically, they assessed life stages A1-A5, representing cradle to practical completion. For comparison to operational carbon, they also reviewed B6 for which they set the internal temperature to not fall below 180C; this is cooler than Passive House’s 200C but aligned both to an important NZ carbon research paper10 and to typical Kiwi occupant behaviour.

The biogenic impact of timber from sustainable forests was not included. Embodied carbon from appliances was also excluded. The latter was in part because the embodied carbon of some items, for example the mechanical ventilation and heat recovery system, are currently unknown. Lastly, as a simplification regarding the embodied carbon in transport, each climate used the same value as that calculated for Auckland.

An important element in calculating operational carbon emissions was to forecast the mix of renewables in New Zealand’s grid. Our building scientists drew on data from BRANZ, selecting the Mixed Renewables scenario11 and extrapolating the last five years of data to 2050 out to 2111 for the 90 years of the house’s life. Emission values for LPG, natural gas and wood came from the NZ Green Building Council.

13 “A carbon budget for New Zealand houses”, BRANZ Research Now: Zero-carbon built environment #1, September 2020 12 Chandrakumar, C., McLaren, S., Dowdell, D., & Jaques, R. (2020). A science-based approach to setting climate targets for buildings: The case of a New Zealand detached house 11 https://www.branz.co.nz/environment-zero-carbon-research/framework/data/ Note that this data was refreshed in December 2021, but the update has not been incorporated into the results presented here

Results

Because of the transport simplification and because FMI uses the same FMI Zero Energy house design across all climates, the embodied carbon results were identical across New Zealand. The FMI Zero Energy house contains about 18.5 tonnes of embodied carbon, and Figure 4 shows most of this amount to be fairly equally split across the aluminium window frames, the steel roof and the aluminium Eurowood walls. Roof, walls, and floor have all been insulated with Heatlok HFO polyurethane foam which is also a significant carbon contributor for the house as a whole. When combined with operational carbon, the FMI Zero Energy house’s total emissions in all climates are well above BRANZ’s 35 tonne limit. Figure 5 shows the total emissions ranging from 49 tonnes in climate zone 1 to approaching 55 tonnes in climate zone 6. The higher operating emissions load in colder climates is driven chiefly, and unsurprisingly, by the difference in overall space conditioning energy demand. The much greater heating demand in climate zone 6 more than offsets the higher cooling demand in zone 1.

FIGURE 4:

Embodied carbon by building component for life stages A1-A5 (Tonnes CO2 eq)

FIGURE 5:

Total carbon emissions by climate zone for FMI Zero Energy house (Tonnes CO2 eq)

Solar power

Where the FMI Zero Energy house comes into its own in terms of net carbon emissions is in the renewable energy off its roof. Even in the coldest climate, and allowing for the embodied carbon in the solar panels and their fittings, a modest 5.5W solar array putting out just under 7000 kWh can create enough surplus energy beyond the needs of the occupants to offset all the embodied carbon – see Figure 6. The 17x 340W panels array modelled is similar to the one we proposed in the previous issue of Fenestration New Zealand where we used 18 x 300W panels on a slightly higher spec’d building envelope. Installed, both these arrays cover a little over 31m2 which is slightly less than half the area of the sunny side of the roof. This leeway to add more panels matters for the reason explained next.

FIGURE 6:

Carbon balance for FMI Zero house over a 90-year life in Zone 6

Caveat

Arguably, there is an issue with the international definition of Zero Energy houses when viewed through a Zero Carbon lens. A house meets the Zero Energy definition if its annual energy demand can be equalled or exceeded by its renewable energygenerating capacity. There is no requirement for battery storage to deal with short term or seasonal imbalances. Seen on a monthly basis, as in Figure 7, it becomes clear that in the warm months the house produces more energy than is needed, while in winter it must draw down on the grid which may be fuelled at peak times by fossil fuels. Battery storage to cope with seasonal imbalances would be expensive and, more importantly for an FMI Zero Carbon house, embody more carbon and use operating energy itself. Our FMI Zero Carbon house would need to offset the battery carbon with more solar panels to produce an even greater renewable energy surplus. We have comfort that this could be possible given the current modelled solar panels, even in Zone 6, cover scarcely half the north-facing roof.

FIGURE 7:

Schematic of monthly energy demand in an energy efficient house12

Code versus FMI Zero

The final insight into the feasibility of achieving FMI Zero Carbon came from using a carbon emissions lens to compare a new H1 Code-built house to our FMI Zero Energy house. As BRANZ has pointed out, houses with high thermal performance are not necessarily low in embodied carbon. Similarly, houses with poorer thermal performance in the building envelope can deliver lower embodied carbon. Clearly there is a sweet spot to be found, where the extra embodied carbon needed to deliver energy efficiency and energy generation is more than repaid in both the kilowatt hours saved over the life of the house and kilowatt hours returned to the grid13 .

Figure 8 shows the differences in carbon emissions by type between the Code-built and FMI Zero Energy houses. The FMI Zero Energy house embodies about 15% more carbon, chiefly due to its walls. The aluminium wall cladding is a key source of extra embodied carbon and so too is the extra Heatlok spray foam needed to achieve FMI’s higher wall R value (3.3 versus Code 2.0). The other aspect of note for embodied carbon in the FMI Zero Energy envelope is the larger profile size of a thermally broken window frame that contributes significantly to thermal performance. The larger frame profile results in a smaller glazing area, and this is enough to offset the higher embodied carbon of triple-glazed units. Consequently, glazing is almost the same for the two builds.

Operating carbon is two thirds less for the FMI Zero Energy house, reflecting the superior thermal performance. Total carbon emissions for the FMI house are consequently about half those of the Code house. And when solar panels are included, the FMI Zero Energy house achieves a 100% advantage over the Code house, even after allowing for the embodied carbon in the panels and fittings. In other words, our FMI Zero Energy design has proved it can qualify as FMI Zero Carbon. Henceforth, we’ll refer to the house simply as “FMI Zero”.

12 https://passipedia.org/basics/energy_and_ecology/primary_energy_renewable_per 13 We are unsure how the FMI Zero householder might be compensated for surplus renewable generation beyond the few cents per kWh currently on offer from electricity retailers. The Emissions Trading Scheme (ETS) does not cover households. Currently, the point of obligation in the scheme occurs as far upstream in the supply chain as possible

FIGURE 8:

Lifetime carbon emissions by type for New H1 Code-built and FMI Zero houses (Tonnes CO2 Eq)

Optimisation

The difference between FMI Zero and the Code house on embodied emissions is relatively small at about 15%. We suspect that with a few adjustments the embodied carbon of our FMI Zero house could be reduced to match or surpass the Code house given that we did not consider this aspect in our original FMI Zero Energy design.

It is tempting to think that a design focus optimising operational carbon is the better path to lower lifetime carbon emissions for New Zealand houses. As the years go by, operational carbon accounts for an everlarger share of a home’s carbon emissions (Figure 1), making it the greater prize. In our FMI Zero house, it represents about 65% of lifetime emissions and is the source of the significant carbon emissions advantage over the Code house, coincidentally also about 65%. However, we recognise that results may shift as measurement of embodied carbon develops and once all stages of the building’s life cycle are able to be included.

In this article we have focused on understanding net zero carbon, its components and how different house designs, including our own FMI Zero approach, stack up. Further optimising would consider the relative costs and payback of reducing embodied and operational carbon, and of being paid for creating surplus renewable energy.

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