It is sprayed on as a liquid which reacts and expands in place. This expansion action also seals all of the cracks and crevasses in your wall's exterior sheathing. The result is that air can no longer slip in – your house will be less drafty and more comfortable. Air leakage can also introduce moisture into the wall cavity, resulting in wet insulation and mold and mildew. With the sealing effects of polyurethane foam insulation, this will not be a concern.