Living in Ontario, we may experience wildlife in some neighborhoods. Either being small rodents to racoons, we have seen it all. When rodents have gain access to the attic, there is a high chance that droppings have been left behind. Over-time these types of damages could cause long-term health problems as well as home structural problems. Droppings from rodents can spread throughout the home just like mold. If this is your situation, you must remove the old material before spraying in the new insulation.
When insulation is exposed to excess amount of water or moisture, there is high chance of mold growth. Homeowners should not insulate over the damaged insulation as it will contaminate throughout the whole attic. It is recommended for the old insulation to be removed before applying the new stuff.
If you have not experience any of the above mentioned problems, you should be able to spray on top of the old insulation.