Hi guys. Client is installing Wunda underfloor heating on the first floor. Ive never done it before so Im looking for some advice. Wunda tech team has given us three options. Tile straight on top of the polystyrene and integrate the ahesive into the exposed pipework. Lay Ditra over the top and then tile. Or lay interlocking hardiebacker board and then lay on that. Problem is the Hardiebacker board is 18 mm thick. The tech tean has also said the we must use the mapei s2 adhesive but my local store can only supply the keraquick with the lstex additive and that works out quite expensive. Any help would greatly appreciated.