Subscriber OnlyProperty Clinic

Moisture is forming between the layers of my double-glazed windows. What should I do?

Property Clinic: How can I test my remaining windows to see if they are working properly?

My house was built in the year 2000 and fitted with double-glazing. Recently one of the glazed units has failed, moisture has accumulated between the two layers. How can I test the remaining units to see if they are operating properly?

Your query reminded me of a pre-purchase building survey I carried out for a client of a number of years ago. It had double-glazed timber casement windows and was about 25 years old. The window frames were poorly maintained and twisted. Typically, this can have an impact on the longevity of the double-glazing. I was surprised that despite the poor condition of the windows, none of the glazing showed any sign of failure. The typical fogging that one would expect to see, indicating failure, was absent.

However, on closer examination, I noticed that the property owner had drilled a hole externally in the bottom corner of each double-glazed panel. This allowed ventilation between the panes of glass, thus removing any mist or fogging. All double-glazing in the property had failed. Ever since, I am extra-cautious when looking at older double-glazed panels.

When double-glazing fails, condensation will occur between the panels of glass, leading to fogging. This is usually easy enough to see if weather conditions are right. Even in warmer weather when condensation may not occur, there will typically be evidence on the inside of the glass that the glazing has “fogged” in the past.


Double-glazing has a relatively limited lifespan when compared to other building products and is usually guaranteed for just 10 years. The date of manufacture is typically stamped on the spacer bar between the internal and external glass panes. Your glazing, at 23 years, is well advanced into its design life, and failure of one unit can mark the beginning of the end of remaining units. This does not mean that the remaining double-glazing will definitely fail in the short term. Rather than testing, you should simply monitor the glass for fogging between the panes, as this shows that the double-glazing has failed and needs to be replaced.

I wouldn’t recommend the drill-hole method of repair or concealment! Although I’m always on the lookout for this, thankfully I only ever came across it once.

Noel Larkin is a chartered building surveyor and a fellow of the Society of Chartered Surveyors Ireland.

Do you have a query? Email

This column is a readers’ service. The content of the Property Clinic is provided for general information only. It is not intended as advice on which readers should rely. Professional or specialist advice should be obtained before persons take or refrain from any action on the basis of the content. The Irish Times and it contributors will not be liable for any loss or damage arising from reliance on any content