Glass condensation and misting – What it is and why it Happens

Misted glass and condensation are far from a rare problem. They have plagued homes, buildings and glazings of all sorts for generations, and as minor as they may seem, condensation and misted windows can be caused by a host of external factors. Although damp is a common problem with single-glazing, modern layered insulated glazing has experienced its own share of condensation, despite the best efforts of manufacturers. This old issue is slightly more complex than one might first imagine.

Condensation or window misting implies one significant thing – lack of thermal consistency in a building’s structure. As thermal images demonstrate, many buildings suffer from gaps in their thermal bridges, holes through which energy escapes, causing conflicts of temperature and depositing condensation on the surfaces. These thermal gaps often exist in the form of windows or wall-floor connections. In minor instances, condensation collects and forms damp or just looks ugly. In rare cases, cavities between layers of glazing can gradually fill with water and need replacing entirely. So why does this occur so often, and what can be done to combat it?

Causes of Condensation

The direct cause of condensation is probably familiar from school science – moisture in the air cools and collects on a surface. But there are a number of factors that impact the ways in which moisture forms on surfaces, and where exactly it manifests. Double glazing is particularly prone to condensation, but placement of moisture and which face it forms on can help you to deduce what your particular problem is, and how to deal with it quickly and effectively.

Inner Condensation

Condensation forming on the internal face of the window (i.e. the one you can touch when you’re inside) can usually be put down to one or a combination of several factors: surface temperature of the glass; external temperature and climate; internal air temperature; internal humidity; internal ventilation rate. All of these factors – except, of course, the external climate – can be controlled and modified if need be. It is often advisable to try to collect water from the source as soon as possible to avoid condensation and damp problems. Maintaining a building well-heated and ventilated will help to prevent condensation, as will warm edge spacer bars, which work to regulate the temperature of the thermal cavity.

Outer Condensation

At first look it may give the impression of poor glazing, but condensation forming on the outer edge of a window suggests entirely the opposite. Outer condensation is caused by a few external factors: the heat being passed from the interior through the glass (which, itself, is caused by the temperature differences between the internal and external surfaces); heat exchange by convection with the external air; heat loss by radiation. Studies have shown that in overcast weather, heat exchange by radiation is fairly minimal, but in clear conditions, considerable amounts of heat are lost to the sky.

The higher the quality of the thermal insulation, or U-Value, of the window, the less heat transfers to the external surface, meaning that the external surface is almost always significantly colder, causing outer condensation. Quite contrary to what some may assume, outer condensation is in fact indicative of highly efficient glazing, and is nothing to worry about.

Sandwiched Condensation

This is the condensation that really causes hassle. Internal or external condensation can be mopped and polished, but it is when moisture somehow seeps its way into the cavity between the two layers of double glazing that annoyance is caused. It looks unsightly, is virtually inaccessible and cannot be remedied by any simpler means than having windows removed and replaced. In extreme cases, this space can fill up with water.

Unfortunately, traces of moisture between the layers of a double-glazed window are a sure sign that sealings have failed and that the glaze is no longer air- or water-tight. Take care to consider conditions: temporary inner condensation can be caused by periods of high humidity or particularly cold weather, or in bathrooms and kitchens where short bursts of humidity are expected. However, should you notice condensation between layers that is still there after some time, or when conditions have changed, it is likely that they need replacing. Once sealing has given way, the whole window will decay quite abruptly, so it is important that any instances of moisture are monitored and acted upon.


Very often, condensation comes down to conditions, some of which are controllable and others which are not. But in any case, the only time condensation becomes a real issue for glazing is when it is sandwiched between the layers, at which point nothing but replacement can solve the problem. But in the meantime, give a bit of extra consideration for the temperature and conditions of the building. Maintain both comfortable heat and ventilation, to sustain thermal efficiency and keep air and heat flowing.