Wood Moisture

By July 19, 2017 August 23rd, 2017 No Comments

Wood Moisture

The goal of the ATMOX System is to have balanced and reduced moisture levels. The best way to measure “moisture” is through direct wood moisture readings in the crawlspace or attic. Since most homeowners don’t own a wood moisture meter, the homeowner will need the assistance of a contractor, mold remediator, pest control company or other professional to provide accurate readings in the home. However, understanding wood moisture is important since the wood in your crawlspace or attic is ultimately what you are trying to protect.

So what is a good number? Wood moisture is measured as a percentage. Most research states that wood in an unfinished structure of the house should range from 10-16%.


To better understand where these numbers come from, there is a correlation between the temperature and the relative humdity and the outcome on wood moisture readings.  Of course, there are always variations in all of these things but it is easier to understand when we look at an equilibrium moisture content where temperature and relative humidity remain fairly constant over time. As an example, at a 70 degree F temperature and relative humidity at 60%, the wood moisture content will be 11.0%.  At the same 70 degree F temperature, if the relative humidity jumps all the way to 80%, then the wood moisture content will be 16.0%.  The 70 degree F temperature range is fairly typical for a summer crawl space.  If looking at an attic at 110 degrees F in temperature and 80% relative humidity, the wood moisture content would correlate to a 14.7% wood moisture content.   This information can get somewhat technical, but for more details on relationship of temperature and relative humidty to wood moisture content, please see pdf on Wood Moisture Relationship.   It also helps to understand that wood moisture in the oudoor atmosphere generally has natural range of 11-15%.  You can see typical wood moisture levels in various climates in this pdf on Outside Wood Moisture.

Why is this important to understand?  Mostly because there are many misunderstandings of what is an appropriate level for wood moisture levels, especially in the crawlspace.  In a vented crawlspace in most geographic locations in the United States, the wood moisture should have a low range of 8-10% in the winter and a high range of 14-16% in the summer. The ideal wood moisture reading for wood is 12%.  This is in line with the natural level of wood moisture content that exists in nature.   Wood getting too dry can lead to problems for the structure. Getting too dry generally does not happen naturally unless an extreme drying action has been placed in a crawlspace or attic. Most homeowners have greater issues with high moisture levels. Fungal growth or mold can start occurring in the 16% wood moisture range but generally needs around 19% wood moisture or higher. In order to have wood rot or decay, the wood moisture levels will need to be even higher. The other important thing to remember is that fungal growth does not happen instantaneously – it takes time. It is okay for there to be variances in the wood moisture as long as the wood is drying out again and averaging below the 16% range over time. As an example, when it rains outside and the humidity level reaches near 100%, mold does not grow all over your wood decks or on trees instantly. It becomes an issue only when the wood does not have a chance to dry out and remains wet.



I hope this information helps you and stay tuned for more to come.

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Mike Potts

Mike Potts

After graduating from Michigan State University with a degree in structural engineering, Mike moved to Tampa Bay and soon discovered a need in the community for trained, professional inspectors. Using his training in the field, Mike established Affordable Inspections, Inc. in 1992 and began offering his services to the public – inspecting commercial and residential properties. Taking on not just inspections, but full-on construction projects, Mike made a name for himself as an expert in structural engineering and worked on large commercial buildings in Tampa Bay and Clearwater.