Why are the windows upstairs in our house steaming up?

Our house is 25 years old and there is ventilation in every room. When the upstairs bedroom windows are closed at night, condensation forms which has to be cleared in the morning. Strangely this never happens in any of our downstairs rooms. Should we replace the windows and/or glass in the bedrooms or can you suggest an alternative solution?

Condensation on windows, particularly non-thermally broken aluminium framed units commonly installed in the late Eighties and Nineties, is a result of a difference between the indoor and outdoor temperature and the internal humidity. When warm, moist, humid air contacts a cold surface, water vapour in the air reverts to water on the cold surface. The higher the humidity, the more condensation forms, typically in kitchens, bathrooms and less-used or upper rooms.

This will occur particularly if you have no mechanical ventilation to extract humidity in rooms where the activities generate lots of water vapour and only rely on the older air brick-type of passive ventilation. The reason it is not happening downstairs is most likely because you have a higher heating regime there and this raises the vapour pressure, which encourages that warm moist air to rise upwards.

In houses built after 2011, the propensity for this to happen is lessened. Changes to part F of the building regulations then promoted whole house heat recovery ventilation systems. These extract the warm moist air from the damp rooms and recover some of the heat energy which is transferred to warm incoming fresh air.

These systems work best where good insulation is provided in an airtight house and “cold bridges”, such as cavity closers or points of less or no insulation, are minimised during the build. Nevertheless, introducing such a system into your house may be the best way to alleviate the symptoms you describe if you have good insulation installed or the following actions don’t help: ensure that “wet” rooms are ventilated, preferably with a humidity-controlled extract fan; ensure that clothes are not dried in the house, try for a more even heating regime with fewer stops and starts, possibly at a lower overall set temperature; when cooking, make sure an extract fan ventilates directly to outside air, particularly if boiling food.

When you have done this, then you might then consider upgrading your windows. Consider triple-glazed and the lowest U value you can afford, however beware that the condensation forming on your windows helps reduce the vapour pressure, acting as a sort of safety valve. Stopping such visible condensation without reducing the production of vapour can lead to trouble elsewhere that might not be so easy to resolve.

The Sustainable Energy Authority (SEAI) is a good source of information for this type of problem. It also advises: “Eliminate the source of the moisture at source, ensure that there is adequate ventilation throughout the house, properly insulate the home to bring up the internal temperature of the house, additional heating (in the cooler rooms) will also raise the internal temperature reducing the likelihood of condensation but adequate ventilation would also be required.”

if you still need help, contact your local building surveyor who can analyse the specifics of your case and provide appropriate solutions that won’t just transfer the problem elsewhere. Remember, the warmer the air the greater capacity it has to hold water, so the more water vapour you give warm air through showers etc, the greater the chance that condensation will form on relatively cold surfaces.

Fergus Merriman is a chartered building surveyor and member of the Society of Chartered Surveyors Ireland, scsi.ie