Most research agrees that ideal crawl space wood moisture content should be between 10% and 16%.
Wood moisture levels will vary some, but they should not remain too low or too high. It is rare to see crawl space wood moisture levels too low, but it can happen in cases where a dehumidifier setting is set so low that it actually begins to over dry wood in an area. Wood moisture that is too high is a concern. According to the Weyerhaeuser Technical Resource Sheet: A moisture content greater than 19% is sufficient to support mold growth. This moisture content is also where you may see decay-producing organisms leading to wood decay and loss of structure.
So as a reminder, there are really two reasons that you actually care about moisture in your crawl space – (1) mold or fungal growth and (2) wood rot or decay. It’s important to always keep the goal of moisture management in mind when addressing issues. For these reasons, we want to keep crawl space wood moisture below the 20% range.
Why should crawl space wood moisture be between 10% and 16%?
The simple reason for the 10-16% range is wood’s natural range within outdoor environments. Sometimes we forget to keep things simple and follow mother nature’s lead when looking for the answer. In a detailed study in the Wood Handbook by the Forest Products Laboratory (USDA), it shows that in cities across the United States during all months of the year the equilibrium moisture contents (EMC) generally fall within this range of 10-16%. Yes, there are some variations in extremely dry areas such as Nevada and Arizona and extremely cold and wet areas such as Alaska. However, overall, the range is remarkably consistent with a seasonal variance of 2-3%. Wood naturally adjusts to moisture is normal, even in a crawl space.
It should be noted that lower is not always better, and that wood moisture levels can actually be too low. At some point this can lead to cracked or split wood, which sometimes becomes evident in hardwood floors.
How does wood moisture content relate to humidity and temperature?
It is important to note that we are including temperature in this question. You will often see attempts to simplify messaging by referring to humidity alone. However, looking at temperature conditions is critically important in understanding moisture.
To keep things as simple as possible, we are going to look at just one temperature – 70 degrees F. This is a fairly common temperature range for a summer crawl space. In reality, humidity levels fluctuate all of the time, but we are going to look at wood moisture content with a constant temperature and relative humidity. Here are the equilibrium moisture contents (EMC) of wood at various humidity levels at 70 degrees:
| Relative Humidity at 70 degrees F (%): | Moisture Content of Wood in Equilibrium (%): |
| 10 | 2.5 |
| 20 | 4.5 |
| 30 | 6.2 |
| 40 | 7.7 |
| 50 | 9.2 |
| 60 | 11.0 |
| 70 | 13.1 |
| 80 | 16.0 |
| 90 | 20.5 |
You can see that as humidity changes over long period of times, the wood moisture content will go up or down. You will also see that the healthy wood moisture levels of 10% – 16% occur when the relative humidity is between 55% and 80% at 70 degrees. Even with lower temperatures, the ideal range for wood keeps relative humidity between 55% and 75%. Let’s repeat that again. Wood moisture levels remain in their ideal range of 10 to 16% when relative humidity is between 55% and 75%!
What should the relative humidity level in my crawl space be?
As stated, a healthy crawl space will have some variation, but as long as humidity levels remain mostly below 75% to 80% in the wettest locations over the long term then the wood moisture will remain in the desired range of 10% to 16% to avoid fungal growth and wood rot. That is where the focus of your crawl space moisture control solution should be.
Many in the crawl space industry will try to oversimplify or confuse messages on relative humidity. Relative humidity inside your home for healthy and comfortable air should be in the 30 to 50% range. This often requires air conditioning in the summer to lower humidity and a humidifier in the winter to add moisture. However, your crawl space is different. It is unrealistic, inefficient and unnecessary to keep a crawl space at such low levels.
You can find many companies quoting the ideal range of 55% relative humidity, which equates to a roughly 10% wood moisture content. They will sometimes set dehumidifiers at these low levels to try to keep the crawl space below this humidity range. Yet, if you follow the science, you will see that the 55% range is really the floor and not the ceiling for the ideal range. There is no reason to attempt to keep the crawl space this dry all year round. (Sometimes the dehumidifiers are being set so low due to some deficiencies in operation, but we’ll save that discussion for another day.)
ATMOX controllers are using data from the conditions of temperature and humidity to determine operation of fans and/or a dehumidifier. Wood moisture can be monitored directly from the controller with a wood moisture sensor. The entire algorithm is set up with the goal of maintaining acceptable wood moisture content within the crawl space in the most efficient manner.
