Should I use new construction windows if the sills are rotting out?

If you're already finding soft spots in those sills, you shold definately move away from the idea of using replacement inserts. Those pocket-style windows are designed to sit inside a structurally sound, weather-tight existing frame. If you just drop a new insert into a rotting opening, you're essentially burying a problem that will continue to spread into your wall studs and potentially your floor joists.

In a 100-year-old house, you’re almost certainly dealing with old weight pockets and sash cords behind that trim. When you go the **new construction** route, you get to rip all of that out, properly insulate those empty cavities with low-expansion foam, and acutally inspect the framing. Given the water damage you described, there's a high chance your jack studs or the framing under the sill have some degree of rot. You simply can't fix that properly without pulling the entire frame.

It sounds like alot more work—and it is—but it’s the only way to ensure the longevity of the repair. You’ll need to remove the exterior casing, which allows you to install proper **flashing tape** and a sub-sill pan. Most of those 80s-era retrofits skipped the flashing entirely, which is why your sills are failing now. If you find the framing is shot once it's open, sistering in a few new studs is a relatively minor task compared to the headache of a leaking window five years from now. I've seen plenty of DIYers try to just "patch" a sill and then pop an insert on top, but it rarely ends well. Once that old wood starts to go, the water finds a path down the trimmer studs. Tearing it back to the rough opening is the only way to be sure you aren't just hiding a mess. Plus, using a window with a nailing fin gives you a much better mechanical bond to the house's weather barrier than any replacement unit ever could.

That makes sense about the rot, but I'm worried about what happens to the siding if I go the new construction route. If I pull the exterior casing to get at the framing, am I going to end up having to redo the clapboards around the window too? I’ve seen people mention that nailing fins are a nightmare if you aren't alredy replacing the siding. Is there a way to do a full frame replacement without destroying the outside look of the house, or do I just have to accept that the siding is getting messed up?

You can skip the nailing fin if you're worried about the clapboards. Just use a "block frame" window. It lets you pull the entire rotted frame out to fix the studs, but you shim and screw it through the jambs instead of nailing to the exterior. You'll still need to build a new sloped sill and use good flashing tape, but it won't mess up your siding.

If you want the extra protection of a fin, just back-cut the siding about two inches with a multi-tool. Install the window, then cover that gap with some new 5/4 trim. It’s way easier than trying to slide a fin under old brittle siding and ussually looks better anyway.

One thing to keep in mind if you pull the whole frame is your jamb depth. Those 100-year-old walls are ussually way thicker than modern 2x4 or 2x6 framing because of the plaster and exterior sheathing. You’ll likely need to add extension jambs to the new window so it actually meets your interior trim. Also, since you're DIYing, grab a lead test kit. If you're cutting into those old sills and trim, you’re definately going to kick up some nasty dust. Better to know what you're dealing with before you start demoing everything.

If you're pulling the old frame, pay close attention to the drip cap above the window. Most of those old houses had a piece of lead or copper flashing tucked behind the siding. If that's missing or mangled, water will just run behind your new window regardless of how well you seal the sill.

Since you're rebuilding, I'd suggest using a piece of PVC for the new sub-sill if you want it to last forever. Wood is classic, but if you've allready got drainage issues, a rot-proof material gives you a lot of peace of mind. Just make sure you back-caulk the stops.