Studies have shown that crawlspace wood should have a moisture content between 7% and 19%. There is some variation in wood moisture content, but it should not be too low or too high. Excessive moisture in wood is a common problem.
According to Weyerhaeuser's Technical Information Sheet: A moisture content of more than 19% is sufficient to support mould growth. Excessive moisture content can lead to wood rot. It is rare for crawlspace wood moisture content to be too low, but it can happen if the dehumidifier setting is so low that the wood in an area begins to over-dry.
Why Should Crawlspace Wood Humidity Be Between 7% And 19%?
The reason for the 7-19% range is simple: that's the natural range of wood in an outdoor environment. There are really two reasons to control moisture in a crawl space: (1) mould or fungal growth and (2) wood rot or decay. For these reasons, we want to keep crawlspace wood humidity below 20%.
A detailed study conducted by the Forest Products Laboratory (USDA) in the Wood Handbook shows that in major U.S. cities, the equilibrium moisture content (EMC) for all months of the year is generally in the 7-19% range.
Of course, there is some variation in extremely dry areas such as Nevada and Arizona, and extremely cold and wet areas such as Alaska. Overall, however, the range is very consistent, with seasonal variations of 2-3%. It is normal for wood to naturally regulate humidity, even in crawlspaces.
It is important to note that lower humidity is not always better, and the humidity of wood can actually be too low. In some cases, this can lead to cracking or splitting of the wood, which is sometimes evident in hardwood floors.
How Does Wood Moisture Content Relate To Humidity And Temperature?
Most of the time we only use humidity as a measure of good or bad condition, ignoring the one factor that affects humidity, which is temperature. Because humidity is not static, there may be different humidity levels at the same temperature.
To make the problem as simple as possible, let's look at the equilibrium moisture content (EMC) of wood at different humidity levels at just one temperature - 70 degrees Fahrenheit:
|
Relative Humidity (%) |
Wood Moisture Content (%) |
|
50% |
9.2% |
|
70% |
11.0% |
|
80% |
13.1% |
|
80% |
16.0% |
|
90% |
20.5% |
As you can see, the moisture content of wood increases or decreases as the humidity changes over time. You can also see that the healthy moisture content of wood is 9% - 20.5% when the relative humidity is between 50% and 90% and the temperature is 70 degrees.
Even at lower temperatures, the ideal relative humidity range for wood remains between 55% and 75%. Let's repeat that. When the relative humidity is between 55% and 75%, wood maintains an ideal moisture content of 7% to 19%!
What Should The Relative Humidity Be In A Crawl Space?
Crawlspace humidity levels stay below 75% to 80% for long periods of time in the wettest areas, then the wood humidity will stay within the ideal range of 7% to 19% to avoid fungal growth and wood rot.
Many in the crawl space industry try to oversimplify or confuse information about relative humidity. For healthy, comfortable air, the relative humidity inside your home should be between 30-50%. This usually requires the use of an air conditioner in the summer to reduce humidity and a humidifier in the winter to increase it. However, your crawlspace is a different story. It is impractical, inefficient, and unnecessary to keep the humidity in your crawlspace at such a low level.
Many companies set their relative humidity in the ideal range of 55%, which corresponds to a wood moisture content of about 10%. They sometimes set their dehumidifiers at such a low humidity level in an attempt to keep the crawlspace humidity below this range.
However, if you get your hands on the science, you'll see that the 55% humidity range is actually the bottom of the ideal humidity range, not the upper limit. There's no reason to keep a crawlspace so dry year-round.
Signs of Excessive Moisture in Your Underground Space
If you have excessive moisture in your crawlspace, you may notice
- Heavy 'sweating' (condensation) of air conditioning ducts
- Discolouration of the subfloor and floor joists in your crawlspace
- Insulation peeling away
- The floor ‘bounces’ when you walk on it
- Floors buckling or denting in the room
- Large amounts of standing water in crawl spaces
- Mouldy odour in the interior and crawlspace
- Termites, powder post beetles or wood-boring bees
When to Take Action Against Excessive Moisture
All underground spaces are different and therefore face different problems. Some flood every time it rains. Some flood every day. Some underground spaces have insulation and loose moisture barriers. Others are completely sealed with no dehumidifier and vents open to the outside.
I think you should follow the instructions on the hygrometer and relative humidity. If you have done all of these things and the RH is still over 55%, then you need to do even more. Here are the 3 must-haves for consistently and effectively controlling the humidity in your crawlspace.
- Seal vents and doors
- Properly sized commercial dehumidifiers and condensate pumps
- Moisture barrier that covers the entire floor
Why not include foundation ventilation fans? We use them to remove odours and soil gases such as methane, ammonia and radon. However, we don't need them for humidity control. Nonetheless, I highly recommend that you install an active ventilation system in your crawlspace to address soil gases.
Product Recommendation: AlorAircrawlspace VentirPro Crawl Space Fans
Maintaining proper air circulation in your crawl space is key to preventing moisture buildup. AlorAircrawlspace VentirPro fans are designed specifically for crawl spaces and help maintain ideal moisture levels by promoting airflow and reducing humidity. Here are three highly recommended products from AlorAircrawlspace that can help you manage crawl space conditions:
1.AlorAircrawlspace VentirPro 260 Pack of 8
This pack of eight powerful fans is ideal for larger crawl spaces. Each VentirPro 260 fan delivers efficient airflow with minimal noise, ensuring your crawl space remains dry and well-ventilated. The fans are equipped with humidity sensors that automatically adjust fan speed to maintain optimal conditions.
2.AlorAircrawlspace VentirPro 540 Pack of 4
This set of four VentirPro 540 fans is perfect for medium-sized crawl spaces. These fans offer superior performance and are easy to install. With their built-in adjustable settings, you can customize airflow based on your crawl space’s specific needs.
3.AlorAircrawlspace VentirPro 720 Pack of 3
For smaller crawl spaces, the VentirPro 720 pack of three is an excellent choice. These fans are compact yet powerful, ensuring that even tight spaces receive adequate ventilation. Like the other models, they feature humidity sensors and energy-efficient operation.
These fans, combined with other moisture control methods like dehumidifiers and vapor barriers, can help keep your crawl space in optimal condition, preventing mold and wood rot.
Related reading: How to Install Crawlspace Ventilation Fans: Step-by-Step Guide
Conclusion
In short, ensuring that your crawl space wood moisture content remains within the 7% to 19% range is critical to preventing mould, wood rot and structural damage. By monitoring relative humidity and temperature and using a reliable product such as an AlorAircrawlspace dehumidifier, you can control moisture and protect the integrity of your home.
