Avoiding mould behind fitted wardrobes on external wall

[cinnabunbun]

Any advice for how to avoid mould on a cold external wall? I currently have open clothes rails in the alcoves of my bedroom and every year we seem to get some mould growing on the wall behind the clothes. I'm hoping to get expensive fitted wardrobes made but I'm worried they will just end up mouldy as there will be even less ventilation against the wall.

I'm already pretty good at using dehumidifier regularly, keeping the trickle vents in windows open, not hanging wet washing around the house, extractor fan when cooking etc.

The wall is the end of terrace onto an alleyway which never sees sunlight so is always very cold and damp.

Is the best thing to add some kind of internal insulation/kingspan type thing along the wall behind the wardrobe and completely seal it?

Or am I better off trying to leave a small space and having extra air brick fitted into the wall?

Current plan is for removal of the chimney breast and a full wall of floor to ceiling cupboards.

The damp you describe is usually due to condensation, when ventilation and heating of the room does not penetrate behind furniture on an external wall. Start by ventilating the room better. Never hang wet washing indoors or over radiators. Use an effective extractor fan in the bathroom so that water vapour cannot drift round your home.

you can deal with it by adding low-level heating, such as a pipe or tubular heater, on the back wall (I advise adding a wire guard) or (surprisingly) by drilling a hole through the wall so that the gap is ventilated with outside air which, being cooler, always contains less water vapour than indoors air.

If you can expose the entire wall (side to side, top to bottom) you can clad it with an insulation layer of rigid foam slab, or with insulation-bonded plasterboard. It must contain an impervious layer (this is often aluminium foil) to prevent humidity penetrating it and causing interstial condensation, or condensation on the wall behind. The plasterboard variety can be decorated like any other plasterboard wall.

however

Look at the outside of the wall first and make absolutely sure that there is no source of penetrating damp, such as a leaking gutter, downpipe or parapet.

If you want to prove the difference between pentrating damp (which comes through the wall) and condensation (which forms on the surface), tape a piece of clear plastic tightly to the wall. Condensation will form beads or mist on the room side; penetrating damp on the wall side.

Thank you so much @PigletJohn for the detailed reply! The tape test is a genius idea