I have been going around the replacing the old caulk where it was cracked on the insides of my windows. I came across this window and it really opened up when scraping away the old caulk. It's by far the biggest gap so far and I'm guessing needs more than just a re-caulk. I stopped scraping after the first bit, got a little worried. As you can see in the photos, the insullation is visible and the wall flexes quite a bit.
Is this a big issue or can I just fill it with a different product? Would something like a foam backer rod work here?