We are wanting to install slate tiles (about 1/2" thick, 12x24") on the exterior of our icf house. Any one done this before? I know the methods of installing stone and stone veneers, but I would think that would be over kill for thin tiles. My thought would be to install 1/2" backerboard via screws to the furring strips and install tile as normal onto that. The other thought was to screw up galvanized mesh and install a thick mortar bed/install tiles that way (not doing a scratch coat first). any thoughts?

Standard backerboards for tiles aren't rated for exterior use, and are likely self destruct over time due to freeze/thaw cycles.

It seems to me that backerboards will store humidity there and with the cold will probably crack. I have to agree with Dana1 on this.

so go with a mesh, scratch coat and then tile?

We are looking into installing a 1x1 slate tile 5' up on the interior of some ICF walls. A tile guy we work with said to use Platinum Thinset directly to clean EPS. Opinions?

My guess is that it would be fine.

The underlayment should be what the tile guy will warranty. A scratch coat will work. Better for you if it is a scratch coat of his choosing. Mesh definitely