Gap under window

[PCAnimal]

Hi, in one of my bedrooms i can feel the window sill is very cold. I took a look outside and see a good 2 to 3 inch gap and screws into the bricks. Those screws are from the window frame. They were fitted before i bought the house and are double glazed
My question is should i put rockwool in the gap or expanding foam? As the gap is under the sill you can't get rain in there. My gut instinct is to foam it

 

[TicTac]

Foam it, but then you need to cover it over, and seal it somehow.

 

[PCAnimal]

Cheers am i being thick why did i need to seal it?

I could use a white pvc strip ive got in the shed. Is that suitable

 

[TicTac]

If you had rockwool in there, it would get wet if not sealed up afterwards. If you put in foam, it'll react to the light, so both solutions need sealing afterwards. A white PVC strip would be fine, but what should have been there; and old window cill, or bricks perhaps.

 

[^woody^]

A 2-3" gap that you never noticed before?

 

[PCAnimal]

Its a first floor window next to a pitched roof. It was only in the recent cold i noticed how chilly it was. When i stuck my head out i spotted the gap. Its not really visible from the ground.
Thanks for the advice I've just given my foam gun a good clean up