CU Boulder — EVEN 2909 — Sustainable Buildings & Transportation
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Sustainable Buildings & Transportation

EVEN 2909: Introduction to Sustainability Engineering — Week 11

University of Colorado Boulder

Buildings: 40% of U.S. Energy

40%
of U.S. energy consumption
13%
of U.S. GHG emissions (direct)
~6B
sq ft built in U.S. per year
50%
of 2050 buildings exist today

Buildings are where we spend 90% of our time. They consume enormous amounts of energy for heating, cooling, lighting, and equipment — and most of this energy still comes from fossil fuels.

Where Energy Goes in Buildings

  • Space heating & cooling: ~40% of building energy (largest share)
  • Water heating: ~18% (residential)
  • Lighting: ~10% (declining with LEDs)
  • Appliances & plug loads: growing rapidly (electronics, data centers)

The Scale of the Challenge

  • The U.S. has ~6 million commercial buildings and ~140 million housing units
  • Average building lifespan: 50–100 years — decisions today lock in emissions for decades
  • ~50% of buildings that will exist in 2050 have already been built
  • The retrofit challenge is as important as new construction

Sources: EIA Annual Energy Outlook 2023; Architecture 2030; DOE Buildings Energy Data Book

Operational vs. Embodied Carbon

Buildings have two distinct carbon footprints. As we decarbonize the grid, their relative importance is shifting dramatically.

Operational Carbon
Emissions from energy used to run the building over its lifetime: heating, cooling, lighting, plug loads. Historically 80–90% of a building’s lifecycle emissions. Shrinks as the grid gets cleaner and buildings get more efficient.
Embodied Carbon
Emissions from manufacturing and transporting building materials: concrete, steel, glass, insulation. Emitted upfront, before anyone occupies the building. For a new high-performance building, now represents 50–70% of lifecycle emissions.

The critical shift: In a net-zero grid scenario, all remaining building emissions come from embodied carbon. This means material choices at design stage become the most important carbon decisions in construction. Yet most building codes still focus exclusively on operational energy.

Engineering implication: Whole-life carbon assessment (operational + embodied) should be standard practice. Optimizing only for operational efficiency can actually increase total emissions if it requires carbon-intensive materials.

Sources: Architecture 2030; Carbon Leadership Forum; WGBC Embodied Carbon Report

Net-Zero Buildings

A net-zero energy building produces as much energy as it consumes over a year. Achieving this requires a “reduce then produce” strategy.

Passive House Standard
Ultra-low energy standard from Germany (Passivhaus). Reduces heating/cooling demand by 75–90% through super-insulation, airtight construction (0.6 ACH50), high-performance windows, and heat recovery ventilation. Comfort without conventional HVAC.
High-Performance Envelope
The building envelope (walls, roof, windows, foundation) is the thermal boundary. Continuous insulation eliminates thermal bridges. Triple-pane windows with low-e coatings. Airtight construction with mechanical ventilation and energy recovery.
Electrification + Solar
Replace gas furnaces and water heaters with heat pumps (3–4x more efficient). Add rooftop solar PV to generate electricity on-site. Heat pump water heaters provide both hot water and space cooling as a byproduct.

The economics: Net-zero buildings often cost only 5–15% more upfront but save money over their lifetime through reduced energy bills. In many markets, the payback period is under 10 years and falling.

Sources: Passive House Institute; RMI; DOE Zero Energy Ready Home

Sustainable Building Materials

Mass Timber
Cross-laminated timber (CLT) and glulam beams can replace steel and concrete in mid- and high-rise buildings. Wood stores carbon (~1 tonne CO₂ per m³). Strong, fire-resistant (chars predictably), and seismically resilient. The tallest mass timber building: Mjøstårnet, Norway (85m).
Low-Carbon Concrete
Cement production emits ~8% of global CO₂. Strategies: supplementary cementitious materials (fly ash, slag, calcined clay), carbon-cured concrete (CarbonCure), geopolymer cements, reduced clinker ratios. Some achieve 30–70% reduction in embodied carbon.
Recycled Steel
Electric arc furnace (EAF) steel from scrap uses ~75% less energy than blast furnace steel from iron ore. The US recycles ~70M tonnes of steel annually. Structural steel is infinitely recyclable without quality loss.

Other Innovations

  • Hempcrete: carbon-negative insulation made from hemp fibers and lime binder
  • Rammed earth & adobe: low-energy, locally sourced, excellent thermal mass
  • Recycled insulation: cellulose from recycled paper, mineral wool from slag
  • Environmental Product Declarations (EPDs): standardized “nutrition labels” for building materials’ embodied carbon

Sources: Carbon Leadership Forum; Architecture 2030; WBCSD Cement Technology Roadmap

The Retrofit Challenge

New buildings get all the attention, but the vast majority of 2050’s building stock already exists. Decarbonizing buildings means retrofitting millions of existing structures.

The Scale
The U.S. has ~140 million homes, most built before modern energy codes. Average home energy use: ~10,500 kWh electricity + 41,000 cubic feet of natural gas per year. Retrofitting at the needed pace means upgrading ~7 million homes per year — we currently do fewer than 500,000.

Key Retrofit Strategies

  • Weatherization: air sealing, insulation, window upgrades — typically 20–30% energy savings at low cost
  • Electrification: replace gas furnaces with heat pumps, gas stoves with induction — cuts direct emissions to zero
  • Energy audits: blower door tests, thermal imaging to identify the biggest leaks and losses
  • Smart controls: programmable thermostats, occupancy sensors, smart lighting — 10–15% additional savings

Barriers

  • Split incentives: landlords pay for upgrades, tenants benefit from lower bills
  • Upfront cost: deep retrofits cost $20K–$80K per home, even with IRA incentives
  • Workforce: not enough trained contractors for heat pump installation and building envelope work

Sources: EIA Residential Energy Consumption Survey; RMI; DOE Weatherization Assistance Program

Smart Buildings

Building automation systems (BAS) use sensors, data analytics, and controls to optimize energy use in real time — ensuring comfort while minimizing waste.

Sensors & Automation
Occupancy sensors turn off lights and reduce HVAC in empty rooms. CO₂ sensors adjust ventilation to actual need (demand-controlled ventilation). Temperature and humidity sensors optimize comfort zones. Daylight harvesting dims electric lights when natural light is sufficient.
Demand Response
Buildings can act as flexible loads on the grid. Pre-cool buildings before peak hours, shift water heating to off-peak times, discharge building batteries during peak demand. This reduces grid stress and avoids firing up peaker plants. Buildings become grid assets, not just grid loads.
Digital Twins
Virtual replicas of buildings that simulate energy performance in real time. Identify inefficiencies, predict equipment failures, and optimize operations. Can reduce energy consumption by 15–25% in commercial buildings.

The data opportunity: Most buildings are operated blindly — no one knows how much energy each system uses. Submetering and analytics make the invisible visible, enabling targeted action.

Sources: DOE Smart Buildings; ASHRAE; Lawrence Berkeley National Laboratory

Part II

Sustainable Transportation

29%
of U.S. GHG emissions
#1
Largest emitting sector in U.S.
~280M
Registered vehicles in U.S.

Transportation is the single largest source of greenhouse gas emissions in the United States — and one of the hardest to decarbonize.

Electric Vehicles

18%
of global new car sales are EVs (2023)
50–70%
Lower lifecycle emissions vs ICE
3–4x
More efficient (energy to wheels)

Battery Technology

  • Lithium-ion dominates: NMC (nickel-manganese-cobalt) for range; LFP (lithium iron phosphate) for cost and safety
  • Battery costs dropped 90% since 2010 (~$139/kWh in 2023)
  • Solid-state batteries promise higher energy density, faster charging, and longer life
  • Vehicle-to-grid (V2G): EVs as distributed storage, feeding power back during peak demand

Lifecycle Emissions

  • EVs have higher manufacturing emissions (battery production) but much lower operational emissions
  • The “break-even point” depends on grid carbon intensity: typically 1–3 years of driving
  • On a clean grid, EVs emit 50–70% less over their lifetime than comparable ICE vehicles
  • Even on a coal-heavy grid, EVs are generally cleaner due to motor efficiency

Charging Infrastructure

  • Level 1 (120V): overnight at home; Level 2 (240V): 4–8 hrs; DC Fast: 20–40 min to 80%
  • Federal NEVI program investing $7.5B in national charging network
  • Equity challenge: renters and apartment dwellers lack access to home charging

Sources: IEA Global EV Outlook 2024; BloombergNEF; DOE Alternative Fuels Data Center

Public Transit

Emissions per Passenger-Mile

Single-occ. car
~400 g
Bus (avg.)
~160 g
Light rail
~100 g
Heavy rail
~70 g
E-bike
~20 g

g CO₂e per passenger-mile (US averages)

Induced Demand

Building more highways does not reduce congestion — it induces more driving. Within 5–10 years, new lanes fill to capacity. This is one of the most robust findings in transportation research.

Transit-Oriented Development (TOD)

  • Concentrate housing, jobs, and services around transit stations
  • Residents of TODs drive 50% less than suburban counterparts
  • Creates walkable, mixed-use neighborhoods
  • Increases land values, helping fund transit infrastructure

Sources: FTA National Transit Database; Duranton & Turner, AER 2011; TCRP Report 128

Active Transportation

Walking and cycling are zero-emission, promote public health, and are the most space-efficient modes of urban transportation.

Protected Bike Infrastructure
Physically separated bike lanes (not just paint) increase cycling rates 75–200%. Cities like Amsterdam, Copenhagen, and Bogota demonstrate that infrastructure drives mode shift, not culture. E-bikes extend cycling range to 10–15 miles, making them viable car replacements.
Walkability
Walkable neighborhoods have higher property values, better health outcomes, lower transportation costs, and stronger social connections. Walk Score is now a standard real estate metric. Key factors: sidewalks, crossings, shade, mixed-use zoning, traffic calming.
The 15-Minute City
Urban planning concept where daily needs (work, school, healthcare, groceries, recreation) are reachable within 15 minutes by foot or bike. Championed by Paris Mayor Anne Hidalgo. Requires mixed-use zoning, distributed services, and dense neighborhoods.

Health co-benefits: Active transportation reduces obesity, cardiovascular disease, depression, and air pollution exposure. The WHO estimates that if everyone cycled as much as the Danish, the world would save $24 trillion in health costs over 25 years.

Sources: NACTO; Pucher & Buehler, Transport Reviews 2017; WHO Health Economic Assessment Tool

Freight & Shipping

Moving goods accounts for ~8% of global emissions and is one of the hardest sectors to decarbonize, especially for long-haul and maritime transport.

Trucking
Moves 70% of U.S. freight by value. Heavy-duty trucks are the fastest-growing source of transport emissions. Battery-electric trucks viable for short-haul (<300 mi). Hydrogen fuel cells may be needed for long-haul. Efficiency gains: aerodynamics, platooning, route optimization.
Rail & Shipping
Rail is 4x more fuel-efficient than trucking per ton-mile. Electrification of rail is proven technology. Maritime shipping carries 80% of global trade by volume but uses heavy fuel oil. Green methanol, ammonia, and wind-assisted propulsion are emerging solutions.
Last-Mile Delivery
E-commerce has exploded last-mile deliveries — the most expensive and emissions-intensive segment. Solutions: electric delivery vans, cargo bikes, micro-fulfillment centers, consolidated delivery. Amazon alone delivers 5.9 billion packages/year in the U.S.

Aviation

  • Aviation accounts for ~2.5% of global CO₂ but ~3.5% of warming (contrails, NOx at altitude)
  • Sustainable Aviation Fuel (SAF): made from waste oils, agricultural residues, or synthetic e-fuels; currently <1% of fuel used
  • Battery-electric aircraft viable for short routes (<500 mi) by ~2030; long-haul remains the hardest challenge

Sources: ITF Transport Outlook 2023; IMO GHG Strategy; ATAG Aviation Climate Goals

Urban Planning & Land Use

How we design cities determines transportation emissions for decades. Urban form is destiny.

Sprawl vs. Density

  • U.S. suburbs generate 2–3x more transport emissions per capita than dense urban areas
  • Low density makes transit economically unviable (need ~15 units/acre for frequent bus service)
  • Single-use zoning (residential here, commercial there) forces car dependence
  • Parking minimums subsidize driving and consume vast urban land (some cities dedicate 30% of land to parking)

Zoning Reform

  • Allowing duplexes, triplexes, and ADUs in single-family zones (Minneapolis, Oregon led the way)
  • Eliminating parking minimums (many cities now doing this)
  • Mixed-use zoning: homes above shops, offices next to parks

Green Infrastructure

  • Urban tree canopy: reduces heat island effect by 2–8°F, improves air quality, sequesters carbon
  • Green roofs & walls: insulation, stormwater management, habitat
  • Permeable surfaces: reduce flooding, recharge groundwater
  • Bioswales & rain gardens: natural stormwater filtration

Urban Heat Islands

  • Cities are 2–10°F warmer than surrounding areas due to concrete, asphalt, and waste heat
  • Disproportionately affects low-income neighborhoods (less tree cover, more pavement)
  • Solutions: cool roofs, urban forests, reflective pavements, green corridors

Sources: EPA Urban Heat Island Effect; Ewing & Cervero, JAPA 2010; Strong Towns

Boulder & CU Examples

Boulder Transportation

Bike Infrastructure
Boulder has 300+ miles of bike lanes, paths, and routes. ~10% of commuters bike to work (vs. national avg. of 0.5%). The city’s multi-use path network connects neighborhoods, parks, and commercial areas. Protected bike lanes expanding on major corridors.
RTD & Transit
Regional Transportation District provides bus service (Flatiron Flyer to Denver). CU provides free EcoPass for all students, faculty, and staff. The long-planned Boulder–Denver rail (B-Line) remains unfunded — a cautionary tale in transit planning.

Boulder Buildings

Building Energy Codes
Boulder adopted some of the strongest building energy codes in Colorado. SmartRegs requires rental properties to meet energy efficiency standards. The city’s Building Performance Standard requires large commercial buildings to report and reduce energy use.
CU Campus Sustainability
CU’s Climate Action Plan targets carbon neutrality. Campus has 2.5 MW of solar capacity. Central plant efficiency upgrades have cut building energy intensity. New buildings target LEED Gold or higher. The Sustainability, Energy, and Environment Complex (SEEC) is a campus showcase.

Sources: City of Boulder Climate Initiatives; CU Environmental Center; RTD

Key Takeaways

1. Buildings and transport together = ~70% of U.S. energy
These two sectors are where most Americans’ carbon footprints are concentrated. Decarbonizing them is non-negotiable.
2. Embodied carbon is the new frontier
As grids clean up and buildings get efficient, the carbon in materials becomes the dominant issue. Material choices at design stage are critical.
3. Retrofits are as important as new construction
Half the buildings that will exist in 2050 are already built. We must weatherize and electrify millions of existing structures.
4. You can’t EV your way out of sprawl
Electric vehicles help, but land use determines how much we need to drive. Dense, mixed-use, transit-oriented development is the structural solution.
5. Infrastructure is destiny
Build bike lanes and people bike. Build highways and people drive. Build walkable neighborhoods and people walk. The built environment shapes behavior for decades.

Further Reading

Next week: Policy, Governance & Systems Change — How do we make all of this actually happen?