Question 4 of 5: Streamlined Comparative LCA – Wood vs. Polymer Construction Materials

Question 4: Streamlined Comparative LCA – Wood vs. Polymer Construction Materials (25 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Topic (vi) is selected: wood-framed vs. polymer (PVC/composite) decking and structural framing.

Topic description, system boundary and functional unit

Both material systems deliver the same structural/aesthetic service: a residential deck (or framing member) that supports a defined load over a defined design life. The comparison is drawn cradle-to-grave for both systems, and the functional unit is chosen as "providing 10 m² of load-bearing deck surface, in service, for 30 years" — not "one board" or "one kilogram of material" — since the two systems have materially different service lives and maintenance regimes and a mass- or count-based comparison would silently favour whichever material happens to be lighter or sold in larger units, exactly the boundary-selection trap illustrated in Question 5(a). The targeted outcome is a relative comparison of embodied energy/GHG, end-of-life fate, and durability-driven replacement burden, with qualitative treatment of the toxicity concerns specific to each material.

System inventory, impact factors and engineering judgment

For the wood system (pressure-treated softwood lumber), the dominant upstream input is renewable biomass: trees sequester atmospheric CO₂ during growth, so the raw-material acquisition stage carries comparatively low embodied energy (harvesting, milling, kiln-drying) relative to any synthetic alternative, and the wood itself continues to store sequestered carbon for as long as the deck remains in service. However, wood requires chemical preservative treatment (historically chromated copper arsenate, now more commonly copper-based alternatives) to resist rot and insect attack, introducing a toxicity/leaching concern into the use phase and complicating end-of-life disposal (treated wood cannot simply be burned for energy recovery or composted, since the preservative metals would be released). Wood also has a shorter maintenance-free service life (typically 10–15 years before refinishing or partial board replacement is needed), so the functional unit's 30-year window likely requires at least one significant maintenance/replacement cycle, adding a second round of harvesting/milling burden not visible in a single-board comparison.

For the polymer/composite system (PVC or wood-plastic composite decking), the dominant upstream burden is the opposite: petrochemical feedstock extraction and polymerization are energy- and GHG-intensive, and PVC production in particular carries a chlorine-chemistry byproduct concern (historically dioxin formation, now tightly controlled but still a process to scrutinize). The use phase strongly favours the polymer system: no preservative treatment, no periodic refinishing, and a substantially longer expected service life (25–30+ years), so within the chosen 30-year functional unit the polymer system likely requires zero replacement cycles versus wood's one. End-of-life is the polymer system's weak point: PVC/composite decking is not biodegradable, is difficult to recycle economically once contaminated with fasteners and weathering byproducts, and the dominant disposal fate is landfill, where it persists essentially indefinitely — the reverse of wood, which is (if untreated) compostable, and even treated wood can at least be landfilled without the multi-century persistence of a synthetic polymer.

Impact-category assignment: global warming potential (biogenic carbon storage credit for wood vs. fossil-feedstock embodied GHGs for polymer), resource depletion (renewable forestry vs. non-renewable petrochemical feedstock), human/ecotoxicity (preservative-metal leaching for wood vs. plasticizer/chlorine-chemistry concerns for polymer), and solid-waste/persistence (compostable/landfill wood vs. essentially permanent polymer landfill mass). Engineering judgment required: the ranking is genuinely sensitive to the assumed maintenance/replacement schedule — a homeowner who diligently refinishes wood on schedule extends its service life and narrows the gap, while a jurisdiction crediting the "avoided virgin material" value of end-of-life recycling can materially improve the polymer system's score if a genuine recycling stream exists locally (rare for weathered PVC decking in practice). No single "winner" should be declared without stating these dependencies.

Impartial assessment

Published comparative LCAs of decking materials generally find that wood's low embodied-energy advantage is eroded, and can be reversed, once a realistic maintenance/replacement schedule and the biogenic-carbon accounting convention (whether temporary storage in a 30-year deck is credited the same as permanent sequestration) are both applied consistently; composite/polymer decking's longer service life can offset its higher production-stage burden over a multi-decade comparison, but its end-of-life persistence remains an unresolved and largely unaddressed environmental liability in most current waste-management infrastructure. The correct engineering conclusion is therefore conditional rather than a declared "winner": the two systems trade a renewable-but-higher-maintenance material against a durable-but-persistent one, the ranking is sensitive to the specific maintenance regime and end-of-life infrastructure assumed, and a defensible LCA report states this sensitivity explicitly rather than asserting an unconditional recommendation.