Heating and cooling are responsible for around 40% of energy consumption in buildings and a corresponding significant amount of greenhouse gas emissions (ERG, 2018). The challenge of reducing thermal demands and greenhouse gas emissions is therefore critical given the continued growth in the world’s building stock.Current research is centred around the possibilities of inventing and developing innovative and robust thermal building envelopes. Highly insulating bricks made from recycled plastic would provide a solution for not only reducing building emissions, it would also help to tackle the world’s 6.3 Gt mountain of plastic waste (Geyer, 2017). Design inspiration and optimisation A team of engineering researchers at De Montfort University in Leicester, UK has designed and prototyped a waste plastic brick that takes as its inspiration the woven construction of the baya weaver bird’s nest, which is renowned for its excellent thermal insulation and mechanical properties.Believed to be the first of its kind, the brick was constructed using three-dimensional printing and lattice architecture, with intersecting strips of plastic forming an internal grid or weave. The relative density of the brick and arrangement of voids played an important role in defining its thermal efficiency. If the holes were too small, then heat would transfer through conduction and give lower thermal efficiency, but if too big, there would be high convection and radiation, also resulting in poor thermal performance.In addition, optimisation of the design required a detailed research into the material characteristics of mixed domestic plastic waste with regard to thermal performance, workability and strength.Thermal insulation characteristics of the prototype bricks were tested in a hot-box calorimeter. They achieved an impressive U value of 0.25 W/m²K − 10 times more than a traditional clay brick, which delivers an average of 2.94 W/m²K, and close to the 0.18 W/m2K required for new external walls by UK building regulations. Global application The World Health Organisation (WHO) estimates nearly two million people die in developing countries due to health issues that arise from inadequate housing, while the United Nations International Covenant on Economic Social and Cultural Rights states that adequate housing is fundamental to improving the living standards among poor and low-income thresholds. Without adequate shelter, families lack security and are vulnerable to natural disaster and the chaos of civil conflict.It is clear that upcycled-plastic bricks can not only help clean up plastic pollution in oceans, they can also empower people in developing countries to build cost-effective and sustainable housing. Based on the research at De Montfort, it is clear that 3D printed bricks made from household plastic waste can be thermally far superior than the existing clay and concrete bricks, which in turn will help to reduce both the cost of building and energy-related manufacturing and operating costs.Further design optimisation is ongoing together with larger-scale tests in real-world settings. These include assessing the fire resistance of the bricks in cavity wall construction.This article is based on the authors’ briefing article in the latest issue (173 CE2) of the ICE Civil Engineering journal.