BWF members such as yourselves frequently use our Member and Technical Helplines for assistance on a wide range of Employment, Health and Safety, Pay and Technical issues. We’re putting up a frequently asked question every Tuesday in order to help members and illustrate the type of advice we are able to provide. If you have any questions that you would like to see featured, we would be happy to assist. This question and many others like it can be found in our Question Centre. If you want further advice on any of the issues, don’t hesitate to ring the helpline or browse our extensive publications library, which includes our advice on resin exudation in joinery timber, care of products on site, and guidance on the new Parts L and F of the building regulations. This week: I Have A Problem With Condensation Forming At The Edges Of Double Glazing. Is There A Fault With The Window? Condensation occurs when water vapour in the atmosphere comes in contact with surfaces that are cool enough to cause the vapour to condense. The temperature at which the vapour will condense is known as the dew point. It varies according to the amount of water vapour that is present and the temperature of the air. As the inside air temperature is increased its ability to hold water increases. The atmosphere in a building can be likened to a bucket that contains a certain volume of water. As the air temperature increases the bucket gets bigger and can contain more water. Relative humidity (R.H.) is effectively an indication of how full the bucket has become. 100% R.H. means the bucket is full. As the air temperature falls, particularly as the air comes into contact with a cool point in the room, the bucket shrinks and the water can start to overflow, i.e. the relative humidity reaches 100% or the dew point. The water that has overflowed from the bucket appears on the cool surfaces as condensation. To reduce the levels of condensation it will be necessary to control the levels of water vapour in the atmosphere. New windows, which incorporate weatherseals, reduce uncontrolled air leakage (drafts which would have previously contributed to the ventilation of the building. If the rooms are aired and extraction is used in the wet rooms the humidity of the air will be reduced and, therefore, the levels of condensation. With a double glazed window, particularly one that has low-e glass and argon gas, the coolest point on the window will be around the spacer bar. If condensation was to occur it would be there, particularly on the lower portions at each side of the glass unit and along the bottom edge. This is not a problem with the window but a property which is common to all double glazing that uses standard, metal, spacer bars. The cooling effect of the spacer bar can be reduced by using glass units with ‘warm edge’ spacer bars.