Hey Fellas! I'm new to this forum and I'm glad that I found it. I've skimmed some of the posts, but can't find something like this question I have.

I have an outside poured concrete slab and I will be making this area into an enclosed 4-season sunroom. We intend to put down ceramic tile over a heated mat (Nu-Heat or something similar). The area to be tiled is 15' x 14'. Unfortunately, the slab is not insulated - why would it have been, it's exterior, right? There's no point in heating the ground and the added height won't be of any concern. Anyway, I'd like to put down 2" of Styrofoam SM, then the Ditra mat, the heat mat and finally the tiles. What I'm concerned about is possible compression of the SM and subsequent cracking of the tile. I thought about installing cement board over the SM, but it just seems like too many layers. Has anyone had experience with something like this? In a perfect world I'd have poured the slab with hydronic heating and the problem would be solved. However....
Thanks for any and all suggestions, guys.
Cheers,
Ken

Look into using Wedi board .... it is the only backerboard I'm familiar with which can be used over concrete. (Wedi has insolation properties)
The ditra should be used over top the heating cables, not below.
If you use Wedi and level your cables with a top quality mortar you might get away with not requiring Ditra over top the system.

Wedi comes up to 2" thick although you'd need to check the specs for this type of installation. Perhaps their website may have further information.

Thanks, Harry. I'll look into it more. I couldn't find your response without a lot of searching. Is there a way to be notified and taken straight to the post? Thanks again.
Cheers,
Ken

There is Ken
Up top of this or any other page of interest, just click on Topic Options and then make it a watched topic.
Here's the link to their site. http://www.wedi.de/home_usa/