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Surface condensation is the most common cause of mould growth in the UK.

Requirements for mould growth on a particular surface/material appear to depend on : water activity (hygoscopicity).

However many moulds found in properties do not require surface condensation for their development.

It is also important to note that long term maintenance of high humidity (in excess of 75% relative humidity), without condensation, will also cause moulds to develop in stagnant areas especially on ‘moisture sensitive materials’, such as leather, some clothes and paper etc

There are over 100,000 species of mould worldwide.

Less than 100 are commonly reported in UK properties, the most common being Penicillium and Cladosporium.

Stachybotrys  chartarum (atra), a so called ‘toxic mould’, is uncommon but not rare in the uk.

Which species of mould grows where depends on a number of factors including the humidity and the particular substrate. Also the colour of the mould will depend on the particular species as not all moulds are black.

Primary and secondary colonizing moulds only require relative humidity maintained in excess of 75% to develop on certain ‘moisture sensitive materials’.

Simple food requirements

Like wood rotting fungi, they produce vast numbers of spores. They can grow quickly under suitable conditions. Moulds have the same requirement for growth and survival as rots.

They require:

  • Oxygen
  • Suitable temperature
  • Nutrients
  • Moisture (condensation/humidity) This is effectively the only controllable factor.

Some fungi produce toxic substances called mycotoxins, these are secondary metabolites: present in fungal spores (and hyphae)
Moulds produce a large number of volatile organic compounds-this gives the typical ‘mouldy’ odour.

There are 4 species of moulds that are considered to be ‘toxic mould’ (this is not a scientific description): –

  • Stachybotrys chartarum
  • Pencillium
  • Aspergillus
  • Fusarium

FUNGI-Life Cycle

  • Spore
  • Germination
  • Hyphae
  • Mycelium/fruiting
  • Colonisation
  • Sporulation
  • Moulds are more prevalent if there is:
  • Lack of air circulation
  • Dampness
  • Darkness

Warm air can retain more water as vapour than cold air, and the proportion of water that is actually present to what could potentially be retained at any given temperature is known as the ‘relative humidity’.

Surface condensation is the result of moisture laden air coming into contact with a suitably cold surface.

As the air cools when it come into contact with a cold surface the capacity of the air to retain water declines,( and therefore the relative humidity increases)until at a specific  temperature, depending on condition’s ,it cannot retain the excess of water now present; the excess water now drops out of the air as condensation onto the cold surface.

It is important to understand that the excess water vapour in the internal environment responsible to surface condensation is not derived from damp walls or floors.

The excess internal water vapour is the result of occupation (‘lifestyle’) as a result of washing, cooking, bathing, and people breathing, etc. As a result the excess water vapour is generated constantly with the property as a ‘normal part of occupation up to approximately 15 litres (nearly 4 gallons being produced by an average occupying family.

Wet/damp walls are not therefore required for surface condensation to occur, and it occurs mostly on dry walls and ceilings and as such it is very much restricted to the surface, frequently not penetrating more than a couple of millimetres on permeable surfaces and on paint films and vinyl paper and other less permeable finishes it will remain distinctly on the surface.

It does not lead to dampness through the whole thickness of the wall like rising damp.

A low relative humidity is very important for evaporation with higher relative humidity reducing this process leading to sub-optimal conditions especially during the colder months of the year.

Given that a property can have can have four external elevations, only two are likely to be exposed to the wind at any one time and two will probably be sheltered.

Indeed the wind speed around the base of most walls is almost certainly much lower than at high levels, even on windy days.

So combinations of low wind speed, low temperatures and high relative humilities, part of the building in shelter and the wind striking the wall at a non-optimal angle would without doubt, subject the system to long term sub-optimal conditions during which time constant and continuous sources of water ingress must simply pass by the units as under such conditions they are incapable of operating effectively.

Environmental Protection Act 1990 Chapter 43, Section 82. States it is a landlords legal responsibility to deal with dampness problems as well as it being a duty of care. The main problem is condensation, which is nearly always seen as mould growth.

Under the law this can be deemed as a statutory nuisance. An action can be brought under the Act, which is of a criminal nature and can attract significant fines.

There are reasons to believe that the growth of fungi and mould in the built environment may affect human health, depending on the extent of the growth, the length of exposure and the general health of the occupants.

Possible reasons for becoming a more widespread issue:

  • Changes in construction methods
  • Introduction of vapour barriers
  • Fitting of draught proofers
  • Construction of air tight buildings
  • High density insulation