The presence of insulation can cause your property to become dramatically more energy efficient. The National Insulation Association claims, for example, that installing roof insulation alone can save 15% of a home’s annual heating bills.
The efficiency of insulation varies immensely between type, thickness, manufacturer and installation quality. If, for example, you squash the insulation material during installation, this will cause it to be less effective.
The Energy Saving Trust has compiled a list of products which they deem to be of high performance. A list of such products is available here: http://www.energysavingtrust.org.uk/Take-action/Find-Energy-Saving-Trust-Recommended-products