Should I hire a structural engineer for my crawl space before encapsulating?

You shoud absolutely bring in a structural engineer before you move forward with the encapsulation. Since you're in Nassau County, you're dealing with anything from heavy clay to sandy loam, and both can wreak havoc on a block foundation if the drainage isn't perfect. A crack that's wide enough to snap a plastic vent cover isn't just standard settling—that's a sign of active movement or significant **hydrostatic pressure** pushing against the wall from the outside. Vents are naturally the weakest points in a foundation wall becuase they're literally gaps in the structural masonry. When a house shifts or the soil expands, the stress follows the path of least resistance, which is why you see those **stair-step cracks** originating at the corners of the vents. If the cracks are only in the mortar joints, it might just need some tuckpointing and better drainage. But if the blocks themselves are cracked or the wall is leaning even a fraction of an inch, you have a bigger issue on your hands. One thing to check right now is where your downspouts are discharging. If you have a gutter dumping water right near that cracked vent, that's likely your culprit. The water saturates the soil, the soil gets heavy, and it pushes against the blocks. In a crawl space, that moisture also contributes to the humidity you're trying to fix with the liner, so you're fighting a two-front war. A licensed structural engineer will give you a stamped report that tells you exactly what’s happening without trying to sell you a specific repair service. If you put that vapor barrier up first, you're essentially burying the evidence. If the wall continues to move, it'll eventually tear the liner or cause a structural failure that's way more expensive to fix once everything is sealed up. Spend the money on the pro assessment first so you know the foundation is acutally worth sealing.

I'm also in Nassau and the soil here is a nightmare after a heavy rain. I never realized that the vents were acutally structural weak spots, but it makes sense why the cracks start there. If I get an engineer out, do they actually test the dirt around the house or do they just look at the masonry? I'm curious if I should be checking for those stair-step cracks on the outside of the house too, or if they mostly show up on the inside first. This whole foundation thing is way more complicated than I thought.

In my expirience, the engineer isn't going to start digging up your yard for soil tests unless you pay for a full geotechnical study. They ussually just use a high-end laser level to check if the wall is bowing inward. If you see more than a half-inch of 'belly' in that block wall, you might need carbon fiber straps or wall anchors, not just a liner. Check if those cracks are wider at the top or the bottom. If they're wider at the top, your footing might be dropping. If they're uniform, it's probly just lateral pressure from wet soil. Get the pro to check the wall's plumbness first.