Life-cycle assessment of redwood lumber products in the US

Global demand for construction materials has grown exponentially in the last century, contributing to climate change and detrimental impacts on the ecosystem. To aid in sustainable growth and reduce our environmental footprint, renewable construction materials, such as lumber, have been incorporated into green building activities. The purpose of this study was to quantify the environmental impacts associated with manufacturing redwood lumber in northern California using the life-cycle assessment (LCA) approach. This study surveyed and visited redwood manufacturing facilities in the US and collected data including lumber production, co-products, resource inputs, and direct emissions to air and water. The life-cycle inventory (LCI) was developed using the mass allocation of products and co-products. Cradle-to-grave (cradle-to-gate and gate-to-grave) LCA method was used to estimate the environmental impacts and energy usage in the production of redwood lumber (1 m3 of lumber), used in a redwood-deck, and its end-of-life (i.e., the deck was demolished after 25 years of its life and redwood lumber disposed of in a landfill that captures methane). About 48% of dry mass in the redwood logs were converted to lumber in the sawmill. Depending on the redwood lumber product analyzed, the cradle-to-gate cumulative fossil energy demand was estimated to be 1862 (522–4877) MJ/m3 of redwood lumber produced. The cradle-to-gate and cradle-to-grave global warming (GW) impact were estimated at 36 (22–65) and 139 (127–167) kgCO2eq/m3 of lumber, respectively. Upstream operations (including silviculture, harvesting, and transport) and mainstream (mill) operations (including sawing, drying, and planing) contributed 53% and 47% of total cradle-to-gate GW impact, respectively. However, the disposal of the redwood lumber products was the most dominant contributor (45–65%) to the cradle-to-grave GW impact of redwood lumber. Carbon stored in the whole lifecycle of redwood lumber is about 4 (range of 3‒5) times more than its cradle-to-grave carbon footprint (CFP), a substantial environmental benefit. Considering credits from co-generation (used mill residues to generate both heat and electricity) supplying renewable electricity to the local grid decreases the net GW impact from − 468 to − 579 kgCO2eq/m3 of lumber. Many redwood lumber products such as decking are used green (freshly-cut), and a large portion of green lumber is only air-dried, which has a much lower GW impact than kiln-dried (force-dried) lumber. Also, even if the lumber requires kiln-drying, the heat comes from burning on-site mill processing residues, considered a carbon–neutral energy source. For lumber production life-cycle stages, kiln-drying of lumber tends to use a lot of thermal energy (albeit mostly from mill residues) compared with the whole life cycle. However, the GW impact from the redwood lumber drying unit process is low, only 27%, because the product tends to be used green. Furthermore, using mill residues to produce on-site combined heat and power (co-generation) was shown to be the most efficient way to reduce the environmental footprints of lumber production. Overall, the results showed that redwood lumber production has a negative GW impact and acts as a carbon sink if used in the construction sector. Specifically, the final products store 3–5 times more greenhouse gas emissions over than what is released from cradle-to-grave. There are large differences in GW impact among five categories of redwood lumber products and the rough-green lumber types have the lowest GW impact (or highest GW reduction potential) among all.