High temperature and low temperature pressure vessels can be efficiently insulated using injected or spray applied polyurethane foam. The most common method is to metal clad the vessel using spacer blocks to create an annular cavity which can then be injected with foam. The use of this method eliminates the need for further vapour barriers when performing cold work. If the foams are to be spray applied to vessels which will be at below freezing temperatures, then adequate vapour barrier coatings need to be applied to prevent water vapour transmission.