Flashing and Sealing Opening for Replacement Insert Windows
I’ve read a ton on how to flash and seal new windows when exterior trim is being removed or there is existing WRB to seal to, but having a hard time finding what to do with old house with no WRB or flashing, sash weight pockets, and replacement windows installed from inside.
I’m renovating a 1900 farmhouse in western Massachusetts and planning to replace original wood single pane double hung windows, probably with Marvin Elevate double hung double pane wood clad windows. The current window jambs are 1x wood with real 2x sill dadoed into them at the bottom at an angle. The side jambs are about three inches from the rough opening to allow space for the old sash weights. The side jambs have a number of openings in them: two pulleys for sash weights and panels cut in the jambs that I think were used to re-attach sash weights. Three pieces of wood held the two window sashes in place: the exterior stop attached to the end of the jamb, a parting bead, and an interior stop screwed into the jamb. We’re planning to remove the inside stops and parting beads and install the new replacement windows inside the existing jambs against the existing exterior stop, and finish with new jambs and sill attached inside to old.
Outside: The exterior sheathing stops at the end of the rough opening; the gap between the rough opening and the window is covered by exterior window trim that appears to be nailed to the sheathing on one side and the exterior stop on the other side and then covered with metal coil stock. Our siding is wood shingles covered by another layer of cement shingles with what looks like metal L flashing between siding and window trim. We are planning to leave the siding and window trim in place and finish trim with new metal coil stock once the new windows are installed.
Inside: We’re removing the plaster and lathe on exterior walls to insulate from the inside, then planning to put a continuous layer of taped foil faced polyiso insulation across studs as air barrier, then build a 2×3 wall inside that for services and additional insulation. Probably going to fill the sash weight cavities with spray foam (thinking I’ll do it with 2 part froth pack or similar). The continuous polyiso should then run over the sash weight cavity to the old jamb.
So trying to figure out best way to flash and air seal. Current thinking:
1. Use flashing tape to cover old jamb from exterior stop in and seal to the interior polyiso. I’ve seen a lot of recommendations for 3M 8067 Flashing Tape as a good balance between quality and cost.
2. Remove old coil stock and use liquid flashing to cover original window trim, sill and exterior stop, connecting to the flashing tape inside the jamb. This will be covered by new metal coil stock running along the trim and into the window. Thinking about using a liquid flashing that can be rolled rather than thicker versions that require troweling would be efficient.
3. Cover sill with liquid flashing and then tape inside of sill to polyiso.
4. Caulk back of exterior stop when new window installed.
5. Caulk interior of window to sill, leaving exterior side open to drain.
6. Spray foam gap between new windows and old.
I’d love any and all feedback on approach, materials, etc. and any alternative suggestions. Thanks.
p.s. pictures show old storm windows, which will be removed.
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