As I mentioned below, my house has a lot of old double-hung windows. They leak. A lot. And it’s endemic to the design of a double-hung window; there’s a lot of surface area to leak with very little way to seal it.
The standard weather-stripping approach is to use something called spring bronze, which is basically a 1″ wide bronze strip that you put between the window and the frame. I tried it with mixed results. Upside: less draft. Downside: it makes the window more sticky and results in a metallic screeching whenever you open or close the window. Screeching == bad.
Then I saw a company offering insulating parting stops (the square bit that keeps the upper and lower panes apart), and it got me wondering… could I do that? It turns out I could.
For reference: said company can be found here.
I looked at two options: weatherstripping the sash and weatherstripping the trim. The sash was attractive because it was bigger, but it turns out there isn’t a good way to get the edges weatherstripped because of how the ropes hung. That left me looking at the parting stop. That piece is 3/4″ wide by 1/2″ deep, and half the 3/4″ is buried in the wall. Was there enough left over? Yes. To the table saw!
I had weatherstripping (from here, specifically WS74) that was designed to fit into a 1/4″ deep, 1/8″ groove. I wanted this to fit as tightly to the stop as possible, so I also needed to hold the plastic shoulder of the weatherstripping. All together, I needed a profile like so:
where the light gray shows the part that’s buried in the wall and the dark grey shows the part I need cut out. Of course, I need to groove the front side for the lower sash and the back side for the upper sash. There’s also some overlap, but it all holds together.
I used the table saw and router. The saw can make a 1/8″ wide groove pretty effectively, but the end won’t be square. So I started with the table saw and made three passes per side, ending before the end of the groove. I then cleaned it up with a router and a 1/8″ bit.
Finally, I snapped in the weatherstripping and installed the parting bead. Miracle of miracles, it worked! Which mean I was halfway there – I also needed the top, bottom, and between the sashes.
I had expected to install the top edge weatherstripping by cutting a groove into the top edge of the upper sash and using tubular weatherstripping (specifically WS10). However, in at least two windows, the trim on the top edge was actually bowed and thus anything on the top edge of the sash would not seal. Instead, I installed it on the back edge of the top parting bead by milling another groove. It seems to work fine.
Niggly little detail: you want to be sure to weatherstrip the entire perimeter, so the pile weatherstrip actually fits into the top stop a bit. I freehanded the groove for that one.
Next up: meeting rail and bottom sash!