40 10 Maintain or Renew, Reuse or Recycle? When does it make sense to stick with an existing building, and when to renew it? When does it make sense to use a new material versus reusing or recycling an existing one? For buildings, the energy costs of existing construction can be weighed against potential savings with renovations or new construction. Similarly, the benefits of reusing and recycling materials can be weighed against other factors like potential reductions in material performance. Recycled aggregates in concrete, for example, have lower performance than natural ones, which can lead to additional cement in the concrete mix, thereby increasing the resource efficiency of the aggregate but also increasing the CO 2 output of the concrete as a whole. Jonathan Cullen frames it as follows: “Two guiding questions to ask when assessing end-of-life options for waste materials or products are: how much energy is required to restore the recovered material back to the desired material or product? And, how does this quantity compare with obtaining the desired material or product from virgin or primary sources?” 1 1 Jonathan M. Cullen, “Circular Economy: Theoretical Benchmark or Perpetual Motion Machine?,” Journal of Industrial Ecology 21, no. 3 (2017): 483–86.