Image courtesy of Daniel Overbey.
I only learned two things in my high school physics class: conversion factors and Euchre.
The former established a lifelong habit of writing out units when dealing with complicated forms of measurement. For instance, it has helped me navigate complicated building energy calculations as there is a lot of interplay between kilowatt-hours, British thermal units, therms, and so on.
Related to building energy modeling, the U.S. building design and construction industry has consistently settled on a common unit by which to compare the annual energy requirement of projects per unit of floor area: energy use intensity (or EUI) is defined a kBtu per square foot per year (kBtu/sf/yr).
However, as the industry has shifted focus to considerations and calculations of carbon dioxide as a proxy for assessing greenhouse gas emissions (GHG) from the building sector, I have found it much more difficult to contextualize units of carbon dioxide equivalent emissions per unit of floor area.
My difficulty in developing a relative sense of carbon emissions attributable to building projects does not stem from kilograms vs. metric tons vs. pounds vs. tons. Rather, the issue is the lack of consistency by the building design and construction industry to use a common unit to define operational and embodied carbon.
From the most reputable domestic sources on the subject of operational and embodied carbon from buildings, I have found inconsistent utility of the imperial units (I-P) of pounds per square foot and the International System of Units (SI) units of kilograms per square meter. Moreover, recently I am seeing an increased use of kilograms per square foot—which is a mix of I-P and SI units (ill advised, in my opinion).
This lack of consistency in how carbon is reported per unit of building floor area has led to confusion across the industry. It is disorienting and confusing to my architecture students and I have personally witnessed highly suspect data among some of best design firms in the country when I have served as a technical reviewer for building awards.
Even through my own work, I have published embodied carbon data in lbs/ft2 and kg/m2.
We have a carbon equivalent of an EUI. Now, we need to be more aware, explicit, and consistent regarding the units we are using for operational and embodied carbon.
Conversion factors for carbon dioxide equivalent (CO2e) emissions
Please use the following to approximately convert common units of CO2e emissions per floor area.
Convert kg/m2
kg/m2 x 0.205 = lbs/ft2
kg/m2 x 0.093 = kg/ft2
Convert lbs/ft2
lbs/ft2 x 4.882 = kg/m2
lbs/ft2 x 0.454 = kg/ft2
Convert kg/ft2
kg/ft2 x 10.764 = kg/m2
kg/ft2 x 2.205 = lbs/ft2
We’d love to hear your thoughts on how we can be more consistent when reporting a building's carbon emissions. Feel free to email the Building Enclosure team at lewisl@bnpmedia.com to continue the conversation.
Daniel Overbey