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Author Topic: Relative humidity vs. moisture content  (Read 380 times)

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Online trimguy

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Relative humidity vs. moisture content
« on: September 16, 2020, 08:41:37 PM »
Wood should be dried to a certain moisture content depending on your specific relative humidity. I looked up the yearly average relative humidity for Augusta Georgia and it is 71%. I know the wood Has to be dried to less then 71% moisture content. How do you figure what it should be?

Offline Ianab

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Re: Relative humidity vs. moisture content
« Reply #1 on: September 16, 2020, 09:23:29 PM »
There is a graph on this page that shows the relationship. 

Equilibrium moisture content - Wikipedia

At 70% RH the EMC is about 13%.  That's for outside air, air in a house might be different, but if usually open (mild climate) then it will be close.  Houses in colder and drier areas will have a lower EMC, more in the 8% range.  

Here in NZ we have a mild and humid climate, around that 70% area, and wood is "Dry" at 12-14%. 
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Offline Larry

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Re: Relative humidity vs. moisture content
« Reply #2 on: September 16, 2020, 09:26:05 PM »
My favorite chart.

https://www.finewoodworking.com/FWNPDFfree/equilibrium-moisture-content-chart.pdf

Print it out and hang it on your shop wall.
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Offline Don P

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Re: Relative humidity vs. moisture content
« Reply #3 on: September 16, 2020, 09:43:10 PM »
Y'all type faster :D, well, here's one chart

You are looking for the "equilibrium moisture content", the moisture content of wood in relation to the relative humidity and to a lesser degree the temperature. This is a table from the Wood Handbook, google it and read the section on wood and moisture.


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Offline Ianab

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Re: Relative humidity vs. moisture content
« Reply #4 on: September 16, 2020, 09:53:57 PM »
What the charts show is that unless you have complete control over the temp and humidity, the wood is going to change it's MC to some extent, hence why we have to take wood movement into account when building furniture, and why a super precise measurement isn't really needed. It just needs to be close to your local conditions, within the seasonal variations you are expecting to see. 
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Re: Relative humidity vs. moisture content
« Reply #5 on: September 17, 2020, 01:21:15 PM »
Big thank you to all! Id know one or both but could never relate to two together. The charts make handy and easy to read. Also, the areas I checked were less than 1% difference between these charts.

Offline GeneWengert-WoodDoc

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Re: Relative humidity vs. moisture content
« Reply #6 on: September 18, 2020, 07:52:57 AM »
The outside EMC  by month for Augusta, GA.
Jan.  13.1%
Feb.  12.4
Mar.  12.4
apr.   12.2
May.  12.6
June   12.8
July    13.2
Aug   14.0
Sept   13.9
Oct   13.2
Nov   13.1
dec   13,2
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Re: Relative humidity vs. moisture content
« Reply #7 on: September 19, 2020, 02:47:48 PM »
Thank you.

Offline firefighter ontheside

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Re: Relative humidity vs. moisture content
« Reply #8 on: September 19, 2020, 06:29:46 PM »
It helps to understand the difference between what RH is and what moisture content of wood is.  RH is the amount of water that can be held in air at a certain temperature.  Thats why it rains at 100% RH the air can't hold any more water and it falls from the sky.  Its not a measure of how much water is in the air per unit volume.  During the day, the RH may be 20% at around 90deg.  As it cools off toward night, the RH goes up as the temp goes down.  The actual volume of moisture in the air has not changed.  This is so important for wildland firefighting and the reason that fire tends to settle down at night.  Of course that can change based on wind and weather factors.  

Moisture content of wood is simply a comparison of the weight of the oven dry wood and the wood as wet as it can get.  
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Re: Relative humidity vs. moisture content
« Reply #9 on: September 19, 2020, 07:39:08 PM »
Yeah, but how does RH figure into the Haines index calculations? :D :D :D ;D
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Re: Relative humidity vs. moisture content
« Reply #10 on: September 19, 2020, 10:06:41 PM »
Well, I don't know for sure, but I would think that low RH plays a part in a high Haines index.
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Offline kantuckid

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Re: Relative humidity vs. moisture content
« Reply #11 on: September 20, 2020, 07:37:24 AM »
I know this humidity factoid- this and last year here in my E KY area of 100 year record rains it's been impossible to dry lumber to ECM not under a roof! or grow good maters...
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Offline Don P

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Re: Relative humidity vs. moisture content
« Reply #12 on: September 20, 2020, 08:40:43 AM »
You are drying to EMC but with higher relative humidity, equilibrium moisture content is also higher.

What FFOS was pointing out is that absolute humidity doesn't change with temperature but because warmer air can hold more moisture we commonly speak of the moisture in air as relative humidity, the amount of moisture present compared to the amount the air can hold at that temperature.

We woke up to the first cold morning today. We've been running in the 80's with high humidity. It was 37 this morning, the windows are sweating inside. The air and building inside is still holding a lot of moisture relative to that warm temperature. It is carrying more moisture than it can hold at 37. When that warm moist air hit the cold glass it condensed. 

We have no AC so run ambient outside conditions, the wood inside is at EMC for late summer so probably around 12-14%, by late winter with wood heat and less than well sealed the RH will be around 35%, EMC will be around 7%, it can then be 10 outside and the windows will not sweat.

A slight correction to the above. Moisture content of wood is the moisture in the wood compared to oven dry wood.

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Re: Relative humidity vs. moisture content
« Reply #13 on: September 20, 2020, 09:27:33 PM »
Thank you. At first a was a little confused, something just wasnt connecting for me, but now I think I understand.

Offline doc henderson

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Re: Relative humidity vs. moisture content
« Reply #14 on: September 20, 2020, 09:52:22 PM »
specifically the weight of the water over the dry weight of the wood.  so MC can be over 100% like in cottonwood.  the EMC is the predicted MC of the wood at a  particular RH, after it gets to equilibrium.  It takes time so the actual MC is moving towards the EMC at that RH.  so in 24 hours the MC may reverse direction.  You can look at the Average RH, and assume that is about where the wood is in terms of MC, or you can measure it.
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