Drying Small Amounts of Wood in the Attic
Thoughts about the airflow and venting required to dry small amounts of wood in an attic. April 20, 2011
Question
I use attic heat to dry lumber also, except I sticker the stack, then cover it air tight, install a couple window fans on the front of the pile, weight it down, then install a couple drain pipes into the stack, up to the inside of the peak. Set my timer to click on at 9 am and off at 8 pm. The gable vents are closed off, the lumber is air dried to 20% before stickering, and the pile is on the second floor of my workshop instead of up in the ceiling joists. You could do this on the ground if you ran your drain all the way up to the peak. It's a great way of storage, also.
I'm thinking of doing this. I'm concerned that when the air cools at night I might get condensation. Should I run fans all night or don't worry about it?
Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
I am not sure why you make it air tight, as we need to get the moisture from the lumber into the air and then exhaust this humid air to the outside. As the outside is humid at night, only run the fans when the RH is low, which means when the attic is heated which means after the sun is up and warmed up.
From the original questioner:
"I'm not sure why you make it air tight?" The air would be discharged back to attic, the thinking being it is still hotter than ambient air and I would be adding no more humidity than is in the air when it first enters attic. Attic has soffit and ridge vents and wood is dried naturally (shed dried) as much as possible.
From contributor X:
Your goal is to achieve EMC, and by wrapping the pile you are inhibiting the moisture from leaving the cells. If your aim is to slow down the drying process this would certainly be one way to do it. Unless I'm not understanding how the setup works, depending on how tight you wrap it and how long the moisture stays trapped, now you have to contend with mildew/fungi/bacteria.
From Gene Wengert, forum technical advisor:
On a good day, you can add about 1/700 pounds of water to a cubic foot of air. As one BF has about two to four pounds of water to be evaporated, you will need 2500 cubic feet of outside air per BF that is dried, green to 7% MC. So, you do need a lot of fresh air in the attic.
From the original questioner:
Gene - got it, thanks. "2500 cubic feet of outside air per board foot that is dried green to 7% MC." Where can I get the formula and information to figure that stuff myself. The wood will start at about 13 or 14% I'm guessing. I have read drying hardwood lumber and air drying wood and a lot of stuff here but can't remember where I read what.
Contributor X - the idea was to have wood in a box with attic air coming in one end of box and discharging back to attic from other end of box. I will have to rethink.
From Gene Wengert, forum technical advisor:
Just make the outflow go directly outside. The info is in drying hardwood lumber but someone has to show you how to make the calculations.
From contributor W:
I dry wood very much the same way you are discussing. Air circulation is the key to success. The wrapping is a concern for me. I understand your theory but I think you are beating yourself up. I cannot help with the formals but what I can say it that I'm a firm believer that slow wins the race as long as you can control the fungus and other slow drying problems. I generally don't see these problems. What I can also say is that this method works for me, but I do not wrap the stack. Sticker it and turn on the fans. My only concern for myself is how much wood I can stack in the second story before I collapse the building. I changed out all the wall studs about a year and half ago.
From contributor S:
If you're going to be adding that much moisture to your attic, you may want to consider the fact that you may end up rotting out your house. Obviously it doesn't happen fast, but if you plan on doing this long term, remember that most houses aren't designed to be used as kilns, and the extra water is bound to have a detrimental effect.
From the original questioner:
The attic is not my house, the wood is air dried first and it is only a small amount of wood (hobby projects). I will take Gene's advice and vent outside. It is just as easy. I just figured the air would still have some energy left in it. I am wondering though why it’s all right to final dry a small amount of wood in your attic but not to use the attic air the way I was thinking of doing it.
From contributor S:
Having done maintenance and building repair, I have often seen buildings deteriorate and ultimately rot where a lot of moisture was induced over a long period of time (even by air drying laundry inside!) and where the moisture was not completely vented - especially modern buildings that are often incredibly well sealed by insulating material.
