Where to attach a storm window?
An outside mount with no sill would at least give you a drip edge there. It's just installation step one: rest on sill. So be careful trying to mount an exterior one. For an inside mount, remove and measure the screen. Have them replicated as storms and hold them in with clips to promote annual use. Caulk it shut if you must. And the rest of your trim while you're at it (the other half of your problem). You may need to install backstops for the new window as your screen is held in place by butterfly latches.





I highly suggest dropping that cash on completely new windows which would be operable and more efficient; inherently greener. (saves you money)