I put an SIP addition onto an existing structure. The SIP part is really plumb, the old structure not so much. There’s a gap between the two structures that varies from 3/4 inch at the top to zero at the bottom. These buildings are 20 feet wide and one story tall where they adjoin each other. One wall is SIP, the mating wall is 2 x 4 with plywood sheathing with joints that have been sealed.

Here’s the question: What to do with the gap, and how to keep critters out of it? I’ve already fought off one attack by ants trying to use it as a pathway into prime nest-building space. I’ve though about pouring vermiculite, powdered styrofoam or dry sand down the gap, just foaming and caulking the edges and leaving it at that, with an airspace in between, but I can’t foresee possible consequences. These mating walls will be living space on both sides of the wall, the 2x 4 wall will be insulated r-11, the climate is 6000 degree-day upstate NY. Any thoughts? Thanks, Nick