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Do metal windows and rooflights really cause condensation?

It is a fact that all metal windows and rooflights are more susceptible to condensation but this is not to be confused with metal windows or roof lights causing condensation.

By Paul Trace, MD, Lumen Rooflight

As the colder weather is now upon us, it is advisable to understand the causes of condensation and how to reduce its impact.

It is a fact that all metal windows and rooflights are more susceptible to condensation but this is not to be confused with metal windows or roof lights causing condensation.

Condensation occurs where humid air comes into contact with air, or a surface, which is at a lower temperature. Air contains water vapour and the warmer the air, the greater is its capacity for carrying water vapour. When warm, moist air comes into contact with a cooler surface, or cooler air, it drops in temperature and loses some of its capacity for storing moisture. Some of this is released to form condensation in the air as steam, or on the surface as water droplets.

It is more common to observe condensation on surfaces that cannot absorb liquid, e.g. windows, ceramic tiles etc, but it can form on any surface and it may not be noticeable until mould appears, or the material starts to rot.

Some examples of where the water vapour comes from:

Breathing: Two sleeping adults produce 1.5 pints of moisture in 8 hours, which is absorbed as water vapour into the atmosphere.

Cooking: Steam clouds can be seen near saucepans and kettles, and then seem to disappear. The clouds have been absorbed into the atmosphere.

The cooker itself may be a source of water vapour; eg. An average gas cooker could produce approximately 1.5 pints of moisture per hour.

Washing up: The vapour clouds given off by the hot water are rapidly absorbed in the atmosphere.

Bathing, laundry, and wet outer clothing: These are often the major sources of water vapour in the home.

Heaters: A flueless gas heater can produce up to 2/3 pint of moisture per hour. Paraffin heaters produce 9 pints of moisture for every 8 pints of fuel burned.

Indoor Plants: A frequently unrecognised but nevertheless significant source of water vapour.

New Property: The bricks, timber, concrete and other materials in an average 3-bedroom house absorb about 1,500 gallons of water during construction. Much of this is dissipated into the indoor atmosphere during the drying out period. The best solution for preventing condensation is ventilation, but installing double-glazing and thermally insulated frames will simply make the existing moisture in the room less apparent. In poorly ventilated rooms and humid environments such as kitchens and bathrooms, condensation can condense on cool steel skylight frames. Of course, it is better that this water vapour forms on the steel frames, where it can do less harm, rather than on internal wall finishes. Condensation forming on our Pilkington double glazed units is a good indication that the moisture content in the room is too high. Our Lumen rooflights are supplied with Pilkington Activ neutral, argon gas cavities and Pilkington Optitherm S3 double glazed units. These provide a centre pane measurement of 1.1.

If condensation does occur it is essential that it is wiped away. Leaving moisture on the metal frames and American ash linings will increase the risk of damaging your rooflight. The time to act is when you see moisture and not when you see mould growth on the liners. Prevention is far less problematic than curing issues that have been left for weeks or months.

It is interesting to note that every window manufacturer and Housing Association has now issued a pamphlet about condensation, and what can be done about it. Interestingly, not one of them says get rid of your metal framed windows or rooflights. The reason? Steel framed windows and rooflights do not cause condensation.

The download section on our site includes a copy of the Glass & Glazing Federation (GGF) guide to condensation. Please take a few minutes to visit this page at it can help you avoid problems in the future.

About Lumen Rooflight The Lumen range of rooflights includes designs for conservation and contemporary projects for both pitched and flat roof applications. Lumen rooflights are manufactured to an industry leading standard and incorporate a wide range of Pilkington glazing, including self clean Activ. Manufactured in the UK to ISO9001:2008 and available in a range of standard or bespoke sizes. Tel: 0330 300 1090