I want to install a glass mosaic backsplash. These are a bunch of small square tiles of different colors on a mesh. Each meash measures 13 x 13. They are so thin, that I want to put some plywood or 1/4 inch tile backer board over the drywall first because my wall is uneven and I'm afraid some gaps will show between the countertop and the wall. My question is is it OK to use 1/4 plywood for the backer or should I use hardibacker or something like that? Someone told me the plywood would expand and contract too much with temperature changes. Someone else also told me that backer board is overkill because its not a wet application and its not a floor. I'm also worried about what the glass tiles would stick best too.
Here are some good points against using plywood, the source is talking about tiling a floor but most of the points are valid for walls as well.
In my opinion it's better to go the "overkill" route, then have to redo the job in the future because something didn't turn out right.
The cost difference between plywood and cement board is negligible, so your not going to save any money using plywood (unless you already have it).
Do the job right and use cement board, when you don't have any problems and you can enjoy the tile for years to come you will never regret the decision.
I just finished installing the backsplash in out kitchen using the mosaic glass tiles like you are describing.
I had no problems with putting the tile directly on the drywall using the usual thinset mortar you would use with ceramic tiles. I used a 1/8 in throwel with the little/thin tiles and the whole sheet of tile would attach pretty easy.
I think that whichever material you chose for the backing will work fine. If you can patch up and sand down the drywall to fix the imperfections, that should work also.
Once you are done grouting make sure to seal the grout since the tile will be exposed to the water.
With the correct adhesive you shouldn't have a problem sticking the tiles to virtually anything. It might be best to use a ready mixed waterproof adhesive. The tiles will get splashed (it is a splash back after all), and you'll have an even consistency of adhesive to work with.
I agree with Tester101 on using cement board to "flatten" your walls. Having recently completed a bathroom where half the tiling was done on Aquapanel (a cement board) and half directly on the wall, it was a lot easier to tile on the panel as the wall was flat and level.