can you supply a bit more information. e.g. age of house, cavity wall or solid. vent shown is basically a big trickle vent and may help a bit but i think you need to get to the real root of the problem. mould and salt deposits inside a living room would suggest a damp proof course problem in the wall rather than a ventilation issue. a photo would be good too.
House is 1920's build with solid walls. Damp proof course has been done recently along the affected external wall. We had had builders round in the past and they said just condensation issues but didn't offer any suggestions. I will supply a photo but main problem is surrounding a upvc window frame which has no vent or opener. The affected room has a gas fire with an open flue and a sufficient sized radiator,
when did they do the DPC and what method was used. Do you have a condensation problem anywhere else in the house?.
Salts coming out of the wall really does not sound like an internal humidity issue. When they did the DPC they should have removed the internal plaster for a couple of feet up from the floor and replaced it with a sand/cement render. often a waterproofer is added to the mix. done properly it is very unlikely that this has not worked. get them back and invoke the guarantee.
if it is a bay window and the patch is high up then check that rain is not leaking in via defective flashing/downpipe.
if the DPC really is ok then you might also want to go the whole hog and consider insulating the wall to remove the coldspot. it will decrease your heating bills as well. I have done this using 70mm celotex PIR board with a 25mm air gap then plasterboard. it is within the capability of a competent DIY person. you can use thinner celotex but the amount of work is the same so unless you are really short on room space it makes sense to go for the thicker board.and the much better insulation it provides.
DPC at the front of the house was done prior to house purchase. DPC on the side of the house was done later when we discovered this problem. The house is rendered externally and previous occupants had rendered down the ground so dpc was breached. Render is now above DPC. We had roof tiles extended, lead flashing replaced and chimney re-pointed etc because we thought water was seeping in from the top. We have a similar issue in the kitchen which is on the same external wall. Could it be that water has penetrated prior to tiles being extended and is evaporating internally and causing a problem. My thoughts are to neutalise the salt and sand back the walls and monitor the problem. I don't want to waste money stripping back to brick to find it doesn't solve the problem.
Minor condensation & mould problems elsewhere in the house but nothing major. My other thought was a loft fitted PIV unit to increase whole whole ventilation.
if wall had got really saturated it could take months for it to dry out and if outside render painted then it harder for the moisture to get out that way. I would be inclined to do as you say, clean it off - leave bare to aid moisture escape then monitor it for a month or two. you could get an electronic temperature/ humidity digital thermometer from Amazon. they only cost a few pounds, and note down the humidity and temperature readings which will tell you if you have excess humidity.
Thanks for the advice. I bought a digital thermometer/humidity reader a while ago. It does seem that the humidity in the living room never drops below around 60% and now the cold weather is here it hovers around 70% even with heating on
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