Wallpaper coming off behind radiator

[mick.n]

Hello, we redecorated last may & i removed the radiator to paper behind it. The wallpaper is the "superfresco" type that we painted silk emulsion over. Now it has started to come away from the wall. The wall is an exterior wall with no cavity as it is an old property,so i put some of the polystyrene veneer on before the wallpaper to help with heat loss. Here is a pic.

Is there any special paste that should be used for behind radiators?

Many thanks.