Condensation within a window
Condensation in double-glazed acrylic windows
Occasional condensation in the windows of a caravan or motorhome often leads to concern that the window pane is leaking and needs to be replaced.
While it is understandable to feel concerned, in almost all cases, the window is not faulty. With double-glazed acrylic RV windows, it is not uncommon for condensation to form between the panes in certain weather conditions. Over time, especially in better weather conditions, the moisture evaporates without a trace.
Why does condensation form in the window?
We humans live in two climates, so to speak; one inside and one outside. With a double-glazed window, however, one can speak of a third climate, namely that between the panes. Climate-determining factors are temperature and humidity, sometimes also the wind. When the temperature drops, relative humidity increases. If warm, humid air is cooled down, the moisture condenses on cold surfaces – for example, on the inside of the window panes.
Double panes for windows are made of high-quality acrylic glass. The panes are permanently glued together. In between is "normal" ambient air, which serves as an insulator. Acrylic glass is an organic material and therefore not diffusion-proof. This means that moisture and/or gases can enter and penetrate the pane. It can be assumed that at +14°C air temperature and 70% relative humidity, 0.3 g of water will diffuse or pass through a 3mm thick acrylic pane (surface area 1m²) in 24 hours; provided that a lower humidity prevails on the other side. The amount of 0.3g does not seem large at first, however, if we consider that 1m³ at 20°C can absorb a maximum of 17g of water, an increase of 0.3g in relation to the relatively small amount of air between the panes is a lot.
In simplified and possibly more understandable terms, the cause of condensation can be explained by the fact that acrylic glass – like almost all organic substances – can absorb water in molecular form. Assuming an originally completely water-free window, the panes will initially absorb water from the air at the surface. The water molecules then gradually penetrate the inside of the material by diffusion and finally reach the inside of the pane. Here, they are absorbed by the air trapped between the panes. If the water vapour content in the area around the window is high enough, this process continues until the air in the window is completely saturated with moisture.
Since the amount of water vapour that can be absorbed by an air volume decreases as the temperature drops, water is precipitated at the coldest points of the window when the temperature drops, where it becomes visible in the form of small droplets. If the pane then warms up again, the water evaporates again and seems to have disappeared. In fact, however, it is invisible – as water vapour – in the heated air between the panes. If the air around the window becomes very dry, the diffusion process through the acrylic glass starts in the opposite direction. The moisture eventually disappears again as far as possible from the window.
Tips for avoiding or reducing condensation
Constant air circulation is the best remedy against the occurrence of condensation on the inner sides of the double panes. Circulating air absorbs humidity, especially when the cold air is heated. Formation and disappearance can, unfortunately, only be influenced within certain limits. The only controllable factor here is the interior temperature in the caravan through heating. The external environmental factors such as warm-cold, humid-dry, sun-clouds etc are, on the other hand, cannot be influenced.
In extreme cases, the plastic plugs on the window can be removed and the two panes carefully pressed together and released again. This pumping action accelerates the exchange of air between the panes via the open vent holes. For panes without plastic plugs, a twist lock can be removed and the pumping process carried out.
Original source: Dethleffs Germany
