Roof insulation prices will always vary. Roof insulation cost will depend on the size of the space you are insulating, the type of insulation materials you end up buying, and your overall budget. If you are insulating a flat roof, cold or warm deck insulation will set you back at approximately £35 per square meter, with other types of roof insulation coming in at a similar price. Exterior insulation, on the other hand, is a touch more costly at around £80 per square meter. In terms of insulating roof materials, fibreglass (glass wool insulation) is the cheapest type of roof insulation you can buy.