You raised my curiosity here. I have a Giro lid, but a while back the yellow 'skin' if that's the way to describe it separated from the helmet. I used a liberal amount of glue around the edge and it's stuck on now.

I did notice the glue began to 'bubble' when I put it around the lid which looks like it is made from polystyrene or something like it?

What do you think? Has the glue perhaps weakened the helmet structurally?