Moisture, when it’s the in air, contributes to the vapour pressure (VP) of that air. The more moisture it contains, the greater the VP and the less moisture, the less VP.

Warm air within a building normally holds more moisture than the air outside, especially in winter months when the building is heated. The amount of moisture in the air and hence the VP depends on the factors mentioned in ‘condensation’ and will vary according to circumstances.

Areas of differing vapour pressure will always try to equalise; with those areas that have a higher VP tending to ‘migrate’ to the lower VP areas. Thus warm, most air, inside a building, will try to find its way to the outside and so equalise the pressure.

In most houses the fabric of the external walls will happily allow moisture laden air to pass through it, and normally this does not cause any problems; but, should the outside surface temperature of the wall be below the dew point of the migrating air, then the phenomenon of interstitial condensation can occur.

Consider the drawing below :

If the migrating air within the room (in this case to the right hand side of the drawing) had a temperature of 21 deg C and a Relative Humidity of 50% then its dew point would be 10 deg C. In other words, as it reaches an area with a temperature at or below 10 deg C, it will condense


If the outside air temperature is only 2 deg C then it's almost certain the outside surface of the wall will also be at, or near, this temperature too; and, if the inside wall has a temperature just a few degrees lower than the inside air temperature of 21 deg C - say 16 deg C, then between the inner and outer faces of the wall will exist a temperature gradient; in this case ranging between 16 and 2 degrees C. It follows that somewhere within that gradient lies the dew point of the air that is migrating from the inside in an attempt to equalise the VP. Naturally enough, at this point the air condenses and moisture forms - inside the wall.

If natural evaporation is occurring that is sufficient to disperse the moisture into the atmosphere - via the outer surface of the wall or into a ventilated cavity - then not a great deal of harm will be done. However there are some circumstances - for instance where there's a dense sand and cement render coat on the outside of a solid wall or waterproof covering on a roof - where evaporation is severely curtailed. When this happens, the problems can be severe.

The areas most liekly to suffer from the effects of interstitial condensation are those that have materials in them that can rot, i.e. those with untreated timbers forming part of their construction. These areas could include flat roofs, pitched roofs and (older) timber framed buildings; and it's these areas we'll consider next:

The above drawing represents a cross-section of a pretty basic flat roof construction. On the upper side is a decking topped by mineral felt - or some other water-proofing agent; the roofing spars, with the insulation between, are beneath; and, below it all, we have the plasterboard and skim. Also contributing to the problem: in some poorly constructed flat roofs, there's usually little, if any, ventilation.

To the left hand side is a bar, calibrated from red to blue, showing the temperature gradient across the roof, just as it was in the wall. You'll also notice that in this instance the dew point temperature is at the underside of the roof decking, although its precise position will vary of course, depending on circumstances.

As the warm, moist air rising from beneath passes through the plasterboard and then the insulation, it comes into contact with the underside of the roof decking. Here it will cool to its dew point and condensation will occur. Water droplets will begin to form and gravity may cause these droplets to fall back onto the insulation and gradually soak into it. Eventually the roofing timbers in contact with the insulation will also be soaked and, when their moisture content rises above 20 percent, they will be in danger from fungal attack. The actual roof decking may also be suffering too of course.

The roof doesn't have to be of the flat kind for this to occur either; I've seen beautifully pitched roofs that have been afflicted with the same problem, usually when the loft space has been converted for use as a further room but little attention has been paid to ventilating the insulation-filled space between the plaster board and the under-felt.

The best way to combat this situation is either to put the insulation on the outside, thus creating a 'warm roof', or to stop the moisture from getting into the void in the first place. This latter method can be carried out by the use of a vapour barrier, which is placed directly behind the plasterboard, as shown below:

The importance of ensuring that there are no holes in the vapour barrier - which can be of medium gauge polythene sheeting - cannot be stressed too much. Because, if there are any gaps, the moisture vapour will eventually find them and get into the roof void anyway - you won't be any better off! Some people rely on foil-backed plasterboards to provide the barrier but these can get damaged at the edges and the foil can easily be torn off, especially when they're being cut to size. If you do use them, be extremely careful - you're better off using a polythene membrane too. Belt and braces it may be but it's a lot better than taking a ceiling down at a later date!

Be especially careful around plumbing and electrical service holes too; make good by taping after any are cut.

A good example of a more modern, and perhaps reliable approach, i.e. warm roofing, is outlined in this link courtesy of ''Kingspan"

Another thing I want to say about interstitial condensation is its role in the problem of rising dampness.

In older buildings there is often a degree of ground water (moisture) in the lower reaches of the internal and external walls. This moisture, which is attracted by capillary action, is continuously evaporating, being replenished, and then evaporating again, especially in the winter months when the ground is wet and the water table is higher than it is in summer. As this cycle occurs, the evaporation will cool the wall and not only will this lower the temperature at the surface of the wall, it will also lower its temperature internally. Not by a large amount perhaps, but when you consider the first example shown above, where the difference between active condensation and no condensation is a small gradation either side of the dew point, then that small amount can be critical.

Even though the internal surface temperature of the wall may be above dew point, and will not promote condensation, beneath the surface, towards the centre of the wall, the situation can be different and interstitial condensation could occur. Thus the overall moisture content of the wall may rise and, without sophisticated analysis, this could be blamed entirely upon 'rising damp'.
Because of the cooling effect of evaporation, interstitial condensation is more likely to occur at the lower reaches of external walls in those older properties that have either nil, or defective damp proof courses. And although by itself a modicum of rising damp might not even be noticeable, when it combines with interstitial and perhaps surface condensation too, an erstwhile problem could occur.

This is why, when you're looking at causes of dampness, it's so important to take time to appreciate the situation as a whole. An injection damp proof course may solve the problem of rising ground water but, unless the condensation is addressed too, you may not even notice. This is why so many walls still display high moisture meter readings long after they've been treated for other causes of dampness. I have quite often seen properties which have had two or more damp proof course treatments; they probably get a new one each time the property is sold!