The actual joist span and height are more important than the joist spacing. the joist spacing does drive the required subfloor stiffness, with 24 inch spacing needing even more thickness than 16 inch spacing. If the joists are at the limit of span, no amount of thickness from the subfloor is likely to eliminate joist flex adequately for tile.

Pressure treated plywood will rot, but alot slower than untreated. I think I used Wonderboard for that one. I didn't fasten it down at all. It is somewhat 'flexible' and heavy, so it just laid flat on its own. The customer installed a floating floor over it. I did patch the seams with a cementious floor patch to even out any slight height difference between panels. You can get a 4' X 5' panel of Wonderboard for about $10.00...that comes to 50 cents a square foot. In my customer's case, because we did two layers, it came to $1.00 a square foot, much less than a self-leveling underlayment would have cost for the same lift.

Every time I run into PB it comes out and is replaced with 5/8 BC grade plywood underlayment. The 1/2" plywood underneath is a CDX grade with no T&G so that is no good for anything without an underlayment over it. Particle board underlayment is fine underneath carpet because it can breath. It has no holding power for staples or cleats, neither does the CDX sheathing grade plywood subfloor. I've seen many floors installed over PB that are very loose and gapped. That said, I have seen a couple that looked fine. Also once PB gets covered over with felt and a wood floor it can't breathe anymore and it can start to do funny things.... well, not funny :), but nothing it may want to do is good. You would have no mfg. warranties either.

Cancork Floor Inc. I could write a book about the moisture issues we have had to deal with over the last 30 years. Ten years ago we were in the process of putting in new windows and a deck and we also knew there was a moisture problem. We called an encapsulation company and they inspected our crawlspace and found standing water and extensive damage due to moisture. In addition, we only had 12” under the beams and 18” under the joists from the soil. Also our crawlspace only had three vents and they were all on one side with no cross ventilation. So began the 10 year journey to force our HOA to address storm water drainage in the complex and rebuild our home from the ground up. We cribbed our townhouse and excavated the crawlspace to increase the distance between the joist and beams, poured all new pads, replaced all beams and joist and installed new 3/4'” T&G STURD-I-FLOOR. We also installed a 6 mil moisture barrier. It is very nice now in our crawlspace and I believe the moisture level is fine. I measured the moisture content of some of the lumber in the crawlspace and it was 12%. I snipped this from a document on Wood Moisture Content. 12% - 16% is not considered wet and is healthy for a crawlspace. http://www.rlcengineering.com/wmc.htm "12% - 16% - Readings in this range are common to lumber during construction, air dried lumber and "healthy" residential substructures (beneath first floor in crawl spaces). These are typical readings for exterior wood." BTW, I used a General pin type moisture meter, Model MMD4E. I understand these are accurate for Doug Fir and plywood. Below are before and after photos in one area of our crawlspace.