Best Moisture Level for Wood in Crawl Space

What is the best moisture level for wood in the crawl space? We are constantly discussing wood moisture levels being too high, but is too low a problem? Many crawl spaces are too wet but once they dry out, what is too dry? Can a damp crawl space that dries out have any issues?

How Does Moisture Level Affect Wood?

Water can affect the wood in many ways, but many overlook the types of moisture that can affect wood. If you have ever seen a water-soaked log floating in the lake, you understand it is heavy and swollen. Take the wood out of the water, and it shrinks and becomes lighter. Relative humidity can affect wood in the exact same way.

Wood is hygroscopic. It gains or loses moisture based on relative humidity. If the relative humidity in the crawl space is high. The subfloor and floor joist will swell and become heavy. As the relative humidity is lowered, the wood will shrink and become lighter. Many homeowners experience squeaky floors and possibly cracked drywall in the home after the crawl space has dried out.

Side Effect of a Dry Crawl Space

I have spoken with customers that noticed bouncing floors and seen cracks in the drywall above door jams after proper crawl space encapsulation. Most of the time, these issues are noticeable by homeowners that had foundation repair prior to encapsulation.

A company was called in to install jack posts but did not address the wet wood first. Then Crawl Space Ninja was called to fix standing water, high humidity, and mold problems. After the crawl space is corrected, the jack posts lost their effectiveness and the floors shifted slightly down.

Why Does the Floor Downshift?

This downshift in the floor is because the wood shrank due to the moisture being removed. If the foundation company would have dried the crawl space before installing the jacks. This issue would have been corrected permanently when the post jacks were installed. The post jacks were adjusted to apply pressure to wet wood. Now the wood is dry and they need to be readjusted to compensate for the density lost in the wood when the moisture is left. Unfortunately, the shift in the wood creates a shift in the walls above.

This cart before the horse approach to fixing crawl spaces is why some see a negative side effect to crawl space encapsulation. Don’t install post jacks or do foundation repair prior to drying out the crawl space if you want to avoid this issue.

Best Wood Moisture Level for the Crawl Space

To obtain the best wood moisture level, you need a crawl space that has controlled humidity all year. The wood in the crawl space will equalize to a good moisture level if the relative humidity is kept between 45% and 55%. Addressing water leaks from above is also recommended. Stopping and removing standing water entering the foundation will also help prevent humidity from getting out of control in the crawl space.

Once the relative humidity and all other moisture problems are addressed, you can expect wood moisture to fall between 6% to 14%. According to Wagner Moisture Meters, interior wood should be 6% to 8% while exterior wood should be 9% to 14%. An argument could be made the crawl space is exterior.

But we want to condition the crawl space.

Which means control the moisture levels, so some crawl spaces may see wood moisture levels as low as 6%. Focusing on the relative humidity levels is one of the easiest and most consistent ways to ensure the wood moisture levels are in an acceptable range.