What does "% Wood Moisture Equivalent" or "%WME" mean exactly?
In the simplest explanation its the percent water content, by weight, of a piece of USDA Douglas Fir wood. If a moisture meter reported that a 1kg wood sample was 20% WME and you then dried that wood sample out it would weigh 0.8kg after the 0.2kg (20% of 1kg) of water was removed.
In a typical residence you can expect to measure 12-15% WME in any piece of interior wood. A vented crawl space during the summer in South Carolina might read 26%+. Saturated wood reads 36+%. Microbial growth (ie fungus) typically starts around 26%.
But why use the word "Equivalent"? Lets use concrete as an example.
Wet concrete: A cubic yard of wet concrete typically weighs around 3,900 lbs
Dry concrete: A cubic yard of dry concrete typically weighs around 3,500 lbs
So maximum concrete moisture content by weight is 400/3900 = ~10%. But if you were to sister a piece of USDA Douglas fir up to that same wet concrete it would read 35% because the vapor transmission from the concrete to the wood would force the wood into equilibrium with the concrete. You would therefor say that the moisture content of that concrete is 35% WME. So you are NOT saying that the concrete is 35% water as that is not possible but what you are saying is that if you mounted a piece of wood to that concrete (think sill plate) that the wood would read 35% moisture content by weight. This is a very useful way to look at moisture content since wood and concrete are frequently in contact with each other. The same is true of drywall.
In the simplest explanation its the percent water content, by weight, of a piece of USDA Douglas Fir wood. If a moisture meter reported that a 1kg wood sample was 20% WME and you then dried that wood sample out it would weigh 0.8kg after the 0.2kg (20% of 1kg) of water was removed.
In a typical residence you can expect to measure 12-15% WME in any piece of interior wood. A vented crawl space during the summer in South Carolina might read 26%+. Saturated wood reads 36+%. Microbial growth (ie fungus) typically starts around 26%.
But why use the word "Equivalent"? Lets use concrete as an example.
Wet concrete: A cubic yard of wet concrete typically weighs around 3,900 lbs
Dry concrete: A cubic yard of dry concrete typically weighs around 3,500 lbs
So maximum concrete moisture content by weight is 400/3900 = ~10%. But if you were to sister a piece of USDA Douglas fir up to that same wet concrete it would read 35% because the vapor transmission from the concrete to the wood would force the wood into equilibrium with the concrete. You would therefor say that the moisture content of that concrete is 35% WME. So you are NOT saying that the concrete is 35% water as that is not possible but what you are saying is that if you mounted a piece of wood to that concrete (think sill plate) that the wood would read 35% moisture content by weight. This is a very useful way to look at moisture content since wood and concrete are frequently in contact with each other. The same is true of drywall.
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