I have a sound concrete floor. If I glued high density EPS to this with EWI adhesive could I then fix floor tiles to the EPS with flexible flooring tile adhesive – or would the tiles crack and lift up in a short time. The aim is to maximise insulation with minimum increase in thickness. If the above is not feasible how do you tile on top of floor insulation.

This is, perhaps, going to sound a little odd, but I've had a lot of experience in making aircraft, and more recently boat parts, from XPS (high density extruded polystyrene foam). You can make very strong structures from XPS, if you clad it with a thin layer of glass fibre woven cloth, laid up with epoxy resin.

If you are really stuck for height, then I'd suggest this may be a way to get a very rigid, tough and watertight surface on top of the foam, with less than 1 mm height build up. I'd suggest one, or maybe two, layers of 300 g/m² plain woven cloth, wet out with the cheapest epoxy resin (must be solvent-free epoxy, polyester or vinylester will attack the foam) that you can find. The resin can be easily applied with a roller.

The resulting floor would be plenty tough enough to walk on even without the tiles, and the laminate would positively prevent any tile movement that might cause cracking.

The only downside would be the cost, epoxy resin isn't that cheap and you'd need roughly the same weight of resin as glass cloth (perhaps a little more if you're new to laminating with this stuff.

<blockquote><cite>Posted By: PeterStarck</cite>Isn't that Marmox without the effort?</blockquote>

Yes, it'd be similar, but Marmox is coated on both sides (not really needed for a flooring application on a rigid base like this) and you still have the potential problem with joints between the boards. Laying the foam on the floor, then covering it with epoxy/glass, would bond the whole floor together into a single waterproof sheet, which might be beneficial.

Fibre glass matting and cloth ontop of the EPS/XPS does should sort of appealing. I too have experienced the strength of fg/foam composite radio controlled 'planes.

3mm ply? I doubt it will stay flat when wetted by the glue.

Use pir instead of ps, gives same insulatio n for less thkness, leaving space for floating floor over the top to tile onto.

I look ed into this, theres lots of info on Gbf how to do it and on celotex kingspan etc manf websites. Google ' celotex floating floor' or variation s.

I ended up getting the extg floor dug out and new slab cast over ins, which was less of a job than I expected.

One point of backer board is, being cementitious, doesn't warp. The other points, behaves like tile as far as expansion/contraction etc; and tile adhesives really bond with it, so tiles and board unite into something stable, what, 15mm thick.

There are plenty of XPS based tilebacker boards out there now with all different thicknesses so I'd recommend one of them. From memory only Lux tilebacker uses EPS the other European ones use XPS. Not sure if Knauf makes one now otherwise all the others come in from Germany or further afield

Hi,
I just checked a piece of 20 mm left over, it came from a bathroom online supplier (dosnt state Marmox) I'd forgotton how rigid and tough it is, thinnest is again 6mm (might flex a bit).

no not unless you have read this first http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=125&page=27#Item_30

even then the answer is still no!

Posted By: ikimikiDoes it get mentioned at some point later on (among the 27! pages) ?

Almost a a year since there has been any posts on that one, still it is getting towards winter it may start up again

Update to original question - I made compromises with the amount of insulation and the floor height and put in XPS under 8cm of concrete with 5mm rebar mesh then tiles.
Thanks for the help

Its what the builder was happy doing!! or to be more precise he did not want to put any less.

Posted By: wookeyMuch simpler is to use two 6mm-10mm boards offset by 100mm to provide an overlap at joints.

Yes. Or bigger overlap and use the spay glue i mention on another GBF discussion to stick the two together creating one continuous sheet. Glues name is ” DM Easy” then as you say top with cementisous board