Vegetal materials were one of the first construction materials used by humans, but the development of petrol-based synthetic materials in the last century replaced them in mainstream construction techniques. However, since environmental impact and resource depletion are increasingly becoming a central issue, vegetal materials are revisited. Natural thermal insulations are mainly developed from woody materials and industrial fibres, but these raw materials are not always locally available. Thus, the use of crop by-products is proposed here. The availability of crop by-products to be used as raw materials for building thermal insulations in Spain is evaluated. It is then compared to demand forecasts based on two different scenarios. Results vary greatly from one scenario to another, but they indicate that the amount of crop by-products is sufficient to meet estimated demand. Barley and wheat straw are by far the most abundant by-products, followed by corn, rice and sunflowers. Available corn by-products would be sufficient to insulate between 250,000 and 450,000 dwellings yearly