Yes please take your dog to the vet! Eating insulation could be fatal! If you can't right away, then feed your dog wet bread and cotton balls. They will cover the tiny pieces of glass and render them (for the most part) harmless, but PLEASE TAKE HIM TO THE VET!
Get him to a vet ASAP~! Insulation normally has fiberglass in it, which are thousands upon thousands of tiny shards that will embed into anything they come into contact with. This is very serious!
Well my biggest worry would be the fact that if my dog chokes on it, the dog may die.