The book presents full-scale studies of how to build, according to absolutely sustainable principles by using biogenic materials such as straw, wood, clay, and thatch. Fire hazard is central when building with biogenic materials. Thus, we have chosen to test vertically thatched surfaces treated with mineral aggregates as fire retardant. Clay proves to have highly effective fire-retardant properties. Based on the results created, several thatched wall constructions treated with clay, have been developed to demonstrate a new biogenic building practice. This forms a backdrop for further research into absolutely sustainable architecture by using environmentally and climate-friendly materials.