Cavity wall insulation in flats

Hi,
I have a one bedroom flat built in the early 1970s that is rented to the tenant. I think the construction is a concrete frame with coal dust brieze block internal walls and outer brick facing.
The flat is double glazed (uPVC), gas centrally heated (balanced flue (?) combi boiler) and carpeted.
On an outer wall in a bedroom mould has appeared twice in the past couple of years. The mould appears in the area behind and adjacent to some lined curtains both on an externally facing wall and the adjacent internal wall. I have in the past cleared the mould using bleach, washed the walls and reemulsioned. I guessed the mould would reappear and it has.
I am going to buy a De Longhi dehumidifier today. This device gets decent reviews for its quiet operation and effective drying ability. All that I have read on this topic tells me
Humidity/moisture in the air > Meets cold surface > condensation > mould
Now, I don't have control over the tenant's life-style and leaving windows open all day isn't practical in a downstairs flat. The trickle vents are open so I suppose some would argue that dehumidifying is absolutely pointless as the air will just get replaced. However, my thinking goes like this. If I can create a local dry spot where the mould forms then I will be tackling the problem where it occurs. Basically I'm taking a 100 gamble on the dehumidifier to improve the situation for the tenant and I can't make the situation worse.
Longer term I'm thinking about cavity wall insulation. The idea being to try to stop the external wall being (so) cold therefore stop the condensation. A useful by-product would be lower heating bills - although the finances probably wouldn't pay back during the life-time of the building. The aim would be to improve the conditions in the building and lower the risk of mould on the inside of external walls.
So after all that does anyone know for the building type I have described if cavity wall insulation can be installed and whether there would be any benefit from doing so? I know I would have to get the freeholder on board too. I'm just trying to understand if the insulation is a possible (allbeit slightly expensive) solution to the mould problem.
Regards
Clive
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A dehumidifier sounds like a good solution. Trickle ventilation doesnt stop it working.
From what you've described I dont see a problem with CWI, and payback is quick in heating savings. I'd be hesitant to rely on it solving the damp issue though.
NT
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Clive wrote:

Agreed; though the problem with buying a humidifier for a tenant is that once they realise how much they cost to run they tend not to want to know; furthermore unless you get a model with a permanant drain they don't work once their reservoir gets full and need emptying...
David
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You could fit a vent fan (replaces internal damp air with dryer outside air but with a heat exchanger to the heat stays inside) through the wall. vent axia make them IIRC. They can be set to be permamently on 'low' thus keeping humidity at bay but not losing heat (becuase of the heat exchanger.
Robert
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Condensation occurs where a surface is below the dew point, curtains are insulatory to some extent and may result in an area of wall being cooler thus local condensation and mould.

That suggests higher humidity than desireable from somewhere.
Can you verify no sources of moisture penetration? - gutter leak, radiator leak - sink/shower/bathroom leak, poor sealing thereof - leak around window frame (wind blown) - leak around window-cill - damaged render or blocked vent(s)
Can you be certain occupants ventilate it properly? - kitchen & shower/bathroom extractor *actually used* - trickle vents not stuffed up *when you are gone*
If all ok then frankly I would spend the 100 on CWI. CWI will make the external walls warmer and they will check the cavity for you for free - perhaps get them to check right where you have the condensation for anything bridging the cavity. CWI will not help the condensation on the internal wall, which makes me suspect there is inadequate ventilation.
It might be better spending 100 on getting a humidistat set on the relevant fan(s).
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Clive wrote:

Hi,
Fit decent extract fans with run on timers connected to the LIGHTS in the bathroom and kitchen. I have a bathroom with no windows, have run on set to 20 mins and no mould. Do not rely on tenants to make use of the fans, they won't....
Regards
James
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