A zero carbon eco-house has been built based on design from the Middle Ages. The framework uses timbrel vaulting, a medieval technique from 14th century Spain, that creates a lightweight curved roof which retains heat. The roof has been covered with earth and plants, to maintain the inside temperature. Modern technologies, such as solar panels and triple glazing complete the design. The house comes at a price, though – it took a year to build and cost the owner £800,000.