Because condensation produces its own water/moisture it often shows up as water running down the windows and pooling on the sills. Moisture obtained from condensation indicates that there are problems with the heating, structural insulation or ventilation (or even all three!).

Windows are less thermally efficient than the surrounding walls (especially in older buildings) making their internal surface temperature cooler by a few degrees than the surface temperature of the walls. Once the dew point occurs (100% relative humidity) it will start condensing on the cooler surfaces. i.e. the windows before condensing on the surrounding walls showing as droplets and the discolouration of window panes. It can be found inside walls and insulation as well as under floors and in some cases, corners of rooms where there is either no ventilation and/or air movement is restricted. It survives on emulsion paint or wallpaper and affected areas then attract black mould which grows on its surface. It can also show as black patches of mildew, wallpaper coming loose from the wall, and soft furnishings and clothes can also smell musty and get mildew when been placed in an un-ventilated airing cupboard.

So what can be done to reduce or eliminate it?

  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. If condensation is produced on cold pipes it can be prevented by using moulded insulation which is thermal insulation pre-moulded to fit plumbing pipes and fittings;
  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.

My block is a 1930’s build and there are always issues with condensation (in varying degrees). Sometimes it can be be partly caused by some flats being like greenhouses, or with lots of washing drying inside  but by far the biggest culprit is that of the fireplaces having been blocked up and vents either replaced by smaller ones, or taken out altogether by individual landlords,

This  is not the remit of the freeholder so putting aside living habits, unless the landlords unblock the fireplaces, and replace the vents to their original size the use of dehumidifiers will continue to be prevalent.


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