Little Pot of Gold
METHODOLOGY
THERMAL MODELLING
The Passive House Planning Package (PHPP) was used to model the same design geometry. The construction methods were changed to represent low, medium and high carbon intensities.
FirstRate was then used to rate those three versions. This was repeated for each of the State capital-cities in Australia.
A fourth version was created using the FirstRate assesor's experience to get the best NatHERS rating for the same geometry but using whichever construction methods and assumptions gave the highest NatHERS rating.
Passivhaus Standard - Mechanical Ventilation Plan, to ensure great indoor air quality
Passivhaus Standard - Thermal Envelope is defined by the airtight membrane.
NatHERS Standard - Thermal Envelope is defined by the ceiling lining, and the external face of wall and floor structure.
EMBODIED CARBON
We focused on the elements of the building that are related to thermal performance.
We took the average amount of Embodied Carbon per square metre from the Footprint Company data (1288kgCO2/m2).
We deducted the percentage that is due to structure and thermal elements (25%).
We calculated the volumes of materials for each of the 24 versions of Little Pot of Gold that impact on thermal performance.
We then used data from the EPIC database (and internet sources for hempcrete and strawbale) to calculate the varying intensities of embodied carbon based upon the construction methodology.
We added this ‘performance related’ embodied carbon to the ‘usual’ 75% of embodied carbon that comes from other aspects of home building to give an overall amount of embodied carbon per building.
This exercise emphasised the lack of suitable tools for using embodied carbon as a driver of design decisions although "renewable good, non-renewable bad" still holds true.
Most interestingly, it debunks the long held belief that the operational savings derived from concrete use in buildings as thermal mass outweighs the embodied carbon impacts; oops! (Yes, even when the lowest carbon versions are used too!)
The data also showed that the embodied carbon in the PV panels can be as significant as that in the ‘performance related’ components of the building, although in the ‘Chasing Rainbows’ versions this drops to ~1/3 due to the higher impacts of the concrete used for thermal mass.
The calculations used can be found on the Downloads page.
COSTINGS
Building quality buildings is expensive!
Rather than randomly assume an optimistic square metre rate, we approached a quantity surveyor who kindly costed our project for us assuming that it was based in Sydney and built to a NCC compliant level.
We took that comprehensive breakdown and removed all of the thermal performance related elements (structure, insulation, doors and windows, ventilation systems, shading devices, air conditioning).
We calculated the costs of those elements based upon the requirements of each of the 24 designs. These costs were added back into the other ‘usual’ elements of building costs to give an overall total.
Our calculations were based on real quotes from a prefabricated panel manufacturer, a Passive House certified window supplier and a MVHR supplier. Previous project data was used for the costings of strawbale and hempcrete construction.
The ProCalc software was used to calculate the relative cost difference between each of the city locations. A single project was entered and moved around the country with each resulting cost being noted; Melbourne and Brisbane being cheapest, Hobart the most expensive.
PAYBACK
The Design Matters competition called for the use of the Victorian Whole of House tool to calculate emissions reductions/offsets. Unfortunately that tool only calculates PV output for Melbourne.
We used the Passive House Planning Package (PHPP) to accurately quantify the PV output for each location. We used a 15kWp system in every case.
We calculated our operational energy use in PHPP (we have little faith in NatHERS accurately doing this). We started with our embodied carbon debt (see methodology for how we calculated that) and then for each year added on the operational energy while also minusing the PV output.
Our embodied carbon debt (kgCO2) was converted in kWh using the underlying data from the Whole of House Tool; 1kgCO2 = 1kWh
We found the moment where the carbon debt went from negative to positive; this is the payback time. It varied from 7 years up to almost 21! (See our matrix of data for each capital city for all the results).
NET EMISSIONS
We calculated the tonnes of CO2 in the building for each of the homes.
We used the data from the Whole of House tool to convert the energy use of the home into CO2 and also to convert the PV energy output into CO2.
Annual numbers were multiplied by 27, the number of years until 2050.
The CO2 avoided by PV output was deducted from that consumed/created by embodied and operational energy use to reach an overall carbon position.
Every home was negative by 2050, the best being 563 tonnes, the worst 248 tonnes.
NatHERS
This tool is used in Australia to create a star rating for homes based upon theoretical energy use modelled on a series of somewhat ridiculous assumptions. It is far from perfect and, in our opinion, is currently not fit for purpose.
There are many technical issues however we choose to outline two.
1. INEQUITY
For, what we assume, are political reasons 7 stars means wildly different things depending on your location. The graph shows how much energy your 7 star home can consume as you move it around the nation.
We see this as discriminatory and unfair as it puts highly variable costs onto households to maintain a healthy, comfortable indoor environment.
NatHERS
2. HEALTH
One of the ridiculous assumptions is around the temperature of the indoor environment. Contrary to World Health Organisation (WHO) recommendations, the Australian government has decided that bedrooms do not need to be healthy.
The graphic below shows the ’set points’ used by NatHERS, contrasted against the passivhaus standard and the WHO advice.
Comically, unless it is the building in which you raise your child, you can game the NatHERS system by calling a room a bedroom or living room as it assumes different temperatures at different times rather than assuming your home is comfortable and healthy all of the time!
ASSUMPTIONS / GLOSSARY
SOLAR PV
Competition criteria was for maximum 15kW of Solar PV array on the roof, so this was used as the assumption for all cases to ensure consistent/comparable results.
Export cap for the competition is a 5kW panel system per phase.
3 phase = 15kW
SOLAR PV - EMBODIED CARBON
Based on data we have, there is more Embodied-Carbon for the Solar PV panels/system alone than there is in the building envelope.
WHOLE OF HOUSE TOOL
(Sustainability Victoria)
Solar PVs performance in this tool is currently based on Victoria data only. Competition criteria advised to use this data/calculations for all locations however this means somewhere sunnier would be generating more Solar Power so numbers total payback period is distorted.
Our solar PV production was generated using the PHPP, it showed a difference of up to 30% between cities (Brisbane to Hobart)
BUSHFIRE PRONE AREA
Sites given in the DMN competition brief were assumed to be not in Bushfire Prone Areas, so no BAL (Bushfire Attack Level) applies to these locations.
TOPOGRAPHY
Sites given in the DMN competition brief were assumed to be flat. However most of our designs are a timber-framed sub-floor which can sit more lightly on the earth with sloped sites without large amounts of cut and fill required.
WINDOW FRAMES
High-perfomance double-glazing and triple-glazing was specified, and calculations were done to compare uPVC frames versus Alu-Clad timber frames. The Emodied Carbon was almost identical.
HOT WATER SERVICE
Energy-efficient, all-electric air-sourced heat-pump hot water service units were specified for all locations.
LIGHTING - APPLIANCES
Operational Energy Use (excluding heating, cooling and hot water) was assumed to be the same. Standard LED light fittings, all-electric appliances, etc.
BUILDING ENVELOPE
Also known as "building enclosure" and "thermal envelope" and is essentially referring to the floor, walls and roof of a building.
It is the physical separator between the conditioned and unconditioned environment of a building including the resistance to air, water, heat, light, and noise transfer.