Originally Posted by diy'er on LI
no experience with fence posts, but I do with wood mail box posts...
When I was a teen (almost 20 years ago), I replaced our mail box post. All I did was dig a hole, pour concrete into the hole, and insert the post into the concrete. It's still totally fine. No rot what so ever, even though it's in partial shade. From my limited knowledge, I believe the post will remain rot free as long as 1) the cement is not poured into a plastic bucket in the ground and 2) the post is not wrapped in plastic. I think the reason is that each case, moisture in the concrete and wood won't be able to escape, and instead will encourage rotting.
I've bumped into a fierce argument about this every time I've searched about it. (Show of hands: anyone surprised?) Let's hear the opposing view on this for a sec:
Turns out that one of the things you're doing when you water-proof seal a PT post is actually lock in
the preservative used in treating it. CCA, or what-have-you. Yes, before the screaming starts, the treatment is on the exterior, so the interior is at risk.
However, in the case of PT, it's not the interior moisture that destroys the wood quite so fast as it is the exterior water leaching out
the preservative (which then
allows any moisture, internal or otherwise, to accelerate the rot).
IOW, absolutely no full consensus is available on this. That said however, it's not the same deal as the vapor escaping a house through the siding. This is treated wood, accelerated by its preservative leaching out, and further, if the preservative is not leached out, the internal moisture can stay there for a much much longer time without any degradable growth destroying the wood.
It seems to me though that something that is often overlooked in all this though is that regardless of any kind of plastic/latex/tar water proofing that goes on, the posts will sooner or later crack, or shift sizes, or something that allows moisture in and out under the sealing compound. This is where things get even MORE sketchy for me.