The roof and attic can be responsible for up to 30% of heat lost in the average home. What may surprise you is how attic insulation can lose its effectiveness over time. Loose-fibre insulators will settle, and a poorly-ventilated attic can cause insulation to become damp and rot.
Upgrading your attic insulation is generally very cost-effective, though the actual cost will vary depending on the size of your home. Usually the money you spend on insulating, you'll make up in energy savings within around 3 years.