You might try Schluter's Ditra and not Kirdi. So the Dirtra will channel the vapor pressure to the sides to still allow it to escape. It also acts as a anti movement membrane (it allows for a little movement) in case your surface is subject to some hairline cracks (if its old, all the cracking has probably happened already). Ive used it before, its easy to put down. Just thinset it down first. Once it goes down, you will need a bit more thinset since the thinset on top must fill into the interlocking texture, but its no big deal. Ask on John Bridge's tile forum for more recommendations.