Writing to share a study Ha/f recently put together for the City of Toronto looking at the relative impacts of using Gross Floor Area (GFA) or Constructed Floor Area (CFA) as the denominator in embodied carbon targets. This study follows a somewhat infamous debate ongoing in Canada’s carbon community about what measurement to use for emerging policies.
In line with the City of Vancouver, Ha/f’s stance has been GFA is the correct metric to use as it serves to prioritize space dedicated to the primary function of a building. In other words we should dividing the total emissions of the building, by the total floor area dedicated to the use of the building (in these examples it is housing).
CFA (sometimes called BFA) on the surface makes sense - divide the total emissions by the total floor area. The problem is that this is disconnected with how our planning and building codes operate. Across North America lot coverage maximums use GFA as the basis, not CFA. In most all North American cities floor area ‘below grade’ is not considered in that maximum allowable area and is thus a loop-hole to build more floor area on a site. Typically this manifests itself in basements in SFHs, and parking garages in larger buildings (often on account of needing to accommodate minimum parking requirements).
Our study illustrates that by using CFA as the denominator there supports the perverse incentivization of digging down and building underground. The ‘greenest’ buildings in the CFA versions is the one with the biggest basement. Importantly this does not include the impacts of excavation and shoring which would drive these figures even higher.
Policies shape design. Sharing this in the hopes of establishing broader alignment across North American policy development. More in our recent report for the City of Toronto here if interested: Research & Policy — ha/f
Note: As a ‘new user’ I am unable to share the graphics here but am happy to send anyone the report directly.
I’ll check it out – thanks for all of your research!
The denominator is a challenging thing to get right. On one project we used ‘fire area’ or area calculated per the fire dept. It includes terraces above grade, as well as an atrium openings, plus areas covered by building overhangs.
Have you found any issues in general with using area in the denominator? I’m interested in whether it would need to be adjusted so it wouldn’t penalize smaller homes? Larger homes have larger rooms may have less embodied carbon per area (in terms of structure and interiors), while I’d expect smaller apartments that still have kitchens and bathrooms but less open space would have a larger embodied carbon per area due to more walls, higher proportion of kitchen/bathroom space, etc.
-Kjell
Kjell Anderson FAIA, LEED Fellow
Principal, Director of Sustainable Design
Excellent study - loving the graphic presentation of aspects like facade intensity.
For building performance in general, we use gross internal area. This is total area within interior finish of exterior walls, including stairs, shafts, basements.
For ASHRAE, this aligns with the conditioned area and metrics like EUI.
For the IBC, this aligns with Building Area (somewhat different than allowable area)
For BOMA it’s about the same as Gross Floor Area (they exclude shafts).
For LCA, this also tracks better with the functional unit of assessment for most real estate - rentable area.
That said - the normalization by number of bedrooms seems very relevant for house LCA - it appears your recommend using gross floor area and not bedrooms?
Couldn’t agree more Stuart. Per bedroom, per desk, per occupant, etc. would be the best denominator as it would drive a sufficiency mindset. As Michelle Addington has pointed out - while our buildings may be using less energy per square foot over the past couple decades, they’ve also doubled in size which cancels out the reductions and drives up embodied carbon.
The gross internal, gross heated, etc is, perhaps, the first step towards that but I’d certainly support leap-frogging to occupation-based denominators where possible.