How should the top of new aluminium window be attached?

Lunielumps

I'm redecorating the bedroom and replacing this ancient uPVC window. 

I've discovered that the existing window frame is not attached to anything along the top?? It is a 1930s house with cavity walls.
Above the window is just the soffit but, there is a ~20mm gap between the soffit and the frame all the way along. I can push my fingers through to the outside in places!

I am replacing it with a new aluminum frame window and dont know what to fix the top to?
The internal skin of brick has a sturdy wooden lintel supporting further brickwork, but this window needs to be fixed the the external skin of brick which seems to have nothing other than the flimsy soffit above it. 

Any help much appreciated as i'd need to create a good seal to make the most of the spec of the window in terms of insulating from heat loss. 

Thanks!

stuart45

Fix it to the jambs, not the top

Lunielumps

stuart45 said:

Fix it to the jambs, not the top

Thanks Stuart. I'd assumed all four sides of the new window frame would need to be fixed to something? 
If it isnt tightly secured to something at the top, won't a lot more cold air/ rain get in, which could be a bigger issue for the new aluminium frame?

stuart45

You've got nothing at the top to fix to, so you can only do 3 sides and use foam for the gaps.

Lunielumps

if there are any suggestions of something i could do to the top of the opening so that the window can be weather proofed on all sides that would be great - i'm really keen to make sure there isn't any water ingress and excessive loss of heat above the window frame.

If a lot of cold air gets in above the window frame, given is it aluminium, this could cause significant condensation issues inside...

stuart45

If there's a gap at the top use foam. 

ytfcmad

You could use straps to secure the top.

stuart45

ytfcmad said:

You could use straps to secure the top.

Yes, I forgot about straps.