Who would've thought that clean plastic waste, such as mineral water bottles, can be used as a thermal insulator in buildings? The bottles are packed in bags and in the ceiling or within cavity walls to keep a house warm in winter and cool in summer. Air is an effective insulator and once it fill the bottles, circulation stops within the walls. A house built with these bottles inside the cavity wall can provide excellent insulation and help to improve comfort and reduce heating expenses. At present a house at Kitini in Godavari is being built using this environment-friendly, cheap and easy technology. (Sjoerd Nienhuys)