These are the four main reasons why window condensation can build up in glass sealed window units. Because sealed window units are not designed to be disassembled, they must be replaced. To attempt to split a sealed window unit, clean it and re-seal it again on site would be far more costly in labour terms, as well as almost certainly failing again in the short term.
The main problem for window condensation is usually the hot barrier sealant around the edges not being added properly. The hot melted window sealant is gunned down the sides of the window units, cools and solidifies. However, if any parts are missed, added at the wrong temperature or from old stock then there will be long and even short-term failure.
A break in the window seal only has to start as a pinhole, enough to draw in moisture, depending on the humidity of the air and temperature, which is common to ‘dry out’ again. However, like a tire puncture, the problem now exists and moisture will return inside the window unit. Eventually dirt builds up between the glass panels and the only long-term solution is to replace the window.
A window unit may appear solid when installed but it is very important to use proper blocking in order to support both window panes of glass and even more crucial the larger the size. Unsupported window glass can slip down 10 to 15mm in relation to the other piece. This will breaks the window seal causing condensation to build up inside the window.
Poor or no drainage seems like a less obvious cause of window condensation.It is not a continuous problem in drier seasons but window frames, uPVC and aluminum windows are designed to shed water away. If this fails and the drainage is inadequate, the bottom of the window glass is sitting in water, and then it acts as a source, drawing the water into the window unit.
Movement of the framework can often be a long-term cause, usually due to distortion of the window frame over time. Window frames can expand and contract by surprising amounts depending on temperature changes.
A window will often not retract to its original position and/or size, and eventually distortion will occur. If part of the frame begins to touch and put pressure on the window glass, the seals can break, or in very extreme cases, the window glass can crack.