The truth about condensation on windows

Condensation that forms on windows and/or doors and in conservatories, along with the damage condensation it can do to paintwork, curtains and wall paper etc, are problems still encountered far too often in different types of English buildings.

Modern living has often created rooms which are warmer but which can often have less ventilation and fewer air changes. The result is that water vapour produced by normal living is no longer able to escape up the chimney or through door jambs, window joints and other outlets. In fact in certain circumstances, such material comforts can combine to create ideal conditions for the formation of condensation, which tends to form on the coldest surfaces within any room. This may not necessarily be on the glazing.

The Glass and Glazing Federation’s own publication about condensation on their website, entitled â€ŔCondensation, Some Causes, Some Advice” goes out of its way to make it clear that if condensation occurs on the outside of the external pane on double or triple glazed windows this is a positive sign that these thermally efficient replacement window products are in fact performing well. Such a phenomena tends to only occur at certain times of the year, depending on a property’s location and aspect. Normally such condensation forms early morning and is gone by lunchtime.

As the Glass and Glazing Federation’s article on condensation advice for UK home owners states â€ŔDue to recent innovations in the efficiency of double and triple glazing, along with updated requirements of building regulations and the lowering of carbon emissions, certain weather conditions may allow the formation of external condensation on energy efficient windows and doors. This is a natural phenomenon and a clear indication that the window or door is preventing heat loss from your house.”

That said, condensation between the panes of glass in a double or triple glazed sealed unit means the unit has â€Ŕblown” and is no longer providing the same level of thermal barrier that a modern high performance hermetically sealed unit can provide. Such blown units should be replaced ASAP, particularly in the colder months.

If UK home owners who have fitted energy rated replacement windows still experience condensation on the inside of their windows, it usually means that their property has a particular problem with condensation that they need to address through improving the balance between heating and ventilation. More often than not, these cases are caused by insufficient ventilation within certain rooms that need addressing, as new windows and doors are designed to retain heat, so if there is not enough ventilation then condensation can build up if warm moist air is trapped with nowhere to escape to. Such condensation can be reduced by leaving open window vents to allow fresh air in, or retro-fitting trickle ventilation (although these trickle vents can be expensive, ugly and defeat the object of fitting thermally efficient window products). Other solutions include adding air bricks, buying a de-humidifier or installing mechanical ventilation.

When fitting replacement double glazing, homes with a known internal condensation issue (e.g. which is quite common in blocks of flats) that need more permanent ventilation can have trickle vents built into the window’s or door’s outer frame. Modern double glazed replacement windows should offer a lockable night vent facility on all opening vents, which is a far better solution to condensation than trickle vents as the window can then be shut to keep out extreme cold if need be. The key factors in reducing condensation are having a) thermally broken, energy efficient windows and doors, b) sufficient heating, and c) enough ventilation.


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