When it comes to roof insulation, it's essential to understand that its effectiveness diminishes with age. Generally, experts suggest replacing it after 15 years in order to maintain the same level of insulation. This is because over time, attic insulation can be compromised due to factors such as humidity, heat, humans, and animals. If the old insulation is dry, dust-free, and not crushed, then replacing it may not be necessary.
In this case, leaving the old insulation in place can be advantageous as it will provide an extra layer of insulation. However, if the old insulation has been converted into a habitat for animals, then it's best to remove it before adding new insulation. If you're looking to improve insulation, you might need to remove the old attic insulation before installing new materials. If the old insulation is misaligned, removing it might be simpler or you could realign it to obtain the insulating properties.
A professional insulation contractor will use the right tools to remove old attic insulation without damaging your home.