Condensation occurs,as you know,when moisture laden air reaches a colder surface, such as windows, or any impervious surface, such as foil behind your radiators. Unless the radiators are not turned on, ie cold, it should not present a problem; however, if the radiators are cold, it could be that there is insufficient air circulation between the radiators and the wall. The house may be high in background dampness, which will not show as visible dampness on walls etc. You could try using a de-humidifier if all else fails. Hope this helps.
davidj
October 2011