XPS foam board instead of Kerdiboard?

[speed51133]

I am building myself an outdoor bar. I was thinking of tiling the countertop and using a thick piece of kerdi board as the substrate. I was then thinking, what really is the difference between Kerdi board and XPS anyways? What would be the issue with using XPS for the counter (2in) or really XPS in the bathroom?

This is more of a thought experiment. I will probably end up molding a solid concrete counter, but I do see Kerdiboard used to make counters and vanities and walls.....why not XPS?

[Lou_MA]

I don’t think thinset would bond too well to the XPS. The thinset flows around the fleece on the Kerdi board to lock everything in.

[jadnashua]

The material on top and bottom of Kerdiboard is there for a few reasons:
- it helps with vapor transmission. While the EPS may be liquid water tight, it may not be vapor proof. And, the density of that in Kerdiboard is fairly high.
- it helps with the bending strength because the material doesn't stretch or bend as easily as the foam, so it helps keep everything straight and flat
- it aids some in the thinset bond. But, there are enough micro pockets in foam for thinset to bond quite well. Another but, though, the crystals grown in the cement are fragile, and the raw foam is more flexibly, which could break them, weakening the link. the bonded on films should be more stable, and thus, promote longevity in the bond.

All of the tileable foam core panels have some sort of coating on them to overcome the above potential problems. There has to be a reason for this.

[speed51133]

thanks. makes sense. now I need to glue some kerdi to XPS with foam board construction adhesive

[Just In Tile LLC]

I like your spirit Mike

In Mikes thought bubble..."So basically encapsulate the foam and...eureka!!!"

[jadnashua]

thanks. makes sense. now I need to glue some kerdi to XPS with foam board construction adhesive

Just use an unmodified thinset. That's exactly what is done with their all-foam pans and curb (some of the newer ones come covered from the factory, but for decades, you did it yourself).

When tiling, you really want 100% coverage, and that's hard to do with a construction adhesive, and much easier and cheaper with thinset.