Recently purchased a chain saw mill to mill logs into lumber to construct a barn. For 2x6 or 2x8 Red Oak how long should I air dry the lumber prior to use?

Red oak is a poor choice for a barn. It is not durable like white oak. When it comes to rot resistance and durability RO and WO are as different as night and day.

Im looking at a barn that is 40 years old and the exterior siding is made from red oak.

the red oak board & batton siding on our house is 35 years old, and still sound. If the wood is in the dry, decay will not be an issue. Our house framing (post & beam) was put up within a week of milling and, other than beams sagging a bit, no problem. If the wood is fairly straight grained, not too many knots and free of the pith (center of the growth rings), building green is an option. The wood will shrink about 5% in width & thickness, but less than 0.1% in length.

An advantage of fresh-cut oak is that you can actually nail into it without pre-drilling.

Texas Timbers is right about white oak being more durable when exposed to the elements. I use it for trailer decking and similar applications.

I'm glad to hear RO lasts so long for some of you. In this part of the country it doesn't fare very well. But of course, any species that is kept dry should be fine. Personally though, I will never use RO or Ash or Cottonwood other such species for exterior siding applications but your mileage may vary.

Hey guys, great to know that white oak is more durable and red oak has been on a barn for 40 years but back to the original question: How long should I let it air dry? And I would think the drying time would be longer in this 80% humidity in the south.

For a barn, green is fine. I've used fresh cut and put to the barn that week. There's TOO MANY barns around in the USA that were done that way and still standing after 100 yrs. THIS is what I've found out from 30 yrs of construction...and being around barns...the best barns have concrete/stone bases and keep the wood off the ground (NO ground contact) and the wood is 18-24" above the ground. This allows good wood drainage and drying for the ends that get soaked with water. UNLESS geographical area and wood species differ we in TN have very few woods that we won't use on exposed walls, cottonwood and buckeye are NO-NOs.
Ground contact is a whole different story, white oak, locust, sassafras, cherry and walnut heart are better but hard to beat ground contact PT lumber.
Build it an use what you have. There's a book by Jack Sobon that says... every region has a different wood that was prime, But when you get done to the facts it was because it's availability at the time of build....and I've found that to be true.

Building techniques make the major difference on durability. I have a hunting cabin I built at the farm with green Tulip Poplar...never sealed, painted or treated it, it's exposed to the elements and after 17 yrs it's still in good shape. There are a few nailing techniques that allow wood to move as it dries and moves in moisture changes.

Avoid sapwood of red oak as, with its sugars, it will attract insects and decay fungi. Keep it as dry as possible...good roof overhang and no shrubs or trees close to the walls to restrict drying breezes. Rain gutters will also help. A water repellant coating is awesome.

Nailing is important so that the wood can move without loosening the nail or screw. Wood movement means the outer surface can have some open joints unless special construction, such as board and batten, is used. Some pieces of oak from today's smaller diameter trees will warp. Today's wood also is not as decay resistant as in the past. So, old barns do not accurately predict the behavior of today's barns made with today's wood.

Woodpeckers, carpenter bees, termites, powder post beetles, etc. are risks, so keep you eye on the wood and fix the any issues promptly to avoid severe damage...it may not happen, but often it does.

Note that in most of the South, the average relative humidity is close to 65% to 75% RH which is 12% to 15% EMC. Our bodies say it is more humid, but our bodies do not estimate humidity well. What large city are you close too? You can look up the condition at
http://www.fpl.fs.fed.us/documnts/fplrn/fplrn268.pdf

Gene, your comment that today's wood is not as rot resistant as it used to be intrigues me. Why is this? Is this true of most species?

For oak board & batton, I've had the best luck with a single nail in the center (width wise) of the board, with the head of the nail against the side of the board that was toward the center of the tree. The single nail lets the wood shrink width wise without shrinking, and nailing against the side toward the inside of the tree keeps the board from cupping.

The Wood Handbook has recently indicated that baldcypress, redwood and pines have a difference. I suspect that we will likely see western red cedar and others added in future additions of this text.

I do not think that anyone has come up with a proven theory on why this is happening.

Oops!

...future editions of this text.

Most folks will tell you a barn can be built from green. If you do that, expect shrinkage and gaps to form where boards meet. 8/4 will take longer to dry than 4/4, for obvious reasons (thicker means more time for moisture to transpire).

Wood dries faster in the warmer summer months than in the cooler winter months. The moisture comes out rapidly at first, then more slowly. So if you cut the wood early enough in the season, sticker and stack it properly to promote airflow, then you can probably use it at the end of the summer and will have less shrinkage than green. You'll still get some, just not as much. I'd recommend tight joints as much as possible. Now, if you are building the hay loft, for example, and want some gaps to promote air flow to dry the hay, just deck it green and let the drying happen, and it will form nice little 1/4" gaps for you ;-)

Hey Eric, I've got a similar project ahead of me usin' green Red Oak (Board and Batt.) for siding. A quarter inch gap you say... is that with 4/4 or 8/4... what dimensions? That would be some handy information for me. Also I've heard kiln dryin' time to kill the bugs at 155 deg. F is 30 minuets and I heard also 4 hours... almost makes my Vertigo come back... so what do you say on that matter? Thanks hoss.

Building techniques make the major difference on durability. I have a hunting cabin I built at the farm with green Tulip Poplar...never sealed, painted or treated it, it's exposed to the elements and after 17 yrs it's still in good shape.