Crawlspace air in the rooms you live in

Why the upstairs smells like the downstairs you never enter

Supply ducts blow. Returns inhale. A pretty register in the hallway can be connected to a boot that was never sealed to the subfloor, hanging in damp air. Every time the fan runs, that air is distributed to bedrooms. People describe it as a smell that arrives with the heat.

What you can see without disappearing under the house

From the access, with someone aware that you are looking, note three things: whether returns run through the space, whether the ground is bare dirt or covered, and whether there is standing water. You do not need to become a crawlspace contractor to answer those.

Holes where pipes pass through the subfloor are smaller paths. They matter, and they are not a substitute for sealing a return that is actively inhaling the crawl.

Vapor barriers and the point where DIY stops

A continuous plastic vapor barrier on dry dirt, seams overlapped and taped, is a standard way to slow ground moisture. It is not a treatment for a flooded crawl, sagging ducts lying in mud, or joists covered in growth. If you would estimate the visible growth at more than about 10 square feet, or you cannot enter safely, hire someone who does this work and ask them how they will handle the water — not only what they will spray.