A couple of quick questions

I can't address the bathroom question, but as far as the basement, a dehumidifier's a good idea. But, check Consumer Reports - I believe they did noise tests on these machines. It's important, especially if you live in a ranch, where you're sleeping right above the basement. Even with the quieter ones, you might want to run it with a heavy duty timer, and have it shut off between certain hours.

For tile concrete board is best, If your basement is below 65f you need a dehumidifier that wont freeze up Sears and other companies have models for cold basements. Sizing it is important for a small 600sq ft basement a 35 pt should be ok, to big a unit will cycle alot, to small may not do enough.

Nails and/or screws. Not a bad idea to bed it into thinset to fill any low spots. _____________

Personally, I wouldn't bother...the thinset used to bed the tiles will fill any gap.

Ditto on this opinion. I finished one last year. The Hardiboard has screws specifically made to install it. Thinset is more a leveling agent rather than a bonding agent. No need to tape joints.

Frank

I would look real hard for a left over dehumidifier with a 5 year guarantee. New ones are coming with just 1 year and my big Sears 70 pint unit hasn't made it that long yet. It was replaced once and fixed once.

I agree with Dado and Frank about the taping.

The hardiebacker has concave areas (dimples) where the nails or screws are to be placed. That is a lot of nails if you are doing it by hand. Last time I did half of a 12x18 room I was worn out from pounding. Most of the pros around here just use 2" galvanized RS nails fired from a paslode or air nailer.

Don't know a thing about the humidifier but the advice you have gotten sounds right.

Colbyt

Those "special" screws are just square drive. But, they *did* do nice job. And they didn't break, either.

Actually, they are a little more than just square drive. The point design and the serated thread is specifically for self tapping in either wood or steel. Additionally, the head has cutting flutes for self countersinking without cracking or stressing the board because of seating torsion. The coating is designed to take the basic ph condition without corrosion even if moisture gets to them in the future.

and square drive is the only way to go.

Expensive, but well worth it in my opinion.

Frank