How old is your home? Does it have asbestos? I think he should be fine provided you don't have asbestos in your home. This was a big problem with my home state (NH) where so many homes still have asbestos insulation and/or lead paint on the walls. Only a problem if you have an older home.
We rebuilt our workshop last year and haven't but up the plywood on the walls upstars yet. Bakker has crawled behind the insulation a few times. He is due for his shots within the next 2-3 weeks so I will ask my vet that question.
Was he covered in it? Could he have possibly ingested some? Unfortunately uncovered fiberglass insulation (which is made with 40% recycled glass) could harm cats. If the answer is yes - I would call your Vet to discuss this - better to be safe than sorry.
Our home is only two years old, so I'm pretty sure I don't have to worry about asbestos. I was more worried about the insulation itself. It's irritating to people, so I thought it must be for him too. He didn't eat any, just sat in it. I'm calling the vet. Seamus has been in to see him a couple times the last few months for skin irritations. I certainly don't want him to have another.