Trimming Existing Windows for Continuous Insulation – GreenBuildingAdvisor

Good day Green Building World,

I come to you with a building question from the esteemed four seasons of climate zone 6A. My question is such:

I’m residing my house and adding 2″ of continuous Timber HP wood fiber insulation over Blueskin VP100 with a 1×3 rain screen and LP Smartside siding. I’m replacing a couple windows, but keeping most of my existing replacement windows (vinyl double-hung pocket windows in original wood frames). I’m seeking some opinions on how to trim these out. For better or worse, I already cut the existing wood sill (from the original windows) flush with the sheathing (some was rotting and/or with peeling lead paint). I attached some pics of the flashed windows for reference. I basically flashed them to the WRB as if they were new construction windows and worked some sill pan flashing underneath. I taped one cut origina sill with 3M but didn’t do the rest until I know my gameplan for certain (see pics).

For my trim process, I’m thinking of picture framing the windows with 2×4 + 1/2″ plywood as a nailer for my extension jambs, leaving me at the same depth as my continuous insulation (2 inches, in fact). I’m thinking of going minimalistic and butting my siding into 5/4 extension jambs to keep things simple and light on materials. 

I’m not sure what to do for my new sill trim. I slid some flashing tape under the original wood frame to act as pansill flashing. I’m thinking of taping the original sill frame with 3M tape and milling a new sill out of 5/4 that I’ll butt into the existing window, above the original cut-back sills. I don’t want to block window drainage (see small weep holes in one of the pics), but those weepholes/drainage gaps will still have a path to the water control layer, even if they are underneath my 5/4 sill.

I also considered planing the wood sill I cut flush with the sheathing, and screwing sill extensions onto that (in essence, cutting back the original sills back to good wood and replacing the exposed portion with fresh material).

Any thoughts, dreams, innovations, or inspiration on how to add sill trim to these existing pocket windows?

May you all build in fortitude and also in peace.

Petey Pablo