Insulation is a way to keep heat inside a room or building, it reduces the loss of heat through certain areas like the walls and the ceiling. There are many different ways to insulate the roof with one of the most popular being Insulation Foam, in fact some insulating foam can be used to not only insulate the roof but repair it at the same time. This gives you a few benefits over re-roofing, for starters it is a much cheaper method, it will save you a substantial amount on your heating bills, has the ability to create a dry, warm, clean loft space and comes with 25 year guarantees.
The foam used is a rigid spray foam that is applied to the underneath of the roof tiles or slats, it fills and seals any gaps that it comes into contact with, creating a very solid foam structured. The council use this method for insulating their properties along with blue chip companies and local authorities. Another benefit to the foam is that after the installation is complete it requires absolutely no maintenance what so ever and can be left alone with peace of mind.
Some insulating methods have a limited lifespan and are not likely to last for the duration that the building will, however the foam insulation has a very long life expectancy and should last for the building life expectancy.
If you are looking in to this for your building make sure you chose a company with a good reputation.