the mystery of the condensation on windows

Condensation is water that collects as droplets on a cold surface when humid air comes into contact. Glass has a low thermal mass and therefore does not retain or store heat, resulting in a surface that generally feels cool to the touch. Imagine a glass of cold refreshing drink on a hot summer day. Perhaps a kettle just before it starts boiling. Or a box of a takeaway meal that got soggy instead of crispy and crunchy. Yes, all of this is called condensation. Another type of condensation is on the window glass, which mostly bothers us early in spring or autumn. 

There are three areas where condensation could occur within a glass installation. It is helpful to understand the difference between them, identify the solution, and offer peace of mind when condensation is not problematic. Keep reading for more information on where you might find condensation.


When condensation forms on the outside, on an insulated glass unit, it is an indication that the double glazed unit is well insulated and is not a problem! In some cases, where glass lifting equipment has been used to install the unit, residual sucker marks can become visible when condensation occurs. The spots are not a glass defect; they will simply fade over time or could be removed using a glass polishing agent. Congratulations, you don’t have to take any action besides putting the kettle on and enjoying your cosy home. 


When condensation happens on the inside of a house, it means there is a collection of water vapour generated inside the space. Several factors can drive this, including radiators, placement of house plants, closed curtains, and the number of people inside the room. Perhaps the construction of your home has just finished ad the house was standing without heating taking in all the humidity and moisture? 

Additional ventilation is required to prevent condensation from occurring on the inside; this could include specifying trickle vents on the window systems or opening a window/ door within the area. Homeowners should consider this during the specification stage of a project to ensure adequate ventilation is provided. Building Regulations Approved Document F outlines the ventilation plan required for various properties. We have several opening systems available, from windows and doors to advanced opening rooflight systems.


(cannot be wiped from the inside nor outside)

Suppose you identify condensation inside the glass cavity of an insulated glass unit. That annoying mist cannot be wiped out; it is obstructing the view and is irritating. There are two possible reasons. It is time to replace your windows, or if you just replaced it- someone was daydreaming when producing your unit and missed a slight gap that lets the air inside the unit. (means the seals are not airtight and needs to be replaced)

This can naturally occur when air gets inside the argon gas cavity because air contains water vapour.