Damp diagnosis

Signs of hygroscopic salts

Before considering standard condensation, there is a little studied, but common form of condensation caused by hygroscopic salts.

A seemingly strange form of dampness can occur in spots or bands on wall, despite there being no rain and no obvious contact with the ground. These damp spots are often worse in summer.

The phenomena is caused by hygroscopic salts causing vapour to condense when relative humidity is at normal levels.

The most common hygroscopic salt in UK houses is Calcium Nitrate, which causes deliquescence at about 50%RH. Calcium Nitrate comes from the burning of fossil fuels and from the historic use of manure, affecting properties built before cars became common place.

Calcium Chloride is less common with a deliquescence humidity of about 30%RH.

It is often said that rising damp causes hygroscopic salts to rise. It is possible, but rising damp is not the only reason for finding hygroscopic salts halfway up a wall. As you have to have groundwater to cause rising damp, if there is a remote risk of groundwater, then it is highly unlikely that rising damp cause the hygroscopic salts to move.

Signs of interstitial condensation

We are now going to consider a very special form of condensation called interstitial condensation. This is condensation that forms within the building material.

This occurs when the temperature within a wall or other building material falls below the dew point.

A common example is condensation around a steel support or joist ("RSJ").

Walls naturally absorb atmospheric water vapour. The amount of vapour that can be absorbed is a function of various things including temperature. At a given temperature vapour condenses in the brick. A wet bricks loses half its insulating capacity, increasing heat loss causing more condensation.

The base of a wall is typically the coldest and the most likely to be damp from rain. Furthermore, gravity will tend to draw condensation in the wall down. Therefore interstitial condensation tends towards to base of a wall. People naturally think the water is rising, but it is not.

Interstitial condensation is commonly mistaken for rising damp, but it is quiet different and requires different treatment.

Ventilation Assessment

Modern living, with central heating, open plan rooms, internal bathrooms, frequent showers, more clothes washing, changes in occupancy, cooking styles, double glazing etc. has dramatically increased the need for ventilation.

The best form of ventilation is extracting humidity at its source.

Very few properties can survive without mechanical ventilation, that is electrically operated extractor fan. Alternatives exist such as Positive Input Ventilation ("PIV"), Heat recovery and continuous flow, but they all have short comings.

Building Regs. sets out a benchmark. But note there is a big difference between manufacturer claims and in-situ performance. Extractor fans are often set-up inefficiently, or lose efficiency over time.

See if you have a ventilation issue. Most can be easily rectified, or you can take a nuance approach, increasing ventilation only when it is cold outside.

Each day everyone contributes:

• ½kg from each shower or bath,
• ½kg from washing clothes,
• ¼kg from cooking,
• ¾kg from respiration (¼Kg at night),

At 20°C (68 degrees F), a cubic meter of air can hold a maximum of 18 grams of water. Or at 100%RH 1kg occupies 56M³. Typical property sizes per person are 150M³. Without sufficient ventilation, a shower, or drying clothes will quickly result in condensation.

Heat Assessment

Balanced heating is nearly as important as ventilation.

Condensation forms when relative humidity rises to 100%RH, which is also to say when the temperature drops to the dew point.

Likewise, mould only grows where relative humidity rises to 85%RH for 6 plus hours at a time.

An alternative is to run dehumidifiers. However, dehumidifiers are untargeted and need water to be constantly removed. People forget and become draw into a false sense of security.