Spray foam insulation contractors assess buildings and apply liquid foam that expands and sets to form an insulating layer. The material is commonly polyurethane and may be open-cell or closed-cell, with the type and thickness chosen to suit the building, the surface and the intended use. Work can include insulating roof slopes, lofts, walls, floors and other hard-to-reach areas in homes and commercial premises. A typical service may involve an initial survey, checking the condition of the roof or other substrate, identifying ventilation and moisture requirements, and preparing the area before application. Contractors may protect nearby surfaces, spray the foam, allow it to cure and trim excess material. They can explain how the installation may affect ventilation, condensation, access to roof timbers and future repair work. Some businesses also assess existing foam and undertake its removal, which can require specialist methods.