The rate of moisture production and the rate of its removal establishes the resulting levels of humidity in a house. Living activities, humidification, ground moisture and the house itself, all contribute to the moisture load. Most of this is removed by exchanging inside air with outside air, and only a small portion is removed by vapour diffusion through the enclosing envelope. Most houses experience some degree of condensation, but only a relatively small percentage will develop serious problems because of it. Attention to detail is required during construction to ensure that there will be as few leaks as possible that will permit room air to enter concealed spaces. The provision of additional ventilation can also be used to control condensation, particularly in houses without the benefit of a flue.