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Flashing and Sealing Opening for Replacement Insert Windows

stolzberg | Posted in General Questions on

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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  1. tallpinescabin | | #1

    I've been dealing with somewhat similar issues in my house.
    I have a 1921 2 story duplex, cheaply built at the time. Similar original window setup to yours, and I'm replacing many of them. In my case, I'm also pulling off the 100 year old deteriorated wood siding on mine AND removing the exterior trim. (Much of the trim has rot from neglect, and is still original wood, no covering.) The window sills are staying. I'm installing Boral boards for trim and water table, and vinyl siding. (Yes, an unusual combo.) I've used Marvin's Essential windows in many of the openings. My sash pockets are lath'd over inside, so I've been insulating them from the outside and then taping them before installing new trim. I also had cellulose blown into the 2x4 wall cavities. I'm not going as deep as you are, energy wise on this project.

    My thoughts...I'll nitpick things based on what sticks out to me..

    3M 8067: I've been using this tape quite a bit, and have used some other tapes elswhere. I like it, and would recommend it. After using it a lot on my old house this year, I like it enough that when I get back to my (started in 2020) cabin build in a couple months, I'll keep using it there.... Just be aware it has a strong smell for the first week or so.

    I question the need for the exterior liquid flashing on the trim. If you're leaving the other siding intact, and putting on new coil stock, etc.....seems like a lot of effort for not a ton of benefit to me? I could be missing something there, though.

    Marvin's Ultrex windows: When using at least the Essential line replacement windows, I've been told they ONLY come in a flat sill....Make sure you know if you're getting angled sills or not. If not, you'll have to solve for that gap at the exterior.
    Also keep in mind that the fiberglass frames are sealed in the corners only by silicone caulk, it appears. I've read reports of leaks through this joint. If I were you, I'd plan for this and have some way to drain whatever you do underneath the window frame. Seems like you have this covered, but becomes a bit of a concern if you are NOT getting angled sills (Where any filler used could stop drainage)

    Overall, I'm finding we're both on a tricky path. Modern flashing, materials, and water management techniques can be tricky to integrate into openings where some of the original components are retained. Deeper retrofits often remove the entire window frame and exterior siding/trim, which allows you to treat the ROs quite differently, and/or install flanged windows, etc. The amount of information on how to "properly" do this is lacking.


  2. stolzberg | | #2

    Thanks for the detailed response, nice to know I'm not alone in finding limited information about detailing openings in old houses, especially if exterior trim and siding is not being removed.

    Can you say more about what you mean by angled sills: are you talking about the bottom of the window unit and how it will rest on the existing wood sill (which is angled down and will be flashed with tape or liquid flashing) and covered in coil stock. If the bottom of the window is flat, then seems like it will be tough to insulate since not supposed to seal bottom of window to allow drainage. Could I put a thin sheet of polyiso on top of the sill and rest the window on that?

    I hear what you're saying about usefulness of liquid flashing under coil stock, thinking of it as belt and suspenders assuming some water will get under coil stock. I need to see what shape the trim is in once I take off the existing coil stock. Also, the house has had storm windows protecting the exterior stop until now, so once we remove those the windows and sill will experience a lot more exterior water. I've also thought about using new construction windows attached with fins to the existing trim and putting new trim over the old trim, but that would add a lot more cost.

  3. tallpinescabin | | #3

    Angled sills: You got it. The original window sill is usually angled (mine are about 7 degrees), but I was told, at least for the Essential line, that flat sills are all they can/will do for replacements. So exactly, as you said, it makes it tough to insulate. I suppose you could put some polyiso on top of the original sill...Maybe a wedge of bevel siding under it a bit to help flatten the angle a bit, too. Just something to check into/be aware of when you order windows.

    I hear you on the water under the coil stock. No good way to know how much can/does/will get under there....Belt and suspenders can't hurt, especially when it can't really dry to the inside.

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