My house was built in the mid-80's and has a number of retrofitted vinyl windows. The bathroom one has begun to bow downwards from the top - there's a wood sill incorporated and it's actually separated from the vinyl where they join.

I can't say if it's always been warped, or if it's just happened (it was never painted, and the factory undercoat is starting to come off, so it clearly needs painting.) It may have warped from the humidity, I'm not sure.

I was going to remove some trim and see if it was sprayfoamed incorrectly, in which case I'd cut some away and re-do it with the correct stuff.

What I thought I'd do is drive a few screws across the top, upwards, to pull it back square (it is opening and closing ok) gently, a few turns at a time.

I also noticed that when they installed it, they put a screw through the side of the vinyl part near the outside, rather than through the wood part, where I'd think the best part to fit a retrofit window would be.

Is this a good idea, or should I leave it? Now I'm aware of it, I hate the look of the bowed section.