Air temperature is one of the main factors affecting human comfort, the internal temperature of a building is dependant on the external temperature and the U-vales of the materials used to construct the building, meaning the lower the U-vale the more effective the material is at retaining heat. This can dramatically affect human comfort as the higher the U-values used, the quicker the room will lose heat through convection. By using the U-values of materials we can calculate the total heat resistivity of a building and then create a comfortable environment in which to live. Another factor affecting human comfort is the humidity of a building, this is the amount of water vapour in the air. Humans cool down mainly by sweating but in an environment with 100% humidity it is not possible for the sweat to evaporate into the air, this is called absolute humidity. Anything below 30% humidity can feel uncomfortably dry and anything above 60% can feel uncomfortably wet or damp. Air velocity is another crucial factor affecting human comfort, this is the movement of air through the room or building. Warm air can enter a room and push the cold air downwards creating a draught, in a cold environment this draught can make a person feel even colder. In a warm environment this draught can increase heat loss through convection and help to cool someone down by increasing the rate that their sweat evaporates.