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Old 09-26-2011, 03:58 PM   #2
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Join Date: Jul 2002
Posts: 30,274
Mike, your joists are fine as long as they are in good condition.

The subfloor is a bit skimpy since it is no longer in pristine condition. I would add a layer of 3/8" plywood, if you can find good quality, non-wavy as a potato chip stuff. Otherwise, use 1/2". I know what you said about floor height, but the idea is to build a tile floor that will last, not an even floor that fails.

Set the heat mat directly on the plywood, using thinset or SLC. If using SLC, follow their directions to the letter.

Use Ditra as your tile underlayment. Kerdi is a waterproofing membrane, not an underlayment for wood floors. Waterproof the floor by covering the Ditra seams with Kerdi-band. Flash the walls with Kerdi-band, too. Keep in mind that this will only direct water out the bathroom door or through any floor penetrations like the toilet.

Your crawlspace vapor barrier would be a plastic sheet over the earthen floor.

One-size-fits-all solutions helps remove confusion and misapplication from the product manufacturer's list of things to worry about.
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