Capstone: a low-carbon, resilient strategy for Cedar Row

This capstone brings the course together. Harrowgate Community Housing has asked Marlowe Builders, as construction manager, to recommend a sustainability strategy for Cedar Row that the board can approve before design development: carbon and energy targets, a structural and mechanical pathway, a certification route, procurement requirements, construction-phase commitments and resilience measures, with costs and risks. The strategy draws on every earlier lesson, and its numbers are those calculated there. As always, every figure is illustrative; a real project would use its own energy model, LCA and estimates.

The owner's goals and the targets

Good strategies begin with measurable targets that reflect the owner's goals and the project's constraints. Harrowgate Community Housing is a non-profit landlord whose tenants are often vulnerable to heat and to housing loss; its funders require third-party certification and low operating costs; and its board has adopted a climate policy committing new buildings to substantial embodied carbon reductions and all-electric operation. The team agreed the following targets.

Each target names a metric, a level and a source, and each will be assigned an owner in the contracts. Targets that depend on operations, such as actual energy use, are framed as design targets with a measurement and verification commitment, rather than as guarantees that Marlowe Builders cannot control.

Embodied carbon pathway

Lesson 3 found Cedar Row's baseline upfront carbon to be about 2,960 metric tons, or 274 kg CO2e per square meter, with the structure dominant. The team compared two pathways. Path 1 keeps the concrete frame (Option A) but uses the low-carbon mixes agreed in Lesson 4 and optimizes rebar detailing to save about 10 % of reinforcing steel. Path 2 uses the mass timber superstructure (Option B) over a concrete podium, with low-carbon mixes for the foundations and podium slabs, smaller transport loads and less winter temporary heat because the timber frame is erected faster.

# Cedar Row capstone: upfront carbon (A1-A5) of two pathways against a 200 kg CO2e/m2 target (illustrative)
GFA, TARGET = 10800, 200
def upfront(a1a3, a4, site_energy, waste=0.05):
    return a1a3 + a4 + waste * (a1a3 + a4) + site_energy
envelope_finishes = 127 + 96 + 52 + 45            # windows, cladding, insulation, gypsum (Lesson 3)
paths = {
    'Baseline (Option A, standard mixes)':  (2305 + envelope_finishes, 83, 120),
    'Path 1: Option A + low-carbon mixes, rebar optimized':
        (2305 - 374 - 47 + envelope_finishes, 83, 120),
    'Path 2: Option B mass timber + low-carbon podium mixes':
        (1573 - 173 + envelope_finishes, 70, 100),
}
for name, (a1a3, a4, site) in paths.items():
    t = upfront(a1a3, a4, site)
    k = t * 1000 / GFA
    print(f"{name:55s} {t:6,.0f} t  {k:4.0f} kg/m2  {'meets' if k <= TARGET else 'misses'} target")

Baseline (Option A, standard mixes)                      2,963 t   274 kg/m2  misses target
Path 1: Option A + low-carbon mixes, rebar optimized     2,521 t   233 kg/m2  misses target
Path 2: Option B mass timber + low-carbon podium mixes   1,980 t   183 kg/m2  meets target

Path 1 reduces upfront carbon by about 15 %, which is worthwhile but misses the 200 kg/m² target. Path 2 reduces it by about a third, to about 183 kg/m², and meets the target with some margin. The board could also reach the target with Path 1 plus further measures, such as a thinner post-tensioned slab, but the team judged those more uncertain. Path 2 becomes the recommended pathway, subject to cost and risk review.

Operational energy and carbon pathway

Lesson 5 showed that the all-electric heat pump design reduces Cedar Row's energy use intensity to about 81 kWh/m²·yr, and Lesson 6 showed that meeting a demand intensity of 30 kWh/m²·yr needs a package of triple glazing, better heat recovery and slab-edge thermal breaks. The recommended strategy includes that package, central air-to-water heat pumps with a low-GWP refrigerant (Lesson 2), heat pump water heating, in-unit HRVs, LED lighting with controls, and a 60 kW rooftop solar array that also supports the cooling refuge during outages (Lesson 11).

# Cedar Row capstone: operational energy and 60-year whole-life carbon of Path 2 (illustrative)
heat, dhw, other = 251, 240, 462                  # MWh: improved envelope, hot water, electric loads
elec = heat / 2.8 + dhw / 2.5 + other             # all-electric with heat pumps
pv = 69                                           # MWh per year from a 60 kW rooftop array
print(f"Delivered electricity {elec:.0f} MWh/yr; EUI {elec * 1000 / 8400:.0f} kWh/m2 "
      f"({elec * 3412 / 90400:.1f} kBtu/ft2); net of solar {elec - pv:.0f} MWh/yr")
path2_embodied = 1980 + 560                       # t CO2e: upfront plus replacements and end of life
base_embodied = 2962 + 620                        # baseline Option A with gas boilers
gas_mwh, gas_t_per_mwh, base_elec = 656, 0.181, 462
for grid, g in [('Ontario-like grid', 70), ('Ohio Valley grid', 416)]:
    p2_op = sum((elec - pv) * g * 0.98 ** y / 1000 for y in range(60))
    base_op = sum(gas_mwh * gas_t_per_mwh + base_elec * g * 0.98 ** y / 1000 for y in range(60))
    p2, base = path2_embodied + p2_op, base_embodied + base_op
    print(f"{grid:17s} Path 2 {p2:6,.0f} t vs baseline {base:6,.0f} t  (reduction {1 - p2 / base:.0%})")

Delivered electricity 648 MWh/yr; EUI 77 kWh/m2 (24.4 kBtu/ft2); net of solar 579 MWh/yr
Ontario-like grid Path 2  3,963 t vs baseline 11,842 t  (reduction 67%)
Ohio Valley grid  Path 2 10,994 t vs baseline 17,456 t  (reduction 37%)

With the envelope package and heat pumps, Cedar Row's site EUI falls to about 24 kBtu/ft²·yr, inside the 25 target, and solar supplies about a tenth of its electricity. Over 60 years on an Ohio Valley grid, the whole-life carbon of Path 2 is a little over a third lower than a conventional concrete building heated with gas. Most of the remaining whole-life carbon is operational, so the owner should also consider more on-site solar, a renewable power contract or community solar when funding allows; as the grid gets cleaner, the all-electric building improves automatically, while a gas building would not.

Certification pathway

The recommended certification is LEED v5 Gold, which the funder requires, combined with ENERGY STAR Multifamily New Construction, which supports the energy targets and provides third-party inspections during construction. LEED v5's emphasis on decarbonization means Cedar Row's low embodied carbon, all-electric systems and low-GWP refrigerants earn a large share of the points, and its resilience and human impact prerequisites align with the owner's goals. Lesson 7 showed that the scorecard needs a buffer of points with clear owners; the team will target about 79 points with credits for embodied carbon reduction, electrification, low-emitting materials, construction IAQ and waste diversion assigned to Marlowe Builders.

Procurement and specifications

Specifications will set maximum GWP values for each concrete mix class, require product-specific EPDs for concrete, steel, mass timber, insulation, gypsum board and glazing with the submittals, require low-emitting product documentation, and require that any substitution be reviewed for carbon and emissions as well as cost and performance. These requirements follow state and local practice rather than federal policy. The federal Buy Clean program, including low-embodied-carbon requirements for federally funded materials, was rescinded in January 2025, but the state partnership continues under the US Climate Alliance, and states such as California, with its Buy Clean California Act setting GWP limits for structural steel, rebar, flat glass and mineral wool insulation, Washington, Oregon, Colorado, Minnesota and New York, as well as cities and counties such as Portland and Marin County, have their own Buy Clean laws or low-carbon concrete codes. Harrowgate Community Housing is not bound by these, but using similar requirements builds supplier experience and helps with funders that ask for embodied carbon reporting.

Marlowe Builders will involve the concrete supplier, the mass timber fabricator and the steel suppliers in preconstruction to confirm availability, lead times and prices for the specified products. Subcontracts and purchase orders will include each documentation requirement so that trades provide EPDs, emissions certificates and quantities with their submittals rather than at the end.

Construction-phase commitments

These commitments are written into a sustainability management plan, reviewed at each monthly owner meeting with a dashboard of embodied carbon, construction emissions, waste diversion, IAQ and commissioning status. Problems are flagged early, when they can still be corrected; Lesson 8 showed that a waste target missed in the first months cannot be recovered later.

Cost, risk and the business case

The strategy has costs as well as benefits, and the board must see both. Marlowe Builders' preliminary estimate, against a conventional baseline construction cost of about $48 million, is shown below.

The premiums total about $2.9 million, about 6 % of construction cost. Part of this is offset by lower operating costs, avoided flood damage and funding that depends on certification; part reflects the owner's mission and climate policy, which a purely financial analysis would not capture. A balanced presentation acknowledges that reasonable boards could choose differently: a lower-cost strategy, such as Path 1 with heat pumps but without the solar and battery, would save about $1.1 million but miss the embodied carbon target and reduce resilience. The team presents both, with their consequences, and lets the board decide.

From handover to operation

Sustainability does not end at substantial completion. The handover package includes the as-built LCA and EPD register, the energy model updated to as-built conditions, commissioning reports, operation and maintenance manuals, refrigerant charge records, warranty information and training for building staff. Seasonal commissioning checks heating in winter and cooling in summer. Measurement and verification compares actual energy use with the model during the first two years, investigating any gap, and post-occupancy surveys ask tenants about comfort, air quality, noise and daylight. Finally, the project team holds a lessons-learned review and records what worked, what did not and what each party would do differently, so that the next project starts further ahead.