D U N D E E PA S S I V E H O U S E
CASE STUDY
EMBODIED CARBON
A
n analysis of the building’s whole life carbon score was conducted by John Butler Sustainable Building Consultancy, using PHribbon. Based on a cradle-to-grave analysis, the building totalled 496.8 kg CO2e/m2 GIA – a total which in large part reflects the fact that the analysis assumes the stored CO2e from the timber and timber-based products is released after sixty years (the standard lifespan for the assessment, in line with the RICS whole life carbon methodology.) From cradle to practical completion, however, the building has a total of 353 kg CO2e/ m2 GIA, with a further 216 kg CO2e/m2 stored in the timber and timber-based products at that point. The analysis also compared the as-built external walls against a cavity wall version to the same spec. The as built walls showed a small reduction compared to the cavity wall when considered on a cradle to grave basis, from 221 kg CO2e down to 194 kg CO2e/m2. But at practical completion, the difference is far more marked: 224 KG CO2e down to 154 kg CO2e – and with 131 kg of stored CO2e in the as-built walls, versus just 15 kg in the cavity walls. The analysis included the external envelope, internal walls, floors, and finishes. The stairs were excluded, as was the balcony, and the only internal fixtures included are the sinks/basins/WCs. Due to the absence of environmental product declarations for the heat pump, MVHR system and ductwork, data from the closest comparable certificates was used, drawing from the Product Environmental Passport database – albeit with data for stainless steel ductwork instead of the polypropylene ductwork that was used. The analysis assumed no replacements of building fabric during the projected sixty-year life
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but assumed three replacements of the hot water heat pump, two replacements of the hot water pipes, sanitary ware and sinks, and one replacement of the PV array and MVHR system. The radiant panels and electric towel rails weren’t included, due to lack of data. The entire superstructure of internal and external walls and roof (and window frames, excluding any aluminium cladding) is made from timber and timber-based products, so it’s perhaps unsurprising that they represent the largest chunk of embodied carbon in the building, at a combined total of almost 84 kg CO2e/m2 – though they also provide over 216 kg of sequestered CO2e, meaning these materials store more than 2.5 times as much CO2 as their manufacturing process emitted – at least for as long as they remain in the building. The next largest chunk of emissions is the zinc cladding and roofing at almost 57 kg CO2e/m2, followed by the PV at 50 kg CO2e/m2, and the concrete from the foundations at 38 kg CO2e/m2, with inert products – including glazing, plasterboard, plaster and ceramics – at 34 kg CO2e/m2, with over 39 kg CO2e/m2 relating to bringing materials from the factory gate to the site. Much of this is specific to the concrete – over 42 per cent of that figure is associated with the concrete supply. A treemap lays the cradle to grave figures bare: building-related services, including the PV array, the hot water heat pump and MVHR system, represent 27 per cent of the total, with the roof, external walls and substructure at 20, 18 and 16 per cent respectively. Although triple glazed windows have been criticised as being too high an embodied carbon cost by some high-profile life cycle assessment experts, in this case the total for windows, doors and rooflights only reaches 5 per cent of the total.
Embodied Co2e Comparison: Cradle to Grave, External Walls Only 120 100 80 60 40 20 0 -20 -40 -60 Cavity As Cavity As Cavity As Cavity As Wall Built Wall Built Wall Built Wall Built
Module A
Stored CO2
Module B
Module A: Concrete Module A: Inert Module A: Oil Based Module A: Zinc Module A: A4 Transport to Site Module A: A5 Construction Module A: Aluminium Module A: Brick Stored CO2: Timber Stored CO2: Timber Based Module C: Demolition & Disposal
Module C