In order to understand what interstitial damp is we firstly need to look at what causes water to condensate. The air within a house or a single room will hold a certain amount of moisture, this is called the relative humidity.
If we imagine the room as a bucket filled with water, at the temperature of 0 degrees the air is fully saturated with moisture, with an increase of 10 degrees the air now warmer can hold the same moisture although is less saturated due to the spaces between the molecules of air becoming greater, so although the amount of moisture remains the same the air is only 50% saturated. At 20 degrees which is nearing a comfortable living space temperature, the air is only 25% saturated, despite the air holding the same level of moisture it would feel more humid at 20 degrees than it would at 0 degrees the difference is at zero the moisture has reached what is known as its dew point.
When moving back towards the cold face of the wall or outside the building, the relative humidity may be more than initially identified due to the added moistures of building life, such as a person breathing which can be up to 400ml per day, then if we add baths showers, drying washing inside the house on the radiators etc. and cooking or boiling kettles this initial 100% saturation becomes higher meaning the air must expel the moisture into droplets or condensation.
DEW TO TEMPERATURE
As the air cools the relative humidity in the space rises until the air cannot hold the moisture which will now condensate into droplets and attach themselves to any cold surface forming condensation, which can result in mould spores collecting, causing the all too familiar black spots.
If you identify brown outlined stains this is likely to be a roof leak or water ingress which is different than interstitial condensation, in this case it is best to identify potential sources above or behind the area the stain has formed to see if a wet patch is visible.
JUST TO BREATHE
So how do we deal with our now identified condensation. Allowing moisture to pass through the structure stops the moisture building on the inner face, in figure 2 the blue line shows the moisture passing through the wall and the red line denotes the temperature, if the two contacted that would cause the dew point to form within the wall. This would not form the mould in the internal face unless the moisture capillaries into the internal face, however due to the frost thaw cycle it could cause spalling of the brick face, which may result in water ingress into the property.
However, when looking at figure 2 we can see that both lines are declining meaning the fabric heat losses from the walls are significant, so insulation to the wall is required. This poses a new problem that should the insulation be placed externally moisture forms behind the insulation. If it is not allowed to pass as a lot of products are designed to be water proofed as a new outer skin to the wall. However once wet from behind the insulation becomes ineffective and the moisture damages this and the structure as seen below in figure 3. Vapour control layers and suitable ventilation internally may be able to deal with the internal build-up of moisture prior to the dew point being met
In summary the best form of stopping the damp build up is not a one stop answer, the main cause is an internal humidity higher than the air can handle which then condensates and releases the moisture on the nearest cold surface or where the temperature is lowered to the airs dew point.
However, this may be as simple as venting the house when cooking bathing or drying washing and may not be a design fault, or it may be that the insulation material or render / cladding finishes are blocking the passage of moisture which may then need to be rectified.
When dealing with the build-up of moisture it is a process of elimination. Heat exchange units and other ventilation services are available which can keep the warmth in the building while exchanging fresh clean air and heating it. Trickle vents are often closed on buildings causing a stagnation of air filling with daily moistures and should also be considered if already installed.