You might think that, but I'd beg to differ. I have a similar gap in a double gate in a section of chain link yard fence, and I've personally witnessed skunks, possums and coons slip through with ease. The barn cats don't even slow down when they go through it........and the gap is only 2 1/2 inches +/-. Chickens won't, large dogs won't......but varmints? Remarkable what they can maneuver through.
Varmints that live in the wild are really talented at getting into places we would think impossible. Most folks look at their chicken house as a means of keeping chickens in and build it accordingly. Better plan is to build something to keep varmints out.......and design and build it accordingly and let the chickens adapt to that. In reality, the difference is really pretty subtle, but significant.
And as for the insulation board......good that you are replacing it, as it was not going to stop anything. The best you could hope for with that was when the coons knocked it down, the pipe holding it up was going to club em in the head when it fell.
The ingenious widowmaker trap?