Even years later, once the conditions are allowed to deteriorate again, without the property having undergone thorough eradication treatment, the mould can quickly re-appear. In other words, the mould spores can lay dormant when the conditions are against them, but will be re-invigorated once cold, damp moisture laden conditions are allowed to re-appear.

Older properties, especially those with solid walls (no cavity and no cavity wall insulation) are more prone to condensation problems. It is not incumbent on the landlord to bring an old property up to modern standards, but merely to show that all reasonable precautions have been taken. This may include educating the tenants as to how to avoid creating condensation conditions.

Interstitial condensation

Interstitial condensation occurs where, after prolonged periods of exposure to excess moisture content in the air, usually coupled with cold damp conditions, moisture is diffused deeply into the fabric of the building. The wallpaper, plaster, mortar, masonry and timber gradually absorb more and more moisture. This will lead to a vicious circle of colder surfaces, increased mould, deterioration of the building materials, wet and dry rot in timber and electrical system problems.

What is Condensation?

Condensation occurs where moist warm air comes into contact with colder dryer air, or a surface, which is at a lower temperature.

Air contains water vapour in varying quantities; its capacity to do so is related to its temperature - warm air holds more moisture than cold air. When moist air comes into contact with either colder air or a colder surface, the air is unable to retain the same amount of moisture and the water is released to form condensation in the air or on the surface.

Condensation is generally noticeable where it forms on non-absorbent surfaces (i.e. windows, window sills, mirrors or tiles) but it can form on any surface and it may not be noticed until mould growth or rotting of material occurs.

Conditions for Condensation

In Britain, condensation in houses is mainly a winter problem particularly where warm moist air is generated in areas like kitchens and bathrooms and then penetrates to colder parts of the building.

The moisture in the air comes from a number of sources within the house. Water vapour is produced in relatively large quantities from normal day to day activities - a 5 person household puts about 10 kg of water into the air every day (without taking into account any heating) - i.e.

• breathing (asleep) 0.3 kg
• breathing (awake) 0.85 kg
• cooking 3 kg
• personal washing 1.0 kg
• washing and drying clothes 5.5 kg
• heating - especially paraffin and flueless gas heaters. For every litre of paraffin burnt over one litre of moisture vaporises into air. Every carbon fuel produces some amount of water from combustion.
• (1 kg of water equates to about 1 litre)

Moisture can also be drawn from the structure of the building into the internal air, from below the floor or through the walls/ceilings.

Problems with the structure of the building can mean that its moisture content is unnecessarily high. This can either be due to the method of original construction or as a result of structural failures.

Older houses may not have a damp proof course (DPC), which prevents soil moisture from rising up into the living areas. Solid floors can also lack a DPC.

Any room that is underground, or partly underground, is prone to condensation and mould. Basement walls and floors, unless they are tanked and thoroughly insulated, present cold surfaces to moving warmer air which invariable results in condensation

DPCs can become damaged or bridged, either externally through soil build-up, or internally through mortar debris in the cavity. This allows moisture to soak into the masonry and rise into the living areas.

Buildings may lack or have insufficient airbricks to allow adequate under floor ventilation, which is vital to prevent mould growth and timber rot problems.

Structural failures or design faults can range from missing or bridged DPCs, damaged gutters, down spout pipes, porous pointing or brickwork, and lack of cavity trays on adjoining extensions, to leaking roofs.

Keeping the moist air in the house through effective draft proofing aggravates the effect of moisture generation. It is theoretically possible to avoid condensation altogether by adequately venting moist air from the room in which it is generated.

In certain areas of a house (such as bathrooms and kitchens) the warm air contains a lot of moisture; if that air then spreads to cooler parts of the house it condenses on any colder surface.

Up until the middle/late part of the twentieth century most houses had high natural ventilation, as the level of home insulation was low, draft proofing was non-existent and open fireplaces and chimneys allowed air circulation.

As Energy Conservation became necessary, natural ventilation was greatly reduced by the introduction of double glazing, draught excluders, fitted carpets (which prevent air movement up through suspended wooden floors) and, with the introduction of central heating, the removal of open fireplaces.

Houses have become effectively sealed boxes, keeping in any moisture produced within the house and providing ideal conditions for condensation to occur.

Ventilation is only effective if it is consistent throughout the whole envelope of the house. Condensation is encouraged by poor air circulation where stagnant air pockets form (behind furniture and in cupboards) and the first evidence is often the appearance of mould growth and a musty smell on clothes in wardrobes.

The warm moist air rises to the highest points in the building, forming condensation in those areas, which are often coldest, including bedrooms, wardrobes and upstairs bathrooms and toilets.

The modern life style, and particularly that of tenants, means that many houses remain unoccupied and unheated throughout the greater part of the day, allowing the fabric of the building to cool right down. The moisture producing activities are then concentrated into a relatively short time period, producing large amounts of steam when the building structure is still relatively cold.