The most common cause of condensation inside a property is inadequate ventilation because it causes moist air to accumulate. It is created when what is known as the the dew point reaches 100% relative humidity and the maximum amount of water the air can hold. Condensation will then start on the cooler surfaces first and usually in a particular order:

  1. Window condensation because windows are less thermally efficient than the surrounding walls, especially in older buildings. which makes their internal surface temperature cooler by a few degrees than that of the walls. Shows up as beads of water on the window;
  2. Pools of water on the windowsill;
  3. Beads of water on external walls;
  4. Damp patches on external walls;
  5. Damp patches on ceilings;
  6. Damp patches in the corners of rooms;
  7. Damp internal walls;
  8. Wet internal walls.

This is followed by:

  1. Black mould on window frames, window recesses and windowsills;
  2. Black mould on walls – particularly corners and recesses with little air flow;
  3. Black mould in cupboards;
  4. Black mould on curtains and clothes;

As well as black mould, affected areas can also show as patches of mildew and wallpaper coming loose from the wall. Mould can also be white and fluffy.  All this will be accompanied by a musty, damp smell.

Note: Black mould can also be caused by rising damp and/or penetrating damp.

These photographs show the level of condensation and mould in a number of rental flats on our block.

Reducing or Eliminating Condensation and Black Mould

  1. There must be some ventilation provided to all rooms to enable moist air to escape;
  2. Excessive use of heating must be reduced, especially if point 1 also applies;
  3. Each litre of oil used produces the equivalent of about a litre of liquid water in the form of water vapour so no portable paraffin or flueless gas heaters must be used;
  4. Condensation can also be produced where there are no cooker hoods/extractors over the cooker. Much of the steam will be removed if they are installed and vented to the outside air and 6″ or 8″ in diameter;
  5. Using pre-moulded thermal insulation on cold pipes that have condensation on them;
  6. The internal surface temperature of walls can be increased by internally insulating with dry lining which is a system for the cladding of internal faces of buildings i.e. walls and ceilings.
  7. External walls can be insulated with an insulated render system;
  8. The addition of more extractor fans such as in the bathroom;
  9. Upgrading the heating system.

Treating Black Mould

The process for treating black mould is as follows:

  1. Clean the area with a solution of anti-mould cleaner to kill the spores and help prevent mould regrowth. This must be carried out before redecoration as just washing off the mould with detergent then redecorating will not kill the spores and mould regrowth is likely, even through anti-condensation paint. This will be most effective where the underlying problem of condensation has been remedied;
  2. Leave the anti-mould cleaner for the recommended time, then thoroughly clean the area, preferably with sugar soap (mixed with water to the manufacturer’s specifications) or detergent with water to remove any grease then rinse well.
  3. Apply anti-condensation paint which is a special paint which is hygroscopic, allowing the paint to ‘breathe’ and makes it ideal for kitchens, bathrooms and shower rooms.


A number of landlords on our block have automatically blamed the tenants and ourselves for any and all damp issues and it is indeed true that I have walked into some flats that were like greenhouses with the heating cranked up and washing drying inside. However, this does not cancel out the fact these same landlords have covered up fireplaces, (without adding air bricks), replaced original wall vents with smaller ones or taken them out altogether. We’ve even had one landlord paint over a kitchen vent!

We on the other hand have left our fireplace open and we have little or no condensation/mould issues at all!


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