We are finding an increasing number of problems with UPVC window seals. The predominant seals are rubber and these go brittle and loose their springiness allowing draughts to enter the building ( see if your curtains move when it is windy outside )
In other cases seals and glazing rubber seals have shrunk back leaving gaps at the corners, this could be as a result of the original seals having been stretched into place rather than pressed into place. Alternatively the seals may be shrinking with age, UV exposure or ozone action.
Some budget windows omit the outer seal on the sash but generally have the grooves for them present. Replacement seals are readily available and if yours have failed for any of the above reasons they should be replaced. Please do this rather than replacing whole windows.
My own windows are wood clad with insulated aluminium sections on the outside with two rubber seals and two foam filled nylon tube seals to each sash. If i had UPVC windows with rubber seals I would seriously consider using encapsulated foam seals.
This video is useful
And a great comment from a supporter, “This happened with the uPVC windows we inherited. Massage the seals back into the corners and add a blob of superglue in the corner sticking the seals together. If they have shrunk a lot do alternate corners in pairs with a day in between. Voila”