Originally Posted by MeasureThrice
Yes, there is a void.
Depending on your intended needs, you may consider filling the gap with 1-1/2" closed cell backer rod. About .50c /ft.
Most of our windows were not sealed well on top where the AL window frame meets the FE bus rail. The side rails are not flush with the drip edge. No water intrudes, just air, which travels over the top of the AL frame and into the interior. A nasty source of convection when calculating the structure's envelope leaks.
The CCBR does not absorb water. Optionally, you can add calk to the exterior. Rope calk or calk cord are easy to pressed into the gap or use Loctite PL S40.