Cavity gap next to upvc window

[Sparky1970]

Hi, Im decorating my hallway, on doing prep work found crack around window frame but in the corner by internal sill there is a large gap about size of tennis ball and can see the cavity, whats the best way of dealing with gap as I need to have it ready for a plasterer?

regards

 

[netty1]

From my understanding expanding foam is the answer. If the foam willcome in contact with the double glazed frame, then you should use a window foam filler, so that is does not react with the frame.
Use a search engine to Check out foam fillers types and usages