gaining humidity after the kiln

curved-wood · October 16, 2018, 09:25:46 AM

What's the gain in humidity could I expect after 4 or 5 days ? Say for 8/4 pine that comes out of the kiln at around 6% still on stickers and stored in a shed. And how many days to go back to 12% (equilibrium with the air around ) about flat pile wrapped in plastic ?
I have another project with sawdust and planer chip. Anybody knows what could be the gain for a pile of kiln dry sawdust after 4-5 days? I expect that the surface of the sawdust will gain a bit of humidity and not much below 4'' or 5 '' the surface. Anybody has some kind of info about gaining humidity after kiln drying ?

GeneWengert-WoodDoc · October 16, 2018, 10:10:49 PM

You have asked a question that many have asked in the past and still there is no good answer. So, I will give you my best intelligent guess.

One key is that dry wood, when it is exposed to 12% EMC, will only come up to roughly 11.25% to 11.5% MC. This difference is due to the hysteresis effect.

Temperature of the air has a big effect on the speed of moisture movement.

End grain has moisture moving 10 to 15 times faster than face grain.

So here is my answer.
Note that when the lumber leaves the kiln, it might have a temperature of 150 F, so initially and for perhaps 12 hours or longer depending on circulation within the pile in the storage facility, the EMC for the hot lumber can be under 1% EMC. The dry-bulb is the wood's temperature and the wet- bulb is the air's wet-bulb, which often means about 100 F depression. So, the wood will actually dry a little more for the first 12 hours as it cools. Then it will begin to gain moisture. So, after 24 hours, you are likely at "no net change" overall.

Note that this drying while the lumber is hot is why it is a terrible idea to process lumber right out of the hot kiln. A minimum of 24 hours is needed before processing.

For 6% MC lumber, still on stickers, exposed outside (protected from rain and sun) to 12% EMC (typical for much of the US summer and winter), at an average of 70 F, I would expect that in four days exposure, the outer 1/8" would be close to 10% MC average. The next 1/8" depth would likely be 7.0 to 7.5% MC. The ends grain looses moisture much faster than face grain, so this effect might be 10 times deeper on the ends; that is, the 10% MC might go about 1-1/4" up the ends.

For lumber that is tight piled, the moisture in the air will have a hard time getting into the pile, unless there is wind. So, I would expect that the pieces of lumber on the outside would change quickly, but on the inside, the change in three weeks would be fairly small throughout. Perhaps, an unstickered, flat piled stack would change 7 to 10 times more slowly than lumber on stickers.

When you wrap lumber in plastic, no moisture can get in or out, so there will be no moisture change overall, except that moisture might move within the wood. The average moisture will stay at the original value for years, even with sunlight and rain (if no holes) if wrapping "seals" all six sides of the pack, whether flat piled or still on sticks.