Instead of running a nail or screw through an overhand knot to attach a sash cord the old-fashioned way, use a keyhole. The first step is to restore a clean solid surface down the both side edges of the sash. Do that by filling the old cord groove and knot hole with new wood and a little Bondo. After the glue dries, run a saw or plane down the length of the sash edge to clean it and square it up. Then, cut a new cord channel and keyhole approximately where the old knot hole used to be. Because you’ll use 4 mm braided polyester sailboat rigging line for sash cord instead of fatter, old-fashioned cotton cord, the 1/2″ wide keyhole can be narrower than the old knot hole. That leaves just enough adjacent surface on which you can attach a weatherseal such as Conservation Technology’s brushseal down the length of the sash edge to block drafts. The keyhole will securely hold a figure 8 knot and make the sash cord easier to attach and detach.
-
Steps 1 & 2. After filling the old sash cord groove and the knot hole, and then routing a new 1/4" groove in their place.
-
Steps 1 & 2. The old cord groove has been cut out with a V-shaped router bit. The resulting trench has been filled with a strip of new wood that mates to the router bit's profile. The old knot hole was cleaned out with a Forstner bit and then plugged with a dowel. After the glue dried, a pass through the table saw cleaned up the edges of the sash. Then route the new 1/4" groove.
-
Step 3: Route a half-inch-wide keyhole approximately where the old knot hole used to be.
-
The keyhole holds a figure 8 knot securely and also releases it effortlessly later. The 4 mm polyester sail line is vastly stronger than fatter, old-fashioned cotton sash cord and, because it's skinnier, the new groove and keyhole are narrow and therefore leave enough adjacent surface on which you can attach weatherseal down the length of the side of the sash.
-
Cut-away view showing dimensions of the keyhole.
