Effective Carbon Reduction Strategies For New Homes?

November, 2023 |
Prompted by an article posted by Christopher Hughes on LinkedIn challenging whether the capital investment for Passivhaus was worthwhile, in carbon terms, especially if you can achieve much higher carbon savings through simple specification switches, especially to timber. Below is a quick comparison of Passivhaus versus Embodied carbon savings.
This focusses on the next twenty-year period (so before 2050) given this is the most critical period for reducing carbon emissions for mitigating negative climate change impacts.
OPERATIONAL CARBON SAVING
What are the operational carbon savings by achieving Passivhaus levels of space heat demand versus a non Passivhaus space heat demand? The House Builders Federation (HBF) reports the average new home has a total energy use of 100 kWh/m² per year (2022). To compare this against Passivhaus performance we can take off the energy needs not related to space heat demand (35 kwh/m2 typical) and adjust for lower occupancy level assumptions (30%) and the treated floor area metric (TFA at circa 90% of GIA). This suggests an equivalent measured figure for the space heating proportion of the energy demand to be around 50kWh/m². To compare, The UK Passivhaus Trust publication ‘PH Route to Zero Carbon’ (2019) calculates that a new Building Regulations compliant house has a space heat demand of 54kWh/m². Let’s then assume the lower figure as the average energy demand of a non-Passivhaus new home. To compare, the space heat demand in a certified Passivhaus must not exceed 15kWh/m² representing around 70% reduction. Assuming both houses have electric heating solutions, with an average carbon cost over the next twenty years of 100gCO2/kWh and that our house is an average size of 100 square meters TFA.
PASSVHAUS Passivhaus with an MVHR system, triple glazed windows, resolution of thermal bridges, excellent U-values, airtightness below 0.6.
15kWh x 100 = 1500 kWh = 0.1kg x 1500 = 150kg x 20 years = 3000 kg = 3 tCO2 EMITTED
NON PASSIVHAUS Non Passivhaus would be no MVHR system, very good double-glazed windows, not a full resolution of thermal bridges, very good U-values, airtightness around 4.0.
50 kWh x 100 = 5000 kWh = 0.1kg x 5000 = 500kg x 20 years = 10,000kg = 10 tCO2 EMITTED
Around 7 tCO2 would be saved relating to space heating, over the initial twenty years, for the new Passivhaus home.
EMBODIED CARBON SAVINGS
What about the embodied carbon savings for a Passivhaus material specification change from brick/block construction to PH15 timber-based construction (frame plus low embodied carbon insulations like wood fibre)? Calculations here are for A1 to A4 stages known as ‘Cradle to Site’ for the raw material, manufacturing, and transport. The calculation method is PH Ribbon. The calculation is for the complete thermal shell. The carbon calculations have been done by us and illustrated below for circa 100m² of internal floor area, end of terrace home.
Embodied Carbon PH15 (timber) or equivalent specification 4.5 tCO2
21kg CO2e/m² x 220m² surface area = 4620kg CO2e
Embodied Carbon Brick and Block to Passivhaus standard 16.5t CO2
75kg CO2e/m² x 220m² surface area = 16,500kg CO2e
By changing the specification to a timber-based construction solution there is a carbon saving potential of 12 tCO2. The overall embodied carbon saving could be larger given there are various other reductions possible. One could make use of low carbon concrete or even better opt for a suspended timber floor, both feasible modifications.
WOW!
Around double the carbon reductions achieved from the specification change compared to the full adoption of Passivhaus. These carbon savings could easily be realised for > 100,000 new homes every year in the UK. Beyond the initial twenty-year period used for these calculations, the annual carbon savings from the lower operational energy demand will be significantly diminished as the electricity grid decarbonises. The embodied carbon savings are achieved at the time of build with immediate benefit.
We need new homes to be Passivhaus plus carbon reducing specification changes. Having signed up to a climate commitment, Passivhaus adoption is not the complete solution to fulfill climate change carbon reduction obligations, specification change requires serious consideration also. Passivhaus homes built with high embodied carbon materials should therefore require some carbon justification and architects/clients not assume they are doing ‘great’ by achieving the Passivhaus energy performance alone.
WHY PASSIVHAUS THEN?
Reasons to adopt Passivhaus are NOT predominantly based on carbon reductions. The arguments for Passivhaus adoption are:
- · Indoor air quality and removal of mould (with health/carbon implications).
- · Quality control and longevity.
- · Appropriate fit to the effective use of renewable technologies.
- · Enabling a workable decarbonised grid, especially demand reduction.
- · Fuel poverty.
- · Comfort.
When adopting the Passivhaus standard it is important to value the broad range of benefits that Passivhaus DOES deliver.
In regard to climate change mitigation, a significant reason for adopting Passivhaus is energy demand reduction. If we are building 230,000 new homes every year, for the next twenty years, the potential demand reduction by adopting Passivhaus would be 3500kWh x 230,000 x 20 = 16.1 terawatt hours (based on the earlier figures in this article). This would currently equate to avoiding the need for around 40 million PV panels to be manufactured and put into place. Alternatively, it could save fitting, maintaining, and replacing several million heat pump installations. Avoiding the need for these high embodied carbon renewable systems for new homes, supports focus on their provision for the retrofit of existing homes. These newer technologies are far from being mature supply chains, so any decrease in supply pressure would be a positive gain.
The above assessments are approximations to provide a useful ‘feel’ on the broader impacts of our choices.
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