The Truth About Condensation In Your Home

Modern living with the latest generation thermally broken, energy rated windows and doors, combined with better loft and cavity wall insulation has created more habitable rooms, but also living spaces that are much warmer. However, these changes if not thought through can lead to a reduction in ventilation and consequently fewer air changes.

Though this sounds all positive (which it is), unless there is a change in how these new homes are heated and vented, it can lead to an increase in internal condensation as the moisture generated by everyday living gets trapped by more thermally efficient products that are designed to keep heat in and cold out.

The net result can be 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 birthing of condensation, which tends to form on the coldest surfaces within any room. This may not necessarily be on the glazing.

Air inside a building contains a certain amount of water vapour, which generally increases for example when kettles are used, baths are run, food is boiled on the hob and especially if clothes are dried indoors. The volume of water vapour that the air can hold is a function of its temperature. Obviously warm air has the capacity to hold more moisture vapour than cool air, therefore, as the air circulates, and comes into contact with surfaces cooler than itself such as walls, woodwork and windows, the water vapour condenses, becoming liquid again. The steaming up of a bathroom mirror at shower time is a good example of how condensation droplets are formed; the mirror surface being cooler than the moisture-laden air. Whilst the condensed liquid will run-off glass or painted timber surfaces it is likely to soak into porous surfaces such as walls and ceilings.

Condensed water that soaks into walls or untreated woodwork will damage the underlying substrate of the building while simultaneously providing the perfect environment in which unpleasant and unhealthy mould colonies can grow. Many studies into the effects of inhaling mould spores unanimously agree that it causes damage to the immune system, skin rashes and respiratory disorders such as sinusitis and pneumonia. The way to combat this is to vent and hear each room to avoid condensation forming, or where mould does start to form (as say trapped between internal wooden shutters and windows), clean the area with a bleach based cleaner which will destroy any mould or spores..

Given new windows and doors are designed to retain heat, 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). Most modern aluminium and UPVC casement windows have a lockable night vent facility, that enables the opening vents to be locked in a slightly open position, and it is possible to fit restrictors on vertical sash sliding windows so that only can be slid open a few centimetres. Other more drastic solutions include adding air bricks, buying a de-humidifier or installing mechanical ventilation.

The Glass and Glazing Federation’s own publication about condensation on their website, entitled “Condensation, 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 it is actually a positive sign that these thermally efficient replacement window products are performing as they should. Such an occurrence tends to only happen at certain times of the year, depending on a property’s location and aspect. Normally such condensation forms during the night and is usually completely gone by mid-morning as the temperature rises or the sun comes up.

The Glass and Glazing Federation’s article on condensation advice for UK home owners states “Due 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 “blown” or failed 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 as soon as is practically possible, particularly during the colder months.

When fitting replacement double glazing, homes with a known pre-existing internal condensation problem (e.g. which is quite common in blocks of flats) that need more permanent ventilation, these can have trickle vents built into the new window’s or door’s outer frame. Modern double-glazed replacement windows whose design incorporate opening sashes, should be able to offer you 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) pay special attention to venting in kitchens, bathrooms and utility rooms, c) having cavity wall and loft insulation, and d) the right mix of heating and ventilation in each room.


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