Crawl spaces fail quietly, then all at once.

The vented crawl space was a mistake

Building code required crawl space vents for decades on the theory that outside air dries the space. In practice, warm humid summer air entering a cool crawl space condenses on the coldest surfaces down there — your joists and subfloor. The vents that were supposed to dry it are what wet it. Modern practice is to seal and condition the space instead, which is what encapsulation means.

What encapsulation actually is

A heavy sealed vapor barrier across the ground and up the stem walls, vents closed, penetrations sealed, and humidity managed with a dedicated dehumidifier. Done properly the crawl space becomes part of the building envelope rather than a damp cavity underneath it. Done improperly — a thin sheet loose-laid on dirt with the vents still open — it accomplishes almost nothing.

Structural work down there

Sagging floors upstairs usually trace to a crawl space problem: a girder or joist that took long-term moisture and lost strength, a post that settled into unprepared soil, or spans that were undersized to begin with. Adjustable steel supports on proper footings fix the sag, but only after the moisture that caused it is handled.

It is an air quality issue too

A large share of the air in the living space above comes up through the floor by stack effect. That means crawl space humidity, mould and soil gas do not stay in the crawl space. Utah also has elevated radon in many areas, and an unsealed dirt crawl space is the easiest path it has into the house.