· Michael Blair
The instinct is that vents dry a crawl space out. In Middle Tennessee summer they do the opposite, and understanding why explains most of what goes wrong under houses here.
The condensation problem
Summer air here is warm and close to saturated. A crawl space is cool, because it is shaded and in contact with the ground. Warm humid air entering a cool space condenses.
The result is water forming on ducts, pipes, joists and the ground itself. The vents are not removing moisture; they are delivering it.
What it damages
Wood at above roughly 20 percent moisture content supports fungal growth. Joists, subfloor and girders soften and lose strength, and the damage is slow enough to be missed for years.
It also reaches the house. Air moves upward through a building, so crawl space air becomes house air, taking humidity and odour with it.
- Condensation on ducts, pipes and joists
- Fungal growth on framing and subfloor
- Insulation falling from between joists
- Musty odour in the house above
- Higher indoor humidity and cooling costs
Encapsulation
Sealing the vents, laying a heavy vapour barrier across the ground and up the walls, sealing the rim joist, and dehumidifying the space. That turns the crawl into a semi-conditioned part of the house.
It is the standard answer in this climate and it addresses the cause rather than managing the symptom.
Doing it in the right order
Bulk water first. A crawl with standing water needs drainage, grading and gutter correction before any barrier goes down, or you seal water in.
Then the barrier, then the sealing, then the dehumidifier sized for the volume. A dehumidifier alone in a vented crawl runs continuously and never wins.