We’re now well and truly into Winter: the leaves have fallen, the temperature dropped and the heating’s on. For us, this typically means the same questions from homeowners about window condensation.

MY WINDOWS ARE ALL MISTED UP OUTSIDE. WHAT’S WRONG WITH THEM?

The simple answer is nothing is wrong with them! In fact, external condensation can be proof that energy-efficient windows are performing well (see below).

IF NOTHING IS WRONG, WHY IS THERE CONDENSATION?

The longer answer is that the windows these days are so much more thermally efficient, that the heat we use to warm our homes doesn’t make it through to the outer pane. This pane of glass is, therefore, colder and when its temperature is below dew point moisture in the air condenses on it.

The Glazing and Glass Federation (GGF) defines condensation as the “physical process by which a gas or vapour changes into a liquid“. In our homes, the air surrounding us contains water vapour which is invisible to the human eye. The warmer the air, the more water vapour it can hold. However, there is a limit to the amount it can hold for a given temperature – think of your glass of beer on a hot holiday and how quickly the glass becomes wet. Once the limit has been reached the air then becomes “saturated.” Once this saturated air comes into contact with a surface at a lower temperature surplus water vapour forms – initially in the form of mist and, if excessive, in the form of droplets of moisture.

WHY DOES CONDENSATION ONLY FROM AT CERTAIN TIMES OF YEAR?

The reason it is worse in Winter (Autumn and Spring too) mornings is that the temperature of the glass can really drop overnight but the dew point can still remain fairly high.

Some windows can be affected more than others and slight changes in orientation or shelter can mean that windows or even panes next to each other can react differently.

WHAT CAN I DO TO STOP EXTERNAL CONDENSATION FORMING?

Unfortunately, there is not much that can be done to stop this phenomenon. It usually doesn’t last long and shows that the heat being used to warm your home is being saved rather than lost through your windows – thus potentially lowering your heating bills and reducing your home’s carbon emissions.

INTERNAL CONDENSATION – THIS IS NOT SO GOOD

If condensation forms internally (on the room side surface of the inner glass) this means that the temperature of the glass surface is lower compared to that in the room and is possibly at the outside temperature. Internal condensation is most likely to occur on single glazed windows and lower performing double glazed units with no gas between the panes as they more easily conduct cold between one pane to the other. There are a number of ways to help reduce condensation.  Firstly look at reducing the amount of water in your home and most importantly increase ventilation and air circulation, best through an opening/vent in the window or ventilating unit. Ensure wall vents are fitted and clear. If condensation continues to be an issue you should install new double glazed units ideally with those that have Argon gas between the panes to reduce conductivity in the window.

CONDENSATION WITHIN THE WINDOW CAVITY OR INSIDE THE GLASS PANES – REALLY NOT GOOD AT ALL

If you find that the condensation on your windows appears within the two panes of glass it indicates that there is a failure within the seal. In older or poor quality units the sealant used to create the seal may become loose over time or be of a low grade. The seal around the panes will start to degrade and crack, allowing condensation to form in between the two panes of glass. Condensation can also occur if there is a fault with the “spacer” bar. A good quality double glazed window will feature a spacer bar which contains desiccant (a substance used as a drying agent), which will absorb any moisture within the air gap and prevent condensation from forming. In this case, we would advise you to replace your old windows with new ones or change the glazing in existing frames.

If the condensation occurs in the cavity, to undertake this work for you, you will probably want to get a professional:

  • Remove the secondary glass pane
  • Discard and remove any desiccant
  • Drill holes to connect the cavity to the outside drier air
  • Dry out the frame area.
  • Seal up any holes or cracks with compound or wood filler
  • Seal completely all wooden surfaces in the cavity with the proprietary wood sealer
  • Replace the secondary pane, taking care to make the seal and all joints as near airtight as possible

 

Main source from FENSA website here – External condensation, Internal condensation.

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