This Company Accreditations Cannot Be Believed
if you've got a new build property that had the windows originally installed by K-Ban Windows from Ashchurch near Tewksbury (ww.k-ban.co.uk) your opening windows (sashes) will probably have been made undersize.
According to the K-Ban Windows Customer Care Manager (now their Managing Director) they install approximately 1000 windows/week.
The windows fitted at my property have the Duraflex Diamond Suite Profile (ww.duraflex.co.uk) and the actual windows were fabricated by The UK Window Group (ww.ukwg.co.uk). The UKWG and Duraflex belong to the same group of companies. According to the UKWG website they manufacture 10,000 window frames/week.
Duraflex the profile manufacturers set the standards for the window fabricator to make the windows. In the Duraflex Diamond Suite Fabrication Manual it shows that the opening window (sash) should be 7.5mm/side bigger than the actual window opening (so 15mm o/all bigger than the opening that it fits into).
The opening window (sash) should be made bigger so it overlaps the rubber seal by 7.5mm on all four sides. You can roughly check this on your windows with the window in the closed position draw a pencil line around the window frame opening onto the closed window (sash) frame. When you open the window (sash) the size from the edge of the opening frame to the pencil line should be 7.5mm. The other way to check is to measure the window frame opening and then the opening window (sash). The opening window(sash) should be 15mm bigger.
Having spoken to both UPVC experts and the hinge manufacturers they both say the maximum manufacturing tolerance on UPVC windows is normally between 1.0 - 1.5mm/side with different companies. They also advised that having the reduced overlap onto the seal may cause the window not to seal or if it does seal initially as soon as the hinge wears will then fail to seal. Based on this the minimum overlap onto the rubber seal would be 6 - 6.5mm or the opening window (sash) should be at least 12 -13mm bigger than the window opening.
In my opinion the opening window (sashes) are deliberately made undersize to reduce the remedial works in the first two years for K-Ban Windows as the new build property drys out with shrinkage.
Having the opening windows (sashes) made undersize also creates other problems apart from the window not sealing or sealing that well. The Duraflex profile has an advertised Stack Height of 13.5mm (the gap between the opening window (sash) and the fixed frame where the hinge is fixed) but this is provided the opening window has been manufactured to the correct size with the 7.5mm overlap onto the seal. If you reduce this it will increase the Stack Height size in-situ. For example if your opening window (sash) only has a 5mm overlap (so 2.5mm undersize) this would increase the Stack Height to 16mm in-situ (13.5 + 2.5). However if like me the hinges installed have a 13mm Stack height with a working tolerance of +/- 1.5mm the hinges will wear very quickly & soon need replacing as they have been installed into an out of tolerance situation. The hinge manufacturer when asked about the set-up stated trying to install any product beyond its limitation will inevitably cause issues.
Likewise on the hinge side of the opening window (sash) if you have the plastic Locking Wedges installed these also come in two sizes 13mm (for Stack Heights 13-14mm) and 17mm (for Stack Heights 17-18mm). The idea of the plastic Locking Wedges is that it pushes the opening window (sash) onto the seal where fitted and stops the opening window (sash) bowing out in the closed position. If your opening window (sash) has been made undersize resulting in it having say a 16mm Stack Height in-situ then the plastic Locking Wedges in theory will have a 3mm gap between them when the window is in the closed position so actually not doing anything like they were designed for and certainly not helping to seal the window.