Because we are now in the full construction season, it is a good idea to refresh our understanding and priority for controlling the moisture content of wood on a job site, whether it is being staged for use, or has been installed but still unprotected from the elements. Why is this important? Elevated moisture in wood allows for naturally occurring mold spores to germinate and create colonies (called a reservoir).
Part of construction site management is to control moisture in building materials by having everything staged off the ground on palates or other wood framing. Then, these materials are covered with plastic (some even put plastic on the ground before staging the materials). It is often thought that if wood is saturated, the moisture content must be 100%! That is a misunderstanding of what we are measuring, because the only item with 100% moisture is a glass of water.
To help you understand moisture content percentage, hold a bundle of pencils in your hands and face the end toward you; the spaces between the pencils are easy to see. The percentage of moisture we talk about is the percentage of open space between the pencils to the amount of that space filled with water. But for almost any wood, the maximum amount of pore space between the wood fibers is 40%; when this pore space is full of water, the wood is considered saturated at 40%. Therefore, if someone says the wood is 60% saturated, I suggest you start talking to someone else. This maximum percentage is the same regardless of if we are talking about OSB, pine, or oak.
There are some general guidelines for the potential mold growth when the materials become elevated in moisture. You become concerned with the possibility of growth when the moisture content exceeds 15%, and then, any reading above 20% is definitely in the high-risk area for mold growth. Normal wood in a finished home ranges from 7 to 10 percent on the first and second floors, around 12 percent in the basement.
I recommend that every builder have an accurate moisture meter. There are many types, but I would suggest you look for one that can have the accuracy checked with a calibration bar. For example, the moisture meter Model BD-10 manufactured by Delmhorst has a calibration bar model MCS-BD that you can check the 15 and 25 percent readings. If for some reason you go to court (I was deposed last month and had to verify the calibration method of my moisture meter), you need a meter that the accuracy can be verified or the data will be dismissed, or more commonly, make incorrect conclusions based on the incorrect measurements. There are many meters on the market, the model BD-2100 has three scales and can read the moisture content of not just wood, but also drywall and a reference scale.
The meters that I have been talking about are called “penetrating” moisture meters. That means it measures the electrical conductance across two pins that are pushed into the material. Water conducts electricity, the more water the higher the reading. But remember that ice does not conduct electricity so you cannot measure the moisture content of frozen wood. Also, remember that the moisture content of pressure treated lumber will almost always read saturated, even when it is not.
The other type of moisture meter is called a “non-penetrating” meter. When you have a material that you cannot use a penetrating meter (i.e. ceramic tile or plastic), you can place this on the surface and it measures the impedance of a signal that is emitted from one end of the device and picked up on the other end. The lower the signal, the more moisture present; this signal penetrates the substrate about 1.25 inches. Of course, there are several measurement scales, and you use the right one for the type of substrate. I use a Tramex meter that have measurement scales for wood, drywall and plaster. Of course, there is a calibration test box labeled MEP.
Knowing how to measure your potential problems is the first step in avoiding them. If you have any questions or would like to hear about a certain topic, you can reach me at BobBennett@usefarsight.com. I will post some of the questions asked at the end of the next article.
Bob Bennett is the President of Farsight Management, and the Indoor Air Quality Association for North East Ohio. He has been provided indoor air quality consulting and abatement for the last 20 years to builders, homeowners, insurance companies, and property management companies. You can follow Mr. Bennett on Facebook under Farsight Management; he frequently posts comments on interesting projects and articles of interest. Contact him through his email if you have any questions.
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