INTERNAL INSULATION ON AN EXTERNAL WALL QUERY

[clementsj]

HI, CAN ANYONE HELP WITH MY QUESTION ?
WE ARE PLANNING TO INTERNALLY INSULATE WITH FOAM BACKED PLASTERBOARD OVER AN EXISTING PLASTERED WALL. IT'S A 1930'S CAVITY FILLED BUNGALOW.
WE SEEM TO SUFFER FROM CONDENSATION AND HAVE SALTS ON WALL IN SOME ROOMS.
WE HAVE HAD THE WALLS CHECK, BUT HAVE BEEN TOLD ON NUMEROUS OCCASIONS WE HAVE NO DAMP, BUT PROBABLY HOT AND COLD POCKETS FROM OLD CAVITY WALL INSULATION.
MY CONCERN FROM READING OTHER QUERIES IS DO WE NEED TO PUT A MEMBRANE OR SMALL GAP BETWEEN THE ORIGINAL PLASTER AND THE NEW INSULATED BOARD TO AVOID ANY MORE PROBLEMS? IF SO, HOW? THE ORINGINAL PLASTER WAS ONLY RENDERED AND PLASTERED ABOUT 4 YEARS AGO.
I HAVE SPENT MANY HOURS RESEARCHING THE CAUSES OF CONDESATION AND FOLLOW ALL THE RECOMMENDED GUIDLINES BUT WE STILL HAVE A PROBLEM.
AT PRESENT WE'RE REMOVING EXISTING FLOOR BOARDS AND PUTTING DOWN CONCRETE AND CELLOTEX ETC SO HOPEFULLY WITH THE NEW DCM JOINING UP WITH THE OLD, THIS SHOULD ALSO HELP THE SITUATION.

[stoneyboy]

clementsj,
Are you sure you have cavity walls - the walls should be about 250mm from inside to outside without the internal plastering.
Replacing the suspended floor with a concrete one will probably make the situation worse.
Lining the outside walls with insulated plasterboard will make the walls colder. Where internal walls are joined to the external ones this will create a cold area subject to condensation.
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