I've been asked to tile a hallway and toilet which is currently concrete. The floor is not very level so i'll sort that with a levelling compound (i've not used this stuff yet but imagine it wont be anything tricky). They have then asked me to lay some plywood down before I start the tiling. The reason they give is that it will insulate more. Is this the right thing to do or should I advise that it will be better to go straight onto the levelled floor? thanks a lot.