Sweat Alumnster - what can you do?

Sweating is a common problem with aluminum windows. Why the windows sweat, what physical processes take place, and what you can do about it, read in detail in this post. In addition, why all possible "remedial measures" usually do not work.

Reasons for sweating aluminum windows

Old aluminum windows are fundamentally different in construction than modern aluminum windows. This not only applies to the glazing, whose U-value is no longer permissible today, but above all the frame.

Aluminum is a good conductor of heat. This means that in the winter, the cold outside from the outside passed unhindered inside and radiated into the interior. This physical process can not be prevented. The high thermal conductivity is a fundamental property of aluminum.


Since the cold is transferred from the outer frame to the inner frame, condensation will form on the inside of the window frame. Warm air flows to the cold window frame and cools down quickly.

Due to the rapid cooling, the moisture in the warm air condenses on the cold window frame and settles there. As more and more warm air flows to the window frame in the attempt to balance the temperature, more and more condensed water is generated.

Condensation between frame and disc

Another point concerns the spacers in old aluminum windows. They are also usually made of aluminum. The spacers connect the (hermetically sealed) glass edge to the window frame. In between there is a gap.

Now comes the warm glass edge (because the inner pane is indeed warm) with the cold spacer in touch cools the glass edge on the inside from strong. As a result, the air present in the gap also cool and it comes to condensation between the frame and glass.

consequential damages

By dripping down the condensate, the adjacent components can be heavily moistened, and it can form mold. Other moisture damage is possible.

Due to the radiated cold and the cooling of the air, aluminum windows act like a reverse heating system when the outside temperature is low: they permanently cool the room air, the heating system has to fight massively against this "anti-heating" in order to keep the room temperature constant. That drives heating costs to unprecedented heights.


The only effective remedy is to replace the windows. Modern window frames are thermally separated, so that the inner frame can not cool anymore. The actual problem is solved, the sweating disappears (unless the new aluminum windows are leaking or installed improperly).

Tips & Tricks

If you calculate how much heating costs you lose through the windows, you can not use classic arithmetic programs. It is not about the low thermal insulation of the discs, but especially about the frame. Its effect as a "cold radiator" leads to significantly higher heating costs than just inferior glazing. It's like having all the windows open all the time.

Video Board: