If you use the concrete backer board and adhesive there shouldn't be a problem with moisture from underneath. It sounds like you're not planning to use these. Be aware that if there's the slightest flex of the floor, your grout will crack. That's why you need the concrete board on top of the existing subfloor - to make it completely rigid.
Read the post from kspero, most recently dated September 3rd.

[This message has been edited by rpxlpx (edited September 06, 2001).]

Hi Heart_2001,

I agree w/rpxlpx. The key to this job is the joist and sub-floor support. I feel 18" OC is WAY to large a stretch unless you have a strong sub-floor. As already stated, any flex in the floor will CRACK the grout! You may need to install 1/2" EXTERIOR grade plywood (since you have moisture down below) perpendicular to the joists, and then a 2nd layer of 1/2" EXTERIOR grade plywood perpendicular to the 1st layer of plywood BEFORE you install your 1/2" backerboard. Be SURE your horizontal seams in the plywood (speaking for each layer) don't line up w/each other. In other words start your 1st column w/a 4' sheet, then go to 8' sheets, stacking them long-ways to each other. WHen you go to the NEXT column, start it w/an 8' sheet. You see, you want your 4' sheet to adjoin an 8' sheet, and you'll see how the seams get staggered. Your 3rd column will start w/the other 4' sheet you cut off to start your 1st column. See what I mean?

Now, about your moisture problem - I'm going to assume that it's a dirt floor. Cover it w/6 mil sheet plastic (which you can find in the Home Centers in the Paint Department. (I didn't find it in the Garden or Building Materials Department where I thought it would be.) The color doesn't matter. Just be SURE that after you've layed it out under your kitchen, you lay some bricks (or something) around the perimeter to keep it down. Cut it 2' LARGER so you have some extra hanging out on the edges. (You can just throw it over the bricks, or even cut it a tad larger than the size of the area in question.) If there's any chance of the plastic being exposed to sunlight, cover that area w/gravel. (UV rays will degrade the plastic REAL QUICK!) Don't go adding vents or fans or dehumidifiers or heaters or anything to the crawlspace. Whatever is there already, leave it alone. Go w/the plastic on the ground and see how it goes. The EXTERIOR grade plywood you use will be a good deterent to the moisture that manages to rise up.

If you have more ?'s, post back. My best to ya and hope this helps.

Jay J -Moderator