Equilibrium Moisture Content of Wood in Outdoor Locations

Dont think we all havent been there. Or a good many of us anyway. When I first joined this site and heard about this three year plan I was quite skeptical. Obviously these guys don’t know how to dry wood. They must be stacking it in the local car wash LOL
Bought a MM and my oak cut last winter was 8% on the outside, beautiful!! Split a couple pieces and found 30-35%
It was also the same year I bought a used cat stove and the oil furnace died. It was a rather cold learning experience.

 

(Raises hand) Yes, I've been there too. My first winter in my house I asked my stepdad for some firewood. He gave me oak that was too big for my crappy stove that I had to trim with a chop saw. Also, it was cut and split in June of that year, thrown in a pile and completely covered with a tarp. He told me it was seasoned and I didn't know any better at the time.
The way I sees it: when it comes down to wood burning there are some indisputable rules, and there are some things where there's a bit of wiggle room. Some things that work for you might not jive with the next guy. Personal preference really. My backyard is one big science experiment in wood drying processes... I have conventional stacks, holz hausens, and some other unorthodox stacking methods that may or may not be efficient in the end, and only time will tell. It's interesting to see what works for you in your corner of the world

 

I am glad these charts/tables were valuable.

If you want to be technical what actually happens in the winter is that the inside relative humidity is typically significantly lower than the relative humidity outside. This is assuming:

• The house isn't super sealed for infiltration. You get at least some infiltration of outside air. I don't believe it has to be a lot of air.
• The amount of moisture being generated inside is minimal (such as a reasonably good bathroom exhaust fan & not water boiling especially day). Another cause of high humidity would be when you have a lot of people over like for Thanksgiving or Christmas vs. regular occupancy.
• You don't have a humidifier operating in the house.

In one example say your outside air conditions are:

• 40 F RH= 60%

Your inside conditions (assuming your inside space temperature is 72 F) should be around:

• 72 F RH=18.8%

Here is that data plotted on what is called a psychrometric chart:




If you notice the line basically goes horizontally from your start conditions. Thus even if say your outside air conditions are:

• 40 F RH= 100 %

Your inside conditions (assuming your inside space temperature is 72 F) should be around:

• 72 F RH=31.3%

I hope this isn't too much information and helps vs. confuses.

Using heat to lower the RH (by raising air temperature) is also how both green houses and kiln drying manage to dry wood more quickly. With those processes I believe instead of straight line from left to right it slopes up due to significant moisture being generated in either the green house or kiln.

Hopefully I am not getting too technical. ;-)

I am curious what people's thoughts are.

 

A rule of thumb that I learned for calculating RH change with respect to temperature is that the relative humidity will double with every drop of 20 degrees F and cut in half for every 20 degree gain.