CHAPTER 14FLEXIBILITY, USE AND LIFE CYCLE ISSUES
FIG 14.0 (chapter opener) Floor slabs make up the majority of a commercial building’s superstructure, by mass, volume and associated lifecycle impact. CLT floor slabs and a glulam frame create flexible interiors and are conspicuous when viewed on approach through extensive glazing at the First Tech Credit Union, Hillsboro, OR, United States by Hacker Architects (2018).
This chapter considers issues around building use, flexibility and life cycle impacts, including carbon accounting aspects and end of life considerations. The stage structure and impact categories used to frame this chapter are based upon definitions within EN 15978 as illustrated by Figure 14.1.1
FIG 14.1 Assessment stage and impact categories relevant to CLT structures.
Life cycle issues can be complex, especially investigated in detail. Teams will need to adopt a practical approach to suit their level of experience and may wish to engage specialist support to assist compiling (or reviewing) data.2
RELEVANT ELEMENTS
As reflected within Life Cycle Assessment (LCA) common tools, CLT use may influence the following materials and sub-groups:3
- Substructure materials: foundations/lower floor elements.
- Superstructure materials: external walls, structural frames, internal walls and partitions, upper floors slabs, roof elements, stairs and ramps.
Floor slabs typically represent the bulk of a superstructure’s material use and impacts and therefore might be used for initial comparative studies and calculations.4
CARBON FOOTPRINTS
The global warming potential (GWP) of a product’s use is typically a key focus, increasingly so regarding net zero carbon aspirations, considered in terms of carbon footprint: the total of the direct and indirect greenhouse gas emissions caused by that product. This is typically expressed in terms of CO2 equivalent (kgCO2e) per cubic metre.
Data sources and variance
Carbon calculation and measurement can be a challenge. Assumptions and simplifications need to be made regarding likely impacts but the underlying issues are complex and frequently misunderstood.
Figures for CLT will vary by geographic location, available suppliers, species used as well as by data source. The means of calculation is key when comparing to inform decision making.5
For this reason, along with the potential for changes over time and regional specificity, specific metrics are not included here. Teams may want to build up their own catalogue of trusted metrics relating to local availability from suppliers, preferably prepared and certified by third parties.
Comparison methods
When comparing likely carbon impacts, it is common practice to assume a benchmark based on the default solution for the element being considered.6 Alternative forms of construction, materials, structural arrangements etc can then be indexed to this when assessing relative value, including embodied carbon impacts.7
Carbon storage and end of life assumptions
Impact metrics may be quoted with and without biogenic carbon storage. Long-term storage may be taken into account if timber is sustainably sourced subject to certain conditions at end of life (and this aspect remains a bone of contention as attitudes towards LCA methods evolve).
Impacts of manufacturing will be varied in magnitude for any material (reflecting impacts from gathering raw materials, transport and manufacturing). Overall, these are typically much lower for CLT than other materials. Impact considerations that include carbon storage assumptions will reflect the sequestration properties of CLT (how much carbon is sunk or locked away in the timber), and will be therefore negative in magnitude. For CLT, such impacts can be significant. Net figures for CLT tend to be negative in magnitude (highly unusual for building materials) – the impacts of manufacturing representing a fraction of potential carbon storage (in some cases less than 10%).
Environmental Product Declarations
Environmental Product Declarations (EPDs) may be produced on behalf of manufacturers by independent third party organisations to communicate comparable product information, including life cycle impacts. The scope of each may vary but they are a good source of key data. Cradle-to-gate figures for various impacts (modules A1–A3) are calculated and information relating to subsequent stages may be included.
Product stage (cradle-to-gate impacts)
Raw material (A1)
Timber is a renewable resource and prehaps the ultimate key material for a circular economy (if used carefully). Powered by sunlight, trees convert CO2 into polymers (such as lignin and cellulose) – particularly during their growing phase – while releasing oxygen. The resultant fibre is a versatile material with a very low environmental impact.
Forest stewardship and responsible environmental assessments schemes vary but specifying certified timber (such as PEFC or FSC) ensures that resources are generally well managed with trees replaced upon harvesting. There are however other concerns over the impacts of monoculture plantations on ecosystems and habitats that are not always considered by such schemes.8
Transport (A2)
CLT manufacturing tends to occur close to forest resources and factories are often co-located with, or close to, sawmills. Timber may be sourced from further afield but transport impacts during manufacturing are typically modest.
Manufacture (A3)
Little material is wasted even though only about half of the tree, by mass, becomes dimensioned timber. By-products are used for lower grade products or as a fuel source, powering heating or kilns. The drying process (required to achieve dimensional stability before manufacture) remains the most energy intensive aspect of manufacture.