The time will vary according to the size of the walls but the process is generally much quicker and simpler than an external wall insulation project. The boards are fitted and decorated and any plug sockets have to be extended due to the extra wall thickness. Very little other work is required. A small property can usually be completed in five days. However, work can only go ahead if the existing wall is sound and free from damp. If it is cracked and letting in moisture, the outside wall will first have to be repaired and made weatherproof. This can obviously add to the time required for the project.