Condensation in double-glazed acrylic windows
Occasional condensation in the windows of a caravan or motorhome often leads to the opinion throughout the recreational vehicle industry that the window pane is leaking and needs to be replaced.
With all due understanding for your quite justified concerns, in almost all cases of complaint, the window complained about is therefore by no means faulty if condensation should form between the panes in certain weather conditions.
Don't worry: Over time, especially in better weather conditions, the water evaporates again on its own without a trace.
Why does water vapour or condensation actually 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.
If the temperature now drops, the 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 3 mm thick acrylic pane (surface area 1 sqm) in 24 hours; provided that a lower humidity prevails on the other side. The amount of 0.3 g does not seem large at first moment. However, if we consider that 1 m³ at 20°C can absorb a maximum of 17 g of water, an increase of 0.3 g 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 also 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 still 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 by the heating. The external environmental factors such as warm-cold, humid-dry, sun-clouds, etc. are, on the other hand, not predetermined by humans and 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.