Dwelling energy & carbon scorecard · D-080
Two accounts for four ways to build and heat a New Zealand dwelling — what each takes in energy and carbon to build (embodied), and what it takes to keep warm for 60 years after (operational). Mass timber flips the embodied account from source to sink; density and a heat pump flip the operational account.
Accounts 1 + 2 — the scorecard
All four archetypes sit in Auckland (HDD18 1,140) so the comparison isolates structure and heating system, not weather — the climate variable gets its own account further down. Frame is either light timber (New Zealand's default detached structure) or mass timber (CLT/LVL — identical intensity whether the building is 3 or 6 levels; height changes the operational account, not this one). Heating is either gas into a pre-2022-minimum building, or a heat pump behind H1-2023 insulation.
Embodied — structural frame, kgCO₂e/m²S / EST
Detached and mass-timber archetypes carry one embodied number regardless of which heating system sits on top — heating doesn't touch the frame. Reinforced concrete is shown dashed for scale only; no archetype here is built in concrete. Mass timber's −266 is sourced [S, MPI/PTL · Buchanan 2022]; detached's +30 and all embodied-energy figures below are estimated [EST].
Operational — delivered energy, kWh/yrS / EST
Heat pumps run at ≈3.0 effective COP throughout (heat demand ÷ delivered electricity) — detached-HP draws 1,586 kWh for 4,758 kWh of heat. H1-2023 insulation alone cuts detached space-heat demand ~40% (3,880 → 2,328 kWh/yr) before the heat pump is even switched on; density then compounds it through form factor, pulling 3- and 6-level mass timber down to 1,056 and 955 kWh/yr delivered.
Account 1, continued — the honesty check
Don't let the carbon story oversell the energy story. Add up every megawatt-hour it takes to grow, mill, kiln-dry and assemble a mass-timber frame and it uses more total primary energy per m² than reinforced concrete — mass timber has no total-energy advantage, and this exhibit won't claim one. What it has is a fuel advantage: about 90% of that energy is renewable biomass (bio-boiler kiln-drying, plus the wood's own biogenic content) — so the fossil share of mass timber's embodied energy runs far below concrete's.
Total primary vs fossil-only embodied energy, MWh/m²EST
Concrete: 1.24 total, ~1.04 fossil — nearly the whole total is fossil. Mass timber: 1.40 total (higher, not lower) but only ~0.11 fossil — ~90% of it is renewable biomass. Net effect: mass timber's fossil/grid embodied energy runs ~89% below concrete's. Source: Alcorn 2003 × Buchanan/PTL 2022 × NZ EPDs [EST].
The finding, at stock scale
Mass timber, medium-density, heat-pumped, H1-insulated: apply that one archetype across an illustrative 2,000,000-dwelling New Zealand stock — a scenario, not a forecast — and it wins the embodied account and the operational account simultaneously, not just per m².
Illustrative stock, EST — scenario, not forecast.
Heat-pumped draw vs the abundance model's electrified-floor heat vector (MWh/cap)S
The dwelling stock's actual heat-pumped electricity draw lands under both ceilings the wider Inari abundance model budgets for — consistent with, not equal to, the electrified-floor heat vector. The gap is the heat pump's efficiency dividend against the floor's conservative resistive-heating assumption.
Account 2, continued — climate load
Everything above holds climate constant at Auckland to isolate structure and system. Move the same H1-insulated, heat-pumped dwelling around the country and heating-degree-days (HDD18 — NIWA's sourced 1981–2010 normal, base 18°C) do the rest of the work. Auckland asks the least of any main centre; Invercargill asks the most.
HDD18 by region, ascendingSOURCED · NIWA 1981–2010
Auckland → Christchurch → Invercargill: 1,140 → 2,370 → 2,994. Sourced per-region from NIWA's "Climate and Weather of [Region]" reports — no single national table exists.
Proven, not hypothetical
None of the archetypes above are extrapolated past what's been consented and lived in. New Zealand has built mass-timber residential buildings across the full 3-to-6-level range this scorecard's medium-density archetypes model. The tallest: Community Lane B (Kāinga Ora), Avondale — 30 apartments, CLT and LVL structure, completed 2025.
All four are real, consented, built and occupied residential buildings — mass-timber housing at density, proven in New Zealand from 3 to 6 storeys, not modelled as a hypothetical. Taller mass-timber buildings exist in NZ (e.g. 8-storey office stock) but the tallest built mass-timber residential is 6 storeys.