Plastic windows crack in the upper corners

Plastic Coefficient of Linear Expansion α (mm/m K)ABS 0.10 PA 0.10 PA 120,144 pe 0.15 - 0.20 pp 0.16 - 0.18 PPS 0.15 PVC-u 0.07 - 0.08 PVC-C 0.06 - 0.07 PVDF 0.12 - 0.18

Per increase in temperature by 1 Kelvin, PVC expands 0.06-0.07 mm. Per meter. There is a steel core inside, which does not expand. Neither does the glass. Karsten

The window fitter was just here because a terrace door no longer opens (something broken). He only said that they have total problems with the dark windows on south-facing sides. They heat up extremely. Then something gets stuck somewhere and they have to come and readjust the gaps. In winter, wind whistles in and they have to come back and readjust the gaps again. And it keeps going back and forth like that. He would not recommend dark windows.

Darin ist einSstahlkern, der dehnt nicht mit. Das Glas auch nicht. Karsten

Sorry if I’m being pedantic now, but steel and glass also expand. No length change in steel would be every bridge builder’s dream. However, the coefficients are lower than for plastic. Ultimately, the difference in the coefficients of expansion is to blame...

But why should the windows jam during expansion, the frame expands homogeneously as well. We have no problems at all with our dark windows. Maybe it is advantageous if the windows are installed in the air layer and not in the masonry, then they can expand more freely.

The frame is connected to the masonry and also underpinned, so the frame cannot expand as it wishes; the sash expands almost freely and then jams in the frame or the sash warps.

Installation errors and/or incorrect installation positions, as well as dark large elements or door fillings cause problems.

Aluminum is a bit more forgiving here, especially with door fillings there are warp-resistant profiles.

Please check the contract to see if the window manufacturer or seller has fulfilled their duty to inform and request a correction.

Regarding my previous post: since the cracking continued (depending on temperature/sunlight), a technician from Velux has by now made three attempts to fix the defect. Without success. During one visit, the Velux technician admitted that we were a kind of "guinea pig" in his defect repair attempts, as this problem had also occurred with other Velux window owners. Since the defect immediately appeared again in sunny weather but also at night due to cooling, Velux was asked to take action again. However, Velux suddenly refused and claimed—completely illogically—that we should hire an expert because the cracking probably did not come from the windows. But this is untrue, as it is still the same cracking and it clearly comes from the windows. Furthermore, only windows of the Velux series Velux-Serie GPU FK and UK are affected. We have a Velux window of the old VU series in the same roof structure that does not crack. It was installed by the same roofer in the same roof structure at the same time, which shows that only windows of a specific Velux production type are affected. Now, in November, there is less cracking audible due to lack of sun, but it will return in summer.