The condition is called hygroscopic dampness. And it will be worse when it's rained or is raining. It's not from the exterior though; what has happened is that thick plaster ( not render) has been used to straighten the wall or patch it. Plaster is hygroscopic and will absorb moisture from humidity.
An effective method is to start by cleaning off the mould with spray containing bleach. This will help remove the staining that persistent mould can leave behind. Leave to dry overnight and then spray the affected area with an anti-fungal wash and allow that to dry.
Use concrete sealer on the wall. Such as drylock. Cover the whole wall. Put a ridge foam insulation against the wall. Leave a gap at the bottom, just in case. Frame the wall, put bat insulation in the framed wall. Put some weep holes in the bottom. Drywall over it all. Again slight gap.
Use fans to move air around the damp walls. Also use dehumidifiers, which can help remove moisture from the air and walls. Professionals use tools like moisture meters, infrared imaging devices, dehumidifiers and heavy-duty fans. That ensures the walls are truly dry before further repair, painting or refinishing.
There are several causes for damp patches appearing on interior walls including damaged guttering, cracked roof tiles and more commonly condensation. Condensation forms when warm moist air within a room touches a cold internal wall or surface.
It may be due to one of the following causes: Cracks in the walls. Broken or leaky tiles on the roof. The damp-proofing of the foundation has been affected due to rain.
Condensation Build-Up
In hot, humid climates especially, the mere presence of excessive indoor condensation accumulating faster than it can evaporate sometimes leads to pooling water sources spreading those unsightly stains around.
The areas where the moisture can evaporate is where the damage is the most visible with the salts damaging the plaster and masonry. Damp patches on walls – stains that may be brownish to light yellowish in colour.
Increase air circulation by using fans and by moving furniture from wall corners to promote air and heat circulation. Be sure that your house has a source of fresh air and can expel excessive moisture from the home.
Dehumidifiers can indeed aid in drying wet walls. After a flood, leak, or a spell of extreme humidity, damp walls can develop. A dehumidifier removes extra moisture from the air, lowers the humidity, and stimulates evaporation.
You will want to clean the stain using a mix of bleach solutions. Start by putting down a drop cloth beneath the condensation stain. Mix one cup of bleach with 3 cups of warm water. This solution will help fade the stain and clean any mildew or dirt.
If rising dampness is left untreated, it can cause serious damage to a building. The moisture that seeps into the walls can rot wood, blister paint and wallpaper, and crumbling plaster. Moreover, rising dampness can also promote the growth of mould and mildew, which may have adverse effects on one's health.
If condensation is not remediated quickly, damp patches will form on hard surfaces. Eventually, they evolve into mold and mildew growth (appearing as dark or black spots).
If your home has poor ventilation, condensation from everyday living might be allowing moisture to build up in your home, leading to damp patches. Your kitchen and bathroom are the most likely causes of moisture build up in the home, but bedrooms are also moisture hotspots.
No mold doesn't always grow after a leak. If the temperature isn't right or if the leak is properly dealt with quickly, there's a chance that mold spores won't be able to take root or grow.
Grab a room fan to dry small damp spots. If the area that is wet is a small one, then it is possible to use a standing oscillating fan to try it out. Place one or more fans in front of the damp spot and turn them to their highest setting. This helps dry the wall and also gets moisture out of the air.