Because warm air rises most of the heat loss in a building is through the roof. Money spent improving the thermal insulation of the roof is therefore money well-spent because every pound you spend will signifcantly decrease your heating bill — and keep you warmer as a result.
However, simply sealing everything up or placing insulation in the wrong position can lead to the build-up of moisture that causes timber rot, loss of thermal efficiency and damage to other materials.
It's really important, therefore, to hire a roofing company or building contractor that understands the details of building construction and consequential performance.
Seal off drafts but maintain controlled ventilation
Ensure insulation is correctly positioned
Consider the use of vapour barriers