What Is Wood Moisture Content (MC)?
Wood moisture content (MC) is the weight of water in a piece of wood, expressed as a percentage of the wood's oven-dry weight. In a crawl space, it is the most direct measure of whether joists, sills and subfloor are dry enough to resist decay and termites.
Explanation
Wood soaks up and releases moisture until it balances with the humidity around it. That is why a damp crawl space raises the moisture content of the framing above it. Inspectors and contractors check MC with a handheld pin or pinless moisture meter. Readings vary with wood species, temperature and the meter's settings, so treat any single number as approximate.
The numbers that matter:
| Reading | What it means |
|---|---|
| Below 20% | The U.S. Forest Products Laboratory's long-standing guideline for preventing decay is to keep wood at or below about 20% MC. |
| 20% or more | On a North Carolina Wood-Destroying Insect Information Report, NCDA&CS treats wood at 20% or more as "excessively wet," a condition that attracts subterranean termites. |
| About 30% | Roughly the fiber saturation point, where cell walls are full of water. Forest Products Laboratory researchers describe decay as occurring above fiber saturation. |
Lowering crawl space humidity with a sealed vapor retarder and a dehumidifier lets framing dry out over time. Fixing leaks, standing water and drainage problems has to come first.
When to seek help
- A home inspection or termite report lists wet wood or "excessive moisture" in the crawl space.
- Joists or the sill plate read 20% or higher, feel soft, or show fuzzy growth or staining.
- Readings stay high after encapsulation. That usually points to a water source or a drying system that isn't keeping up.
Try our moisture and mold risk checker, read signs of crawl space moisture, or see mold remediation.