Mapping construction sector greenhouse gas emissions: a crucial step in sustainably meeting increasing housing demands

Using Canada as a case study, this paper quantifies the construction sector's greenhouse gas (GHG) emission footprint to support a discussion on how to meet GHG reduction commitments amidst growing housing and infrastructure demands. Canada is currently experiencing its fastest population growth in nearly 70 years with commensurate pressure to build more housing and infrastructure. In 2023, Canada's population grew by 3.2% and has become one of the fastest-growing countries in the world (Marion and Ducharme 2024). As such, Canada is a microcosm of the challenges facing much of the globe; the Earth's population is expected to increase by 1.5 billion people through 2030, with most of the population growth in cities (United Nations Department of Economic and Social Affairs Population Division 2022). This growth requires increased construction of housing and associated infrastructure. At the same time, to limit the most catastrophic impacts of climate change, all sectors must rapidly decrease atmospheric emissions (United Nations Human Settlements Programme (UN-Habitat) 2023). This is a wicked combination of problems; expanding construction activities requires more materials, construction energy, transportation and services, which results in currently unavoidable GHG emissions from process emissions and energy inputs, consequently driving climate change and biodiversity loss (Zu Ermgassen et al 2022). However, failure to build sufficiently to meet the needs of a growing population would undermine the basic social contract and economy in many countries.

Construction sector emissions are slow to decarbonize, and there is currently no evidence rapid reductions will occur soon (Berrill et al 2022, Soonsawad et al 2022). Quantitatively, the tensions are stark: construction material consumption is expected to double by 2060, while global emissions must decrease to net-zero by 2050. Accordingly, construction sector emissions are at risk of increasingly dominating, or even exceeding, national emission budgets (United Nations Environment Programme 2022, Drewniok et al 2023). Expected construction growth in nearly all countries means the appropriate GHG budget for construction products (e.g. 1 bridge, 1 home) must be considered as a part of a total national or at least sectoral GHG budgets, and that per-product reductions will be insufficient. Even a 50% reduction in GHG intensity per product—currently, a very ambitious goal for construction—will be negated by a doubling of the number of products produced (e.g. doubling housing construction).

Balancing the need to both build more and pollute less requires understanding the current contribution of the construction sector to climate emissions as a starting point. While available data indicates construction is a major driver of GHG emissions (International Resource Panel 2022), specific global, national or regional data on construction emissions are difficult to obtain (Crawford 2022). For example, the Intergovernmental Panel on Climate Change (IPCC) national inventory submissions reporting obscures construction emissions within manufacturing industries, transport, building and transport operations (United Nations Framework Convention on Climate Change (UNFCCC) 2023). Insufficient data makes it difficult to set sectoral budgets and goals, hindering informed climate policy for the construction sector at the national, regional, and municipal levels.

In this study, we use a multi-regional environmentally extended input-output (EEIO) analysis to calculate Canada's construction sector GHG emissions on a national and regional level and consider the implications for future construction GHG budgeting. The EEIO approach captures emissions from raw material supply, transport, and material manufacturing, transport of construction materials and assemblies to site, onsite construction energy and physical maintenance, repair, and refurbishment of the built environment. These relate to life cycle stages A1-A3 material/resource production, A4-A5 transport and onsite emissions and B2-B5 resources and energy used in maintenance and refurbishment in the use stage (BSI 2011). We use the latest available historical data (2018), mapping 54 construction sub-sectors (broadly grouped in infrastructure, housing, and government services) across the ten provinces and three territories that make up Canada. This data is then used as a foundation to consider the GHG consequences of building much more housing, infrastructure and services in Canada (especially following business-as-usual construction patterns) in comparison to Canada's GHG commitments and what emissions that allows from the construction sector.

Scope-based analysis covering direct and indirect emissions of construction have been published (Onat and Kucukvar 2020) but there has been limited detailed work examining the total emissions associated with the construction sector (United Nations Environment Programme 2022). To our knowledge, this is the first study to comprehensively cover Canada's 13 provinces and territories, with elaborated details on material emissions (16 192 commodities), including imported emissions, and one of the first studies to examine the GHG footprint of the construction sector for any country at this level of detail (e.g. 54 construction sub-sectors). As the largest contributor to the construction sector GHG total, and as key area of required growth in Canada, we provide additional analysis focused on the Canadian housing sector; Canada needs to build 5.8 million additional homes by 2030 (Canada Mortgage and Housing Corporation 2022) to meet housing demand and restore housing affordability. Simultaneously, Canada has committed to reducing GHG emissions by 40% below 2005 levels by 2030 (Environment and Climate Change Canada 2022a). We examine the potential tension between these goals and lay out an agenda for future work to reconcile these conflicting objectives.

The following sections are organized as follows: Section 2 reviews literature on construction emissions, how these emissions are measured using EEIO and the challenges in creating construction sector GHG emission budgets. Section 3 explains the multi-regional EEIO approach (3.1), data preparation (3.2) and analysis (3.3). Section 4 presents Canada's construction emissions by subsector (4.1), origins of emissions and final construction emissions by province and lifecycle stage (4.2). The results conclude with an estimation of residential construction per-unit calculation (4.3 ) and potential future construction emissions, based on Canada's housing development goals compared to Canada's international emission reduction commitments (4.4).

The construction industry is a major global consumer of materials and associated GHG emissions (Hertwich et al 2020). In 2009, construction activities alone were already estimated to make up 23% of global emissions (Huang et al 2018). Within construction GHG emissions, building material manufacturing and construction are the largest contributor, accounting for approximately 11% of global GHG emissions and rising (Global Alliance for Buildings and Construction, I.E.A. and the U.N.E.P 2019, Hertwich et al 2020). Acknowledging the escalating demand for construction driven by population growth, changes in family formation, housing, and infrastructure needs, there is widespread recognition of the need for substantial global decarbonization within the construction sector to align with net-zero emission goals (Pauliuk et al 2021, Zhong et al 2021). However, construction sector emissions (buildings and infrastructure) are not identified in reported emission statistics like the United Nations' IPCC Nationally Determined Contributions Registry (NDC) reports (Hertwich et al 2020). The NDC reports are the main way national emissions are documented and understood, meaning the dividing of construction emissions into other sectors (e.g. energy, manufacturing) obscures the collective impact of this high resource sector. Moreover, the availability of building and construction data is lacking in resolution, making it challenging to comprehend trends and demands across regions, construction sub-sectors and other industry systems (United Nations Environment Programme 2022).

GHG emissions in construction are evaluated in the literature using bottom-up, top-down or hybrid approaches (Ward et al 2018). Bottom-up lifecycle assessment approaches that measure and estimate material process emissions and operational energy at the granular level in buildings have been extensively applied to quantify the embodied emissions in construction (Greer et al 2023). However, these approaches underestimate embodied upstream emissions ('truncation error') and expansion of this approach to a national level has proven challenging due to multiple assumptions relating to boundary setting, consistency in methods used and variations in data availability (Yu et al 2017, Ward et al 2018, Nahangi et al 2021). EEIO is a macroeconomic-based, top-down approach, which has been used to calculate the main drivers for environmental impacts of products and industries since the late 1960s, notably expanding these approaches to measure GHG emissions during the 2000s (Miller and Blair 2022). Increasing availability and higher resolution of economic and environmental impact datasets at the national, regional and city levels make the EEIO approach a relevant tool to tackle lifecycle emission mapping challenges across different sectors and provide input to GHG budgeting.

EEIO models have been employed to measure GHG emissions across sectors, including household consumption (Castellani et al 2019), tourism (Sun et al 2020), the food industry (Reutter et al 2017), and high-level intersectoral distribution within economies (Xia et al 2022). Within construction, EEIO has been used to evaluate economic and energy impacts (Chan et al 2020), environmental impact of the road industry (Agez et al 2020), and increasingly applied to calculate the total construction emissions of countries mostly on a broad level analysis. Huang and Bohne (2012) calculated Norway's construction emissions over time (2003–2007). Acquaye and Duffy (2010) analyzed Ireland's direct and indirect emissions including five construction sub-sectors. Onat and Kucukvar (2020) compared a scope-based analysis between five countries (China, the USA, India, Japan and Canada). A limitation associated with EEIO analysis is dependency on data availability, specifically on input-output table details and lack of representation of countries, sectors and sub-sectors. (Wood et al 2014, Huo et al 2022). Consequently, most top-down GHG calculation approaches provide an aggregated representation of the construction industry. Within construction emissions studies, detailed EEIO studies typically focus on buildings (Berrill et al 2022, Zu Ermgassen et al 2022, Drewniok et al 2023). Meeting sectoral emission reduction commitments requires expanding construction sector analysis to non-building elements like infrastructure and extending the product stage analysis (e.g. A1–A3) to include more aspects of the full lifecycle (e.g. maintenance) (Soonsawad et al 2022).

Despite the importance of the construction sector to achieving net-zero emissions commitments, most countries, including Canada, have yet to comprehensively measure construction industry emissions, let alone establish a construction emission carbon budget to meet its emission reduction commitments. A notable exception is the UK, which has set a construction sector GHG budget within their Sixth Carbon Budget (2033–2037) (Climate Change Committee 2022). Carbon budgets are defined as 'The net amount of GHG that could be released in the future by human activities while keeping global warming to a specific level, accounting for other non-GHG warming contributions' (Intergovernmental Panel on Climate Change (IPCC) 2023). The global carbon budget, calculated by the IPCC, is translated into national carbon budget commitments that account for production emissions in each country. Sectoral carbon budgets are the allocation of emissions targets for specific sectors within the broader economy. These targets and indicators help policymakers generate balanced and informed policies and standards toward reaching emission commitments (Vogt-Schilb and Hallegatte 2017).

Methods for allocating GHG budget within sectors vary. One prevalent approach is based on economic sector size and proportional emissions per dollar. However, some researchers argue for sectoral adjustments based on sectors ease of decarbonization (Steininger et al 2020). Examples for relatively easy-to-decarbonize sectors are light-duty transport, heating, cooling and lighting, classified as such because they can be electrified and powered by renewable energy (e.g. wind and solar). Challenging sectors include manufacturing of carbon-intensive materials like steel and cement, aviation and long-distance transport and shipping (Davis et al 2018), many of which feature in the construction sector. In the construction sector, the centrality of material decarbonization is key in decarbonization efforts as these indirect emissions are the lion's share of total emissions, ranging from 77%–95% (Hung et al 2019). However, construction materials remain hard to decarbonize due to process emissions in the manufacturing of materials and the high temperatures required in manufacturing that have been challenging to produce without fossil fuel energy sources. Emerging hydrogen and carbon capture technologies which address these challenges have been slow to deploy and are unlikely to meet reduction goals on time (Nelson and Allwood 2021).

Another less-discussed challenge for GHG reduction in the construction sector is increase in construction demands and the growth in stock (Steininger et al 2020, Berrill et al 2022, Drewniok et al 2023). Meeting these growing demands will require both increases in material efficiency and recycling along with decreased GHG intensity of material production. Global scenarios suggest, however, that decarbonization of steel and cement (the most used construction materials globally) will rely heavily on carbon capture and other strategies that implicitly require large-scale deployment of infrastructure (Watari et al 2023). This in turn, places increasing strain on the supply of construction materials in a carbon constrained world, meaning that their feasible supply within GHG emission commitments will likely fall short of demand (Watari et al 2023). Such constraints further increase the urgency to set realistic GHG budgets for the construction sector and examine potential frictions with growing infrastructure needs.

Our method was split into two phases (1) data preparation and (2) analysis (figure 1). In the data preparation phase, we used multi-regional EEIO tables for Canada to generate life cycle inventory data for the entire construction sector, generating datasets representing different cross sections of disaggregated GHG results (e.g. by geography, by life cycle stage,) to understand the construction sector's footprint within the economy. In the analysis phase, we (i) analyzed the emission profile of the construction sector to categorize current sources and drivers of emissions; (ii) explored per-m² GHG emissions for housing and the challenges of top-down versus bottom-up accounting and policy setting; and (iii) discussed the implications of construction emissions on infrastructure and housing policy going forward.

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3.1. Environmentally extended input-output (EEIO) models

3.2. Data preparation

3.3. Analysis

4.1. GHG emission from construction in Canada in 2018

The EEIO model for Canada's construction sector GHG emissions showed that the construction sector was responsible for 90 MtCO₂e of emissions using a consumption based accounting approach. This calculation, including all GFCF construction sub-sectors, showed that residential buildings is the highest emitting construction sub-sector (37.7 MtCO₂e), followed by 'Other municipal government services' (7.2 MtCO₂e), 'Transportation and warehousing' (7.2 MtCO₂e), and 'Oil and gas extraction' (7.1 MtCO₂e) (figure 2). The combined distribution across development types shows Residential (42%) and Infrastructure (39%) developments each are roughly double that of the service sector (19%) (figure 2).

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Of these 90 MtCO₂e, 35%, are imported as illustrated in figure 3. The top countries of origin include China (9.78 MtCO₂e), the United States of America (USA) (8.61 MtCO₂e), India (1.02 MtCO₂e) and Mexico (0.9 MtCO₂e). If material sourcing norms continue, Canada's construction sector emissions reductions will be partially dependent on climate commitments and follow-through in other countries, particularly China and the United States. Individual international sectors with the highest direct emissions in the Canadian construction sector supply chain include Electricity by coal (2.9 MtCO₂e) used for off-shore manufacturing, Iron and Steel (1.6 MtCO₂e, process emissions), and Cement, lime and plaster (1 MtCO₂e, process emissions) from China; as well as Crude oil extraction processes (1.5 MtCO₂e), Electricity by coal (1.3 MtCO₂e) and Gas (0.6 MtCO₂e) from the USA (figure 3).

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At a provincial level, residential construction dominates construction GHG emissions in most provinces and is particularly prominent in Prince Edward Island, Nova Scotia, Quebec, Ontario and British Columbia, where it exceeded 50% of the total (table 1). In Newfoundland and Labrador, Saskatchewan, Alberta, and the three territories, oil and gas infrastructure is the largest sectoral contributor to construction GHG emissions. These findings are in line with population growth trends in Canada and the regional investment in oil and gas infrastructure. For example, Alberta—Canada's largest oil and natural gas producer—had construction emissions for oil and gas in 2018 equal to 83% of British Columbia's residential construction and 93% of Quebec's. In New Brunswick, infrastructure/buildings for government services are notably large contributors, though as the data provides a one-year snapshot and it is not clear if New Brunswick always builds a lot of government buildings/services or if a large project in 2018 influenced these results (figure 4).

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Table 1. Residential construction emissions by province (roughly arranged from East to West) and territories in comparison to total GHG emissions in the province or territory (Consumption. MtCO₂e, 2018).

Province/Territory	Total Construction GHG (MtCO2e)	Residential structures	%
Newfoundland and Labrador	1.93	0.28	15%
Prince Edward Island	0.22	0.12	52%
Nova Scotia	1.78	0.98	55%
New Brunswick	1.16	0.42	36%
Quebec	13.7	6.84	50%
Ontario	26.8	13.5	50%
Manitoba	3.12	1.17	38%
Saskatchewan	5.49	1.00	18%
Alberta	20.0	5.76	29%
British Columbia	14.5	7.61	53%
Yukon	0.20	0.05	24%
Northwest Territories	0.29	0.03	10%
Nunavut	0.36	0.01	3%

4.2. Emission sources by category and lifecycle stage

While figure 2 focused on sectors responsible for direct emissions (point of emission basis), table 2 presents results aggregated across the supply chain for each major input to the final construction process (i.e. on a cradle-to-gate basis). In this cradle-to-gate emission analysis of Canada's construction sectors (consumption) for the product and construction stages, we found that 84% of emissions are from the product stage, 5% for transportation to site, and 11% are direct emissions of the construction stage: 10% onsite operation, 1% design services and margins. This reinforced the imperative for efforts to reduce construction sector emissions to focus on reducing material use and the GHG intensity of materials. Nationwide, the most dominant construction materials are Concrete (14%), metals (12%) and wood (9%) (table 2). For the full definitions of emission sources including each class and examples of prominent commodities see the SI (SI Sheet 2, Sheet 3).

Table 2. Emission distribution across all construction in Canada (MTCO₂e 2018, consumption).

	Emission type (all construction sectors)	MTCO2e	% Total
Product stage	Concrete	13	14%
	Metals	11	12%
	Wood	8.2	9%
	Energy	8.2	9%
	Design services & margins (product stage)	6.7	7%
	MEP	6.3	7%
	Manufacturing services	5.8	6%
	Asphalt	3.2	4%
	Other	3.1	3%
	Plastics & composites	2.8	3%
	Finishes	2.2	2%
	Masonry	1.4	2%
	Openings	1.4	2%
	Thermal & moisture protection	1.2	1%
	Earthwork & landscaping materials	1.1	1%
Construction stage	Transport	4.8	5%
	Design services & margins (construction stage)	1.0	1%
	Direct emissions	8.7	10%
	Total	90

A further breakdown of these same life cycle stages and emission types is available in SI table S4 for each construction sub-sector (residential, mining, etc), and in table S5 for each province/territory. In residential construction, the top three emitting materials are wood (19%), concrete (12%), metals (9%). The inter-province analysis shows that the more urbanized provinces (Ontario, Quebec, British Columbia and Alberta) see a larger share of emissions from concrete (12%–18%) than more rural provinces (Manitoba, Saskatchewan, New Brunswick and the Territories) (1%–8%). Normalizing per capita identifies big ranges in residential emissions. The lowest emission per capita are in Nunavut (249 kgCO₂e/capita) and Newfoundland and Labrador 540 (kgCO₂e/capita) and the highest were British Columbia (1525 kgCO₂e/capita) and Alberta (1336 kgCO₂e/capita). For context, the fastest-growing provinces/territories (on a percent basis) in 2018 (Statistics Canada 2018) were (in order) PEI (1.8%), Ontario (1.8%), Alberta (1.5%), British Columbia (1.4%), Nunavut (2.2%) and The Yukon (2.1%), which aligns but only loosely with the top residential construction emissions per capita (table 3).

Table 3. Residential emission per capita by province (kgCO₂e, 2018, consumption).

Emission source	NU	NT	YT	BC	AB	SK	MB	ON	QC	NB	NS	PE	NL	Canada total
Wood	29	102	180	300	304	187	186	151	170	102	214	151	113	195
Concrete	10	6	106	278	157	30	36	104	99	39	153	75	46	126
Metals	18	31	96	91	100	84	102	104	60	51	69	73	46	88
MEP	11	10	67	94	65	49	45	66	43	34	59	47	37	62
Manufacturing services	8	25	82	76	95	68	56	60	42	34	69	45	34	61
Design services and margins (product stages)	15	43	86	99	114	76	57	58	51	38	72	53	44	69
Plastics & composites	10	7	43	55	63	38	41	54	39	25	38	26	23	49
Finishes	29	48	39	66	56	41	45	42	41	23	42	28	21	46
Other	38	51	73	68	44	35	38	35	29	26	33	30	23	39
Energy	8	29	36	47	52	44	28	31	27	29	59	52	15	36
Openings	6	34	23	41	15	9	30	29	35	18	13	12	7	29
Thermal & moisture protection	0	0	0	24	22	9	24	21	17	33	21	14	6	20
Asphalt	41	56	66	43	36	47	12	19	23	27	20	15	9	26
Masonry	2	2	18	29	18	13	13	8	9	11	16	14	6	13
Earthwork & landscaping materials	2	2	9	12	10	9	4	7	7	5	13	8	9	8
Transport	23	127	138	96	81	58	60	68	53	31	67	58	49	68
Design services and margins (product stages)	2	20	36	31	68	19	14	21	13	6	11	9	14	25
Direct emissions	—	32	40	77	36	47	75	61	58	13	49	55	39	58
Total	249	626	1140	1525	1336	863	868	939	816	544	1016	765	540	1018

Figure 5 summarises this section by illustrating the emission flow through Canada's construction sector to emission source, construction subsector and consuming province.

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4.3. Residential construction per-m² GHG emission

In 2018, Canada completed 200 262 new units of housing (Statistics Canada 2023b). Overall, that equates to 188 369 KgCO2_e per unit completed nationally on a consumption basis, including the impact of renovations on total housing emissions. To compare emissions intensity of residential construction across the country, we normalize our province and territory residential emission results (table S5) by built floor area in 2018. Results are adjusted to account for different emission boundaries for residential buildings, including consumption and territorial emissions, total, new construction and renovations. The emissions associated with new residential floor-area varies between provinces. Prince Eduard Island reported the lowest GHG intensity (391 KgCO₂e m⁻²) and Alberta the highest (1251 KgCO₂e m⁻²). The monetary share of new construction (construction lifecycle phases A1–A5) compared to renovation or refurbishment (phases B2–B5) ranges between 64% new construction in New Brunswick to 85% new construction in British Columbia and Prince Edward Island. The largest amount of new construction was reported in Ontario, representing ∼43% of all new floor area in Canada in 2018. We apply the monetary relationship to emissions 1–1 in the absence of better data on the different GHG intensity of each dollar of renovation or new construction. The share of emissions that were emitted locally (e.g. in the same province as construction) ranges from 14% in the territories to 50% in Quebec and 61% in Alberta (figure 6)—raising important questions about how national GHG budgets can/should be allocated regionally and again highlighting the difference in production versus consumption-based accounting.

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Published literature on bottom-up GHG accounting finds that cradle-to-grave embodied emissions in residential buildings vary greatly between 179 and 1050 kgCO₂e m⁻² (Chastas et al 2018). Regulations in Canada and around the world are increasingly based on per-m² benchmarks. For example, Vancouver, Canada now sets a limit of 400 kgCO₂e m⁻² (City of Vancouver 2023); Toronto, Canada's Green Standard sets a limit of 350 kgCO₂e m⁻² for new residential buildings (A1–A5) (City of Toronto 2022). Though broadly aligned with past results, our top-down accounting skews toward the upper end of previously reported bottom-up ranges, especially when considering the full consumption-based accounting. Although these results are highly aggregated and cannot easily be adapted to evaluate any single building project, they suggest meaningful truncation errors that could undermine the success of bottom-up emission limits if those limits fail to capture all activities and emission sources within the sector (e.g. renovation). However, using this top-down number to set regulation would be much too broad if bottom-up calculation including truncation were used to meet them. The gap between bottom-up regulation and top-down budgeting is an area that needs further research, thought and policy.

4.4. Future construction emission estimation

Finally, we compared Canada's territorial 2018 construction emissions (58 MtCO₂e) to three scenarios which contrast allowable annual emissions with prospective future scenarios for the construction sector. Based on proportionally allocating emissions, in 2030 Canada's construction sector can emit 34 MtCO₂e. We compare this to an upper bound 'meeting housing demand' emissions scenario if business as usual is scaled up (3.625) and ratios between housing and infrastructure construction hold with no changes in material use or manufacturing. We add a 25% material emission decarbonization while meeting 2030 housing demand to consider the likely reductions in material GHG intensity by 2030. The results (figure 7) highlight that in a business-as-usual scenario, to meet 2030 commitments, Canada would need to reduce its territorial emissions by 83% for every existing unit of construction. Even after factoring in 25% material decarbonization, a 78% reduction in GHG emissions would still be needed—requiring substantial new efficiencies to be found. These reductions concern only the territorial emissions under Canada's international reduction commitment, the 35% of emissions imported are excluded (and shown in hashed grey).

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This study calculated national construction GHG emissions in high resolution using a top-down EEIO approach to inform the discussion of the tensions between meeting future construction needs, in light of national GHG reduction commitments, using Canada as a case study. Our findings show that the status quo of how Canada is currently building cannot be scaled up to 5.8 million homes by 2030 without disproportionate impacts on the environment, and that big changes in how Canada builds will be needed to simultaneously meet housing and climate commitment. To meet the 2030 climate and housing commitments in parallel, an 83% GHG reduction per product (e.g. per home) is needed to achieve the 40% sector-wide emissions implied by Canada's international climate commitments.

Our study showed that in Canada, material production is responsible for the majority share (84%) of the construction sector's consumption-based GHG emissions, which aligns with the international 77%–95% range (Hung et al 2019). The main challenges associated with these emission reductions include slow decarbonization of material intensity, which are expected to only rapidly reduce after carbon capture and storage technology is broadly employed. However, the delivery pace of such technology has been much slower than expected (Watari et al 2022, Climate Change Committee 2023), suggesting an urgent need to rely more on other approaches like material efficiency. Another challenge will be negotiating offshore and territorial emissions. We found that 35% of construction emissions are imported; a regulatory framework to tackle the construction sector GHG will need to address both domestic production and imports. On a transnational level, this highlights the importance of fostering global collaborations for low GHG material manufacturing, resource efficiency and transborder monitoring of material flows and environmental impacts (International Resource Panel 2022). On the national level, care needs to be taken to deter offshoring of emissions to create the appearance of territorial GHG reductions.

The GHG mitigation gap between building more and a 40% sectoral GHG reduction for construction can be tackled in a few ways:

• 1)  
  Changing the ratio between housing and other infrastructure. For example, building more residential buildings while minimizing new roads, water or extraction infrastructure. Reducing the material investment in future infrastructure will allow for a higher proportion of construction GHGs to be directed towards housing. This can be accomplished, for example, by increasing the percentage of new housing built within the existing urban boundary, reducing the need for new transport and water infrastructure. New neighbourhoods could also be built with less infrastructure, for example, narrower and straighter roads. At the national scale choices can be made between infrastructure types. Within Canada, 13% of construction emissions are related to the extraction industry, particularly oil and gas. This means Canada may face a choice of whether to 'spend' future GHG emissions budgets on resource extraction or on housing—or else search for other ways to offset these emissions.
• 2)  
  Changing the amount of new material used to build and or choosing lower GHG materials. e.g. more efficiently designed structures, building smaller, more efficient reuse of existing structures and materials, selecting low GHG materials. Tackling material efficiency focuses on the larger share of materials and is immediately deployable without waiting for improvements in material manufacturing and/or carbon capture. Material efficiency and material choice options are discussed in more length in other work (Hertwich et al 2019, Berrill et al 2022, Arceo et al 2023).
• 3)  
  Enhancing material circularity in building stocks by reusing material in end-of-life through redevelopment and retrofits (London Energy Transformation Initiative 2020), repurposing non-residential buildings into apartments (Gursel et al 2023), or other building adaptation projects (Shahi et al 2020). These approaches save on structural-related and embodied emissions like cement, and steel in new construction by reusing elements in situ. Though it is important to note this is not a panacea as Canada, like many fast-growing countries, currently builds much more new buildings/infrastructure than there are end-of-life resources available for repurposing.
• 4)  
  Allocating more GHG budget to construction from other sectors (e.g. require faster mitigation in the transportation or energy sectors). This paper has worked with the assumption that the construction sector must reduce its GHG emissions by 40%, in line with Canada's economy-wide commitments. However, Canada could choose to allocate more GHG budget to construction in line with its economic importance and difficulty in decarbonizing. However, allowing slower reductions in the construction sector would demand faster reductions elsewhere. Further, construction sector emissions are poised to increase rapidly, so even with the reallocation of emissions from other sectors, large changes will be needed in how and what gets built.

While this paper focused on Canada, the findings here are informative for other countries which need to build more while polluting less. Construction will need much faster per-product GHG reductions than the overall mitigation goals in countries experiencing construction growth (nearly all of the world), which will need to make key decisions and more consideration on what gets built (e.g. housing versus vs. transport vs. extraction infrastructure).

The GHG associated with each m² of new construction is much higher when taking a top-down approach compared to the bottom-up approach, where the narrower boundaries exclude upstream impacts like services and indirect energy. Existing embodied GHG regulations generally apply only to new buildings, narrowly focusing on per m² metrics, which do not consider the total amount being constructed. For example, they overlook up to one-third of the emissions coming from renovations in Canadian provinces required supporting infrastructure. A top-down approach could set a total budget and then divide by the total expected amount of construction to set an annual GHG limit per m². However, starting with the wider boundaries of a top-down approach like EEIO would mean results which are skewed high. The truncation error of top-down EEIO to bottom-up LCA and hybrid approaches is between 30% and 80% (Ward et al 2018). Designers and builders submit documentation to meet regulations based on bottom-up accounting, so a top-down to bottom-up adjustment would be needed.

In this study, we use an EEIO approach to provide the first comprehensive mapping of Canada's construction sector emissions, identifying a total of 90 MtCO₂e in 2018. Residential construction emerged as the highest emitting sub-sector, accounting for 42% of these emissions. This sets a baseline for understanding Canada's potential future emissions as it aims to rapidly scale up construction, particularly housing construction. Meeting both housing demand and rapid decarbonization goals will require changing Canada's construction portfolio (e.g. more housing, less oil and gas, more housing, fewer roads), being more efficient and/or transferring GHG budget from other sectors to construction. As a third of Canada's construction GHG emissions are imported, this is not a problem Canada will be able to solve alone without large scale onshoring. In the next decade, Canada's construction sector emissions are likely to continue to depend in part on manufacturing choices in other countries, particularly the United States and China. While focusing on Canada, this study also provides insights relevant to all other countries struggling with increasing construction demands. It highlights the urgency for transformative changes in the construction sector to align with emission reduction goals and the tension between choices on what to build (e.g. housing or other infrastructure) within a limited emissions budgets.

This research is funded by the Centre for the Sustainable Built Environment (CSBE) at the University of Toronto, the Clean Economy Fund for submission to the Task Force for Housing & Climate, a grant from Infrastructure Canada under the Research Knowledge Initiative (RKI), a Lyon Sachs Postdoctoral Fellowship at the University of Toronto, and the Canada Research Chair in Sustainable Infrastructure, Grant/Award Number: 232970. CSBE in turn is funded by an NSERC Alliance Grant (ALLRP 582941–23), the Climate Positive Energy Initiative and the School of Cities both at the University of Toronto, and 12 industry partners (Colliers; the Cement Association of Canada; Chandos Construction; Mattamy Homes; Northcrest; Pomerleau; Purpose Building, Inc.; ZGF Architects; Arup; SvN Architects + Planners; Entuitive; and KPMB Architects).

All data that support the findings of this study are included within the article (and any supplementary information files).

• 1.  
  Hatzav Yoffe: Writing—Original Draft, Data Collection, Analysis, Visualization/Graphics
• 2.  
  Keagan H. Rankin—writing—Review & Editing, Technical Validation, Visualization/Graphics
• 3.  
  Chris Bachmann—writing—Review & Editing, Methodology
• 4.  
  I Daniel Posen—writing—Review & Editing, Methodology
• 5.  
  Shoshanna Saxe: Conceptualization, Methodology, Writing—Review & Editing, Funding Acquisition, Supervision