Installation of Polystyrene External Wall Insulation - over old render?

Question

I am considering installing a Polystyrene-block based External Wall Insulation system. Currently the house is blockwork with a rough render.

One contractor has told be that the existing render on the house will not need to be removed (i.e. they advise glueing the Polystyrene blocks directly to the old render). This seems a bit odd to me, especially since the render is so rough (there's about a 20mm difference between peaks and troughs in the render).

I would have thought the old render would need to be removed, so that the blocks are attached flush with the blockwork (e.g. so that there is no cold airflow between the blockwork and the polystyrene, and simply to achieve an even/level covering with the blocks). Can anyone offer an opinion either way?

Answer 1

If you want the job done properly (minimize the risk of any potential problems later on down the road), get the existing render removed, install your new external wall system, then apply new render to your new external wall.