The U.S. Department of Energy recognises the enormous potential that exists for improving the energy efficiency, safety and comfort of existing American homes. Air and water vapour are transported into and within single-family homes as a result of pathways and driving forces. Without pathways, no air or water vapour transport will occur. Homes always have pathways and driving forces. This book focuses on managing the driving forces that move air and water vapour across the building envelope and also identifies ways to make homes more comfortable, more energy efficient, and healthier to live in.